J introtojava1-pdf
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This document provides an introduction to Java programming, including setting up a development environment. It discusses downloading and installing the Java Development Kit (JDK) and Eclipse integrated development environment. It then provides an overview of key concepts in the Java language and platform, such as object-oriented programming, the Java compiler, Java virtual machine, garbage collection, and Java Development Kit. The document is intended to help new Java developers get started with building Java applications.
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• volatile
• while
Note that true, false, and null are technically not reserved words. Although they are literals, I
included them in this list because you can't use them to name Java constructs.
One advantage of programming with an IDE is that it can use syntax coloring for reserved words,
as I show later in this tutorial.
Structure of a Java class
A class is a blueprint for a discrete entity (object) that contains attributes and behavior. The class
defines the object's basic structure, and at runtime, your application creates an instance of the
object. An object has a crisp boundary and a state, and it can do things when correctly asked.
Every object-oriented language has rules about how to define a class.
In the Java language, classes are defined as shown in Listing 1:
Listing 1. Class definition
package packageName;
import ClassNameToImport; accessSpecifier class ClassName {
accessSpecifier dataType variableName [= initialValue];
accessSpecifier ClassName([argumentList]) {
constructorStatement(s)
}
accessSpecifier returnType methodName ([argumentList]) {
methodStatement(s)
}
// This is a comment
/* This is a comment too */
/* This is a
multiline
comment */
}
Listing 1 contains various types of constructs, which I've differentiated with font formatting. The
constructs shown in bold (which you'll find in the list of reserved words) are literals; in any object
definition, they must be exactly what they are in the listing. The names that I've given the other
constructs describe the concepts they represent. I explain all of the constructs in detail in the rest
of this section.
Note: In Listing 1 and some other code examples in this section, square brackets indicate that the
constructs within them are not required. The brackets themselves (unlike { and }) are not part of
the Java syntax.
Comments in code
Notice that Listing 1 also includes some comment lines:
// This is a comment
/* This is a comment too */
/* This is a
multiline
comment */](https://image.slidesharecdn.com/j-introtojava1-pdf-151217013816/85/J-introtojava1-pdf-14-320.jpg)
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automatically. If Eclipse finds two classes with the same name, it displays a dialog box asking
you which class you want to add imports for.
Up next in the class definition (referring back to Listing 1) is the import statement. An import
statement tells the Java compiler where to find classes that you reference inside of your code. Any
nontrivial class uses other classes for some functionality, and the import statement is how you tell
the Java compiler about them.
An import statement usually looks like this:
import ClassNameToImport;
You specify the import keyword, followed by the class that you want to import, followed by a
semicolon. The class name should be fully qualified, meaning that it should include its package.
To import all classes within a package, you can put .* after the package name. For example, this
statement imports every class in the com.makotojava package:
import com.makotojava.*;
Importing an entire package can make your code less readable, however, so I recommend that
you import only the classes that you need, using their fully qualified names.
Class declaration
To define an object in the Java language, you must declare a class. Again, think of a class as a
template for an object, like a cookie cutter.
Listing 1 includes this class declaration:
accessSpecifier class ClassName {
accessSpecifier dataType variableName [= initialValue];
accessSpecifier ClassName([argumentList]) {
constructorStatement(s)
}
accessSpecifier returnType methodName([argumentList]) {
methodStatement(s)
}
}
A class's accessSpecifier can have several values, but usually it is public. You look at other
values of accessSpecifier soon.
Class-naming conventions
You can name classes pretty much however you want, but the convention is to use camel case:
Start with an uppercase letter, capitalize the first letter of each concatenated word, and make all
the other letters lowercase. Class names should contain only letters and numbers. Sticking to
these guidelines ensures that your code is more accessible to other developers who are following
the same conventions.
Classes can have two types of members: variables and methods.](https://image.slidesharecdn.com/j-introtojava1-pdf-151217013816/85/J-introtojava1-pdf-16-320.jpg)
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The basic class definition for Person isn't useful at this point, because it defines only its attributes
(and private ones at that).
To be more interesting, the Person class needs behavior — and that means methods.
Methods
A class's methods define its behavior.
Methods fall into two main categories: constructors or all other methods, of which there are many
types. A constructor method is used only to create an instance of a class. Other types of methods
can be used for virtually any application behavior.
The class definition back in Listing 1 shows the way to define the structure of a method, which
includes elements like:
• accessSpecifier
• returnType
• methodName
• argumentList
The combination of these structural elements in a method's definition is called the method's
signature.
Next, you look in more detail at the two types of methods, starting with constructors.
Constructor methods
You use constructors to specify how to instantiate a class. Listing 1 shows the constructor-
declaration syntax in abstract form; here it is again:
accessSpecifier ClassName([argumentList]) {
constructorStatement(s)
}
Constructors are optional
If you don't provide a constructor, the compiler provides one for you, called the default (or no-
argument) constructor. However, if you provide a constructor other than a no-argument (or
no-arg) constructor, the compiler does not automatically generate one for you.
A constructor's accessSpecifier is the same as for variables. The name of the constructor must
match the name of the class. So if you call your class Person, the name of the constructor must
also be Person.
For any constructor other than the default constructor, you pass an argumentList, which is one or
more of:
argumentType argumentName](https://image.slidesharecdn.com/j-introtojava1-pdf-151217013816/85/J-introtojava1-pdf-18-320.jpg)
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Arguments in an argumentList are separated by commas, and no two arguments can have the
same name. argumentType is either a primitive type or another class type (the same as with
variable types).
Class definition with a constructor
Now, you see what happens when you add the capability to create a Person object in two ways: by
using a no-arg constructor and by initializing a partial list of attributes.
Listing 3 shows how to create constructors and also how to use argumentList:
Listing 3. Person class definition with a constructor
package com.makotojava.intro;
public class Person {
private String name;
private int age;
private int height;
private int weight;
private String eyeColor;
private String gender;
public Person() {
// Nothing to do...
}
public Person(String name, int age, int height, int weight String eyeColor, String gender) {
this.name = name;
this.age = age;
this.height = height;
this.weight = weight;
this.eyeColor = eyeColor;
this.gender = gender;
}
}
Note the use of the this keyword in making the variable assignments in Listing 3. The this
keyword is Java shorthand for "this object," and you must use it when you reference two variables
with the same name. In this case, age is both a constructor parameter and a class variable, so the
this keyword helps the compiler to disambiguate the reference.
The Person object is getting more interesting, but it needs more behavior. And for that, you need
more methods.
Other methods
A constructor is a particular kind of method with a particular function. Similarly, many other types of
methods perform particular functions in Java programs. Exploration of other method types begins
in this section and continues throughout the tutorial.
Back in Listing 1, I showed you how to declare a method:
accessSpecifier returnType methodName ([argumentList]) {
methodStatement(s)
}](https://image.slidesharecdn.com/j-introtojava1-pdf-151217013816/85/J-introtojava1-pdf-19-320.jpg)
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What is a loop?
A loop is a programming construct that executes repeatedly while some condition (or set of
conditions) is met. For instance, you might ask a program to read all records until the end of a
file, or loop over all the elements of an array, processing each one. (You learn about arrays in this
tutorial's "Java Collections" section.)
for loops
The basic loop construct in the Java language is the for statement, which you can use to iterate
over a range of values to determine how many times to execute a loop. The abstract syntax for a
for loop is:
for (initialization; loopWhileTrue; executeAtBottomOfEachLoop) {
statementsToExecute
}
At the beginning of the loop, the initialization statement is executed (multiple initialization
statements can be separated by commas). Provided that loopWhileTrue (a Java conditional
expression that must evaluate to either true or false) is true, the loop executes. At the bottom of
the loop, executeAtBottomOfEachLoop executes.
Example of a for loop
If you wanted to change a main() method to execute three times, you can use a for loop, as
shown in Listing 8.
Listing 8. A for loop
public static void main(String[] args) {
Logger l = Logger.getLogger(Person.class.getName());
for (int aa = 0; aa < 3; aa++) {
Person p = new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE");
l.info("Loop executing iteration# " + aa);
l.info("Name: " + p.getName());
l.info("Age:" + p.getAge());
l.info("Height (cm):" + p.getHeight());
l.info("Weight (kg):" + p.getWeight());
l.info("Eye Color:" + p.getEyeColor());
l.info("Gender:" + p.getGender());
}
}
The local variable aa is initialized to zero at the beginning of Listing 8. This statement executes
only once, when the loop is initialized. The loop then continues three times, and each time aa is
incremented by one.
As you'll see later, an alternate for loop syntax is available for looping over constructs that
implement the Iterable interface (such as arrays and other Java utility classes). For now, just note
the use of the for loop syntax in Listing 8.
while loops
The syntax for a while loop is:](https://image.slidesharecdn.com/j-introtojava1-pdf-151217013816/85/J-introtojava1-pdf-37-320.jpg)
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while (condition) {
statementsToExecute
}
As you might suspect, while condition evaluates to true, so the loop executes. At the top of
each iteration (that is, before any statements execute), the condition is evaluated. If the condition
evaluates to true, the loop executes. So it's possible that a while loop will never execute if its
conditional expression is not true at least once.
Look again at the for loop in Listing 8. For comparison, Listing 9 uses a while loop to obtain the
same result.
Listing 9. A while loop
public static void main(String[] args) {
Logger l = Logger.getLogger(Person.class.getName());
int aa = 0;
while (aa < 3) {
Person p = new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE");
l.info("Loop executing iteration# " + aa);
l.info("Name: " + p.getName());
l.info("Age:" + p.getAge());
l.info("Height (cm):" + p.getHeight());
l.info("Weight (kg):" + p.getWeight());
l.info("Eye Color:" + p.getEyeColor());
l.info("Gender:" + p.getGender());
aa++;
}
}
As you can see, a while loop requires a bit more housekeeping than a for loop. You must initialize
the aa variable and also remember to increment it at the bottom of the loop.
do...while loops
If you want a loop that always executes once and then checks its conditional expression, try using
a do...while loop, as shown in Listing 10.
Listing 10. A do...while loop
int aa = 0;
do {
Person p = new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE");
l.info("Loop executing iteration# " + aa);
l.info("Name: " + p.getName());
l.info("Age:" + p.getAge());
l.info("Height (cm):" + p.getHeight());
l.info("Weight (kg):" + p.getWeight());
l.info("Eye Color:" + p.getEyeColor());
l.info("Gender:" + p.getGender());
aa++;
} while (aa < 3);
The conditional expression (aa < 3) is not checked until the end of the loop.](https://image.slidesharecdn.com/j-introtojava1-pdf-151217013816/85/J-introtojava1-pdf-38-320.jpg)
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Loop branching
At times, you need to bail out of a loop before the conditional expression evaluates to false. This
situation can occur if you are searching an array of Strings for a particular value, and once you
find it, you don't care about the other elements of the array. For the times when you want to bail,
the Java language provides the break statement, shown in Listing 11.
Listing 11. A break statement
public static void main(String[] args) {
Logger l = Logger.getLogger(Person.class.getName());
int aa = 0;
while (aa < 3) {
if (aa == 1)
break;
Person p = new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE");
l.info("Loop executing iteration# " + aa);
l.info("Name: " + p.getName());
l.info("Age:" + p.getAge());
l.info("Height (cm):" + p.getHeight());
l.info("Weight (kg):" + p.getWeight());
l.info("Eye Color:" + p.getEyeColor());
l.info("Gender:" + p.getGender());
aa++;
}
}
The break statement takes you to the next executable statement outside of the loop in which it's
located.
Loop continuation
In the (simplistic) example in Listing 11, you only want to execute the loop once and bail. You can
also skip a single iteration of a loop but continue executing the loop. For that purpose, you need
the continue statement, shown in Listing 12.
Listing 12. A continue statement
public static void main(String[] args) {
Logger l = Logger.getLogger(Person.class.getName());
int aa = 0;
while (aa < 3) {
if (aa == 1)
continue;
else
aa++;
Person p = new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE");
l.info("Loop executing iteration# " + aa);
l.info("Name: " + p.getName());
l.info("Age:" + p.getAge());
l.info("Height (cm):" + p.getHeight());
l.info("Weight (kg):" + p.getWeight());
l.info("Eye Color:" + p.getEyeColor());
l.info("Gender:" +
p.getGender());
}
}](https://image.slidesharecdn.com/j-introtojava1-pdf-151217013816/85/J-introtojava1-pdf-39-320.jpg)
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In Listing 12, you skip the second iteration of a loop but continue to the third. continue comes in
handy when you are, say, processing records and come across a record you definitely don't want
to process. You can skip that record and move on to the next one.
Java Collections
Most real-world applications deal with collections of things like files, variables, records from files,
or database result sets. The Java language has a sophisticated Collections Framework that
enables you to create and manage collections of objects of various types. This section won't teach
you everything about Java Collections, but it does introduce the most commonly used collection
classes and get you started with using them.
Arrays
Most programming languages include the concept of an array to hold a collection of things, and the
Java language is no exception. An array is nothing more than a collection of elements of the same
type.
Note: The square brackets in this section's code examples are part of the required syntax for
Java Collections, not indicators of optional elements.
You can declare an array in one of two ways:
• Create it with a certain size, which is fixed for the life of the array.
• Create it with a certain set of initial values. The size of this set determines the size of the array
— it is exactly large enough to hold all of those values, and its size is fixed for the life of the
array.
Declaring an array
In general, you declare an array like this:
new elementType [arraySize]
You can create an integer array of elements in two ways. This statement creates an array that has
space for five elements but is empty:
// creates an empty array of 5 elements:
int[] integers = new int[5];
This statement creates the array and initializes it all at once:
// creates an array of 5 elements with values:
int[] integers = new int[] { 1, 2, 3, 4, 5 };
The initial values go between the curly braces and are separated by commas.
Another way to create an array is to create it and then code a loop to initialize it:](https://image.slidesharecdn.com/j-introtojava1-pdf-151217013816/85/J-introtojava1-pdf-40-320.jpg)
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int[] integers = new int[5];
for (int aa = 0; aa < integers.length; aa++) {
integers[aa] = aa+1;
}
The preceding code declares an integer array of five elements. If you try to put more than five
elements in the array, the Java runtime will throw an exception. You'll learn about exceptions and
how to handle them in Part 2.
Loading an array
To load the array, you loop through the integers from 1 through the length of the array (which you
get by calling .length on the array — more about that in a minute). In this case, you stop when
you hit 5.
Once the array is loaded, you can access it as before:
Logger l = Logger.getLogger("Test");
for (int aa = 0; aa < integers.length; aa++) {
l.info("This little integer's value is: " + integers[aa]);
}
This newer (available since JDK 5) syntax also works:
Logger l = Logger.getLogger("Test");
for (int i : integers) {
l.info("This little integer's value is: " + i);
}
I find the newer syntax simpler to work with, and I use it throughout this section.
The element index
Think of an array as a series of buckets, and into each bucket goes an element of a certain type.
Access to each bucket is gained using an index:
element = arrayName [elementIndex];
To access an element, you need the reference to the array (its name) and the index where the
element that you want resides.
The length method
A handy method, as you've already seen, is length. It's a built-in method, so its syntax doesn't
include the usual parentheses. Just type the word length, and it will return — as you would expect
— the size of the array.
Arrays in the Java language are zero-based. So, for some array named array, the first element in
the array always resides at array[0], and the last resides at array[array.length - 1].](https://image.slidesharecdn.com/j-introtojava1-pdf-151217013816/85/J-introtojava1-pdf-41-320.jpg)
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An array of objects
You've seen how arrays can hold primitive types, but it's worth mentioning that they can also hold
objects. In that sense, the array is the Java language's most utilitarian collection.
Creating an array of java.lang.Integer objects isn't much different from creating an array of
primitive types. Once again, you have two ways to do it:
// creates an empty array of 5 elements:
Integer[] integers = new Integer[5];
// creates an array of 5 elements with values:
Integer[] integers = new Integer[] { Integer.valueOf(1),
Integer.valueOf(2)
Integer.valueOf(3)
Integer.valueOf(4)
Integer.valueOf(5));
Boxing and unboxing
Every primitive type in the Java language has a JDK counterpart class, which you can see in Table
4.
Table 4. Primitives and JDK counterparts
Primitive JDK counterpart
boolean java.lang.Boolean
byte java.lang.Byte
char java.lang.Character
short java.lang.Short
int java.lang.Integer
long java.lang.Long
float java.lang.Float
double java.lang.Double
Each JDK class provides methods to parse and convert from its internal representation to a
corresponding primitive type. For example, this code converts the decimal value 238 to an
Integer:
int value = 238;
Integer boxedValue = Integer.valueOf(value);
This technique is known as boxing, because you're putting the primitive into a wrapper, or box.
Similarly, to convert the Integer representation back to its int counterpart, you unbox it:
Integer boxedValue = Integer.valueOf(238);
int intValue = boxedValue.intValue();](https://image.slidesharecdn.com/j-introtojava1-pdf-151217013816/85/J-introtojava1-pdf-42-320.jpg)
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Use a consistent style
Coding style is a matter of personal preference, but I advise you to use standard Java syntax for
braces:
public static void main(String[] args) {
}
Don't use this style:
public static void main(String[] args)
{
}
Or this one:
public static void main(String[] args)
{
}
Why? Well, it's standard, so most code you run across (as in, code you didn't write, but might be
paid to maintain) will most likely be written that way. Having said that, Eclipse does allow you to
define code styles and format your code any way you like. The main thing is that you pick a style
and stick with it.
Use built-in logging
Before Java 1.4 introduced built-in logging, the canonical way to find out what your program was
doing was to make a system call like this one:
public void someMethod() {
// Do some stuff...
// Now tell all about it
System.out.println("Telling you all about it:");
// Etc...
}
The Java language's built-in logging facility (refer back to "Your first Java object") is a better
alternative. I never use System.out.println() in my code, and I suggest you don't use it either.
Another alternative is the commonly used log4j replacement library, part of the Apache umbrella
project.
In the footsteps of Fowler
The best book in the industry (in my opinion, and I'm not alone) is Refactoring: Improving the
Design of Existing Code by Martin Fowler et al. This book is even fun to read. The authors talk
about "code smells" that beg for refactoring, and they go into great detail about the various
techniques for fixing them.
Refactoring and the ability to write test-first code are the most important skills for new
programmers to learn. If everybody were good at both, it would revolutionize the industry. If you](https://image.slidesharecdn.com/j-introtojava1-pdf-151217013816/85/J-introtojava1-pdf-51-320.jpg)
J introtojava1-pdf
- 1. © Copyright IBM Corporation 2010, 2015 Trademarks Introduction to Java programming, Part 1: Java language basics Page 1 of 54 Introduction to Java programming, Part 1: Java language basics Object-oriented programming on the Java platform J Steven Perry ([email protected]) Principal Consultant Makoto Consulting Group, Inc. 11 November 2015 (First published 19 July 2010) Get an introduction to the structure, syntax, and programming paradigm of the Java™ language and platform in this two-part tutorial. Learn the Java syntax that you’re most likely to encounter professionally and Java programming idioms you can use to build robust, maintainable Java applications. In Part 1, master the essentials of object-oriented programming on the Java platform, including fundamental Java syntax. Get started with creating Java objects and adding behavior to them, and conclude with a summary of Java coding best practices, covering ample ground in-between. View more content in this series Before you begin Find out what to expect from this tutorial and how to get the most out of it. About this tutorial The two-part Introduction to Java programming tutorial is meant for software developers who are new to Java technology. Work through both parts to get up and running with object-oriented programming (OOP) and real-world application development using the Java language and platform. This first part is a step-by-step introduction to OOP using the Java language. The tutorial begins with an overview of the Java platform and language, followed by instructions for setting up a development environment consisting of a Java Development Kit (JDK) and the Eclipse IDE. After you're introduced to your development environment's components, you begin learning basic Java syntax hands-on. Part 2 covers more-advanced language features, including regular expressions, generics, I/O, and serialization. Programming examples in Part 2 build on the Person object that you begin developing in Part 1.
- 2. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 2 of 54 Objectives When you finish Part 1, you will be familiar with basic Java language syntax and able to write simple Java programs. Follow up with "Introduction to Java programming, Part 2: Constructs for real-world applications" to build on this foundation. Learn more. Develop more. Connect more. The new developerWorks Premium membership program provides an all-access pass to powerful development tools and resources, including 500 top technical titles (dozens specifically for Java developers) through Safari Books Online, deep discounts on premier developer events, video replays of recent O'Reilly conferences, and more. Sign up today. Prerequisites This tutorial is for software developers who are not yet experienced with Java code or the Java platform. The tutorial includes an overview of OOP concepts. System requirements To complete the exercises in this tutorial, install and set up a development environment consisting of: • JDK 8 from Oracle • Eclipse IDE for Java Developers Download and installation instructions for both are included in the tutorial. The recommended system configuration is: • A system supporting Java SE 8 with at least 2GB of memory. Java 8 is supported on Linux®, Windows®, Solaris®, and Mac OS X. • At least 200MB of disk space to install the software components and examples. Java platform overview Java technology is used to develop applications for a wide range of environments, from consumer devices to heterogeneous enterprise systems. In this section, get a high-level view of the Java platform and its components. The Java language Get to know the Java APIs Most Java developers constantly reference the official online Java API documentation— also called the Javadoc. By default, you see three panes in the Javadoc. The top-left pane shows all of the packages in the API, and the bottom-left pane shows the classes in each package. The main pane (to the right) shows details for the currently selected package or class. For example, if you click the java.util package in the top-left pane and then click
- 3. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 3 of 54 the ArrayList class listed below it, you see details about ArrayList in the right pane, including a description of what it does, how to use it, and its methods. Like any programming language, the Java language has its own structure, syntax rules, and programming paradigm. The Java language's programming paradigm is based on the concept of OOP, which the language's features support. The Java language is a C-language derivative, so its syntax rules look much like C's. For example, code blocks are modularized into methods and delimited by braces ({ and }), and variables are declared before they are used. Structurally, the Java language starts with packages. A package is the Java language's namespace mechanism. Within packages are classes, and within classes are methods, variables, constants, and more. You learn about the parts of the Java language in this tutorial. The Java compiler When you program for the Java platform, you write source code in .java files and then compile them. The compiler checks your code against the language's syntax rules, then writes out bytecode in .class files. Bytecode is a set of instructions targeted to run on a Java virtual machine (JVM). In adding this level of abstraction, the Java compiler differs from other language compilers, which write out instructions suitable for the CPU chipset the program will run on. The JVM At runtime, the JVM reads and interprets .class files and executes the program's instructions on the native hardware platform for which the JVM was written. The JVM interprets the bytecode just as a CPU would interpret assembly-language instructions. The difference is that the JVM is a piece of software written specifically for a particular platform. The JVM is the heart of the Java language's "write-once, run-anywhere" principle. Your code can run on any chipset for which a suitable JVM implementation is available. JVMs are available for major platforms like Linux and Windows, and subsets of the Java language have been implemented in JVMs for mobile phones and hobbyist chips. The garbage collector Rather than forcing you to keep up with memory allocation (or use a third-party library to do so), the Java platform provides memory management out of the box. When your Java application creates an object instance at runtime, the JVM automatically allocates memory space for that object from the heap— a pool of memory set aside for your program to use. The Java garbage collector runs in the background, keeping track of which objects the application no longer needs and reclaiming memory from them. This approach to memory handling is called implicit memory management because it doesn't require you to write any memory-handling code. Garbage collection is one of the essential features of Java platform performance. The Java Development Kit When you download a Java Development Kit (JDK), you get — in addition to the compiler and other tools — a complete class library of prebuilt utilities that help you accomplish most common
- 4. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 4 of 54 application-development tasks. The best way to get an idea of the scope of the JDK packages and libraries is to check out the JDK API documentation. The Java Runtime Environment The Java Runtime Environment (JRE; also known as the Java runtime) includes the JVM, code libraries, and components that are necessary for running programs that are written in the Java language. The JRE is available for multiple platforms. You can freely redistribute the JRE with your applications, according to the terms of the JRE license, to give the application's users a platform on which to run your software. The JRE is included in the JDK. Setting up your Java development environment In this section, you'll download and install the JDK and the current release of the Eclipse IDE, and you'll set up your Eclipse development environment. If you already have the JDK and Eclipse IDE installed, you might want to skip to the "Getting started with Eclipse" section or to the one after that, "Object-oriented programming concepts." Your development environment The JDK includes a set of command-line tools for compiling and running your Java code, including a complete copy of the JRE. Although you can use these tools to develop your applications, most developers appreciate the additional functionality, task management, and visual interface of an IDE. Eclipse is a popular open source IDE for Java development. Eclipse handles basic tasks, such as code compilation and debugging, so that you can focus on writing and testing code. In addition, you can use Eclipse to organize source code files into projects, compile and test those projects, and store project files in any number of source repositories. You need an installed JDK to use Eclipse for Java development. If you download one of the Eclipse bundles, it will come with the JDK already. Install the JDK Follow these steps to download and install the JDK: 1. Browse to Java SE Downloads and click the Java Platform (JDK) box to display the download page for the latest version of the JDK. 2. Agree to the license terms. 3. Under Java SE Development Kit, choose the download that matches your operating system and chip architecture. Windows 1. Save the file to your hard drive when prompted.
- 5. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 5 of 54 2. When the download is complete, run the install program. Install the JDK to your hard drive in an easy-to-remember location such as C:homeJavajdk1.8.0_60. It's a good idea to encode the update number in the name of the install directory that you choose. OS X 1. When the download is complete, double-click it to mount it. 2. Run the install program. You do not get to choose where the JDK is installed. You can run / usr/libexec/java_home -1.8 to see the location of JDK 8 on your Mac. The path isdisplay similar to /Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home. See JDK 8 and JRE 8 Installation for more information. You now have a Java environment on your computer. Next, you'll install the Eclipse IDE. Install Eclipse To download and install Eclipse, follow these steps: 1. Browse to the Eclipse IDE downloads page. 2. Click Eclipse IDE for Java Developers. 3. Under Download Links on the right side, choose your platform (the site might already have sniffed out your OS type). 4. Click the mirror you want to download from; then, save the file to your hard drive. 5. Extract the contents of the .zip file to a location on your hard drive that you'll be able to remember easily (such as C:homeeclipse on Windows or ~/home/eclipse on Mac or Linux). Set up Eclipse The Eclipse IDE sits atop the JDK as a useful abstraction, but it still needs to access the JDK and its various tools. Before you can use Eclipse to write Java code, you must tell it where the JDK is located. To set up your Eclipse development environment: 1. Launch Eclipse by double-clicking eclipse.exe (or the equivalent executable for your platform). 2. The Workspace Launcher opens, allowing you to choose a root folder for your Eclipse projects. Use a folder you can easily remember, such as C:homeworkspace on Windows or ~/home/workspace on Mac or Linux. 3. Close the Welcome to Eclipse window. 4. Click Window > Preferences > Java > Installed JREs. Figure 1 shows this selection highlighted in the Eclipse setup window for the JRE.
- 6. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 6 of 54 Figure 1. Configuring the JDK that Eclipse uses 5. Eclipse points to an installed JRE. You must use the JRE that you downloaded with the JDK. If Eclipse does not automatically detect the JDK you installed, click Add..., and in the next dialog box, click Standard VM and then click Next. 6. Specify the JDK's home directory (such as C:homejdk1.8.0_60 on Windows), and then click Finish. 7. Confirm that the JDK that you want to use is selected and click OK. Eclipse is now set up and ready for you to create projects, and compile and run Java code. The next section familiarizes you with Eclipse. Getting started with Eclipse Eclipse is more than an IDE; it's an entire development ecosystem. This section is a brief hands-on introduction to using Eclipse for Java development. The Eclipse development environment The Eclipse development environment has four main components: • Workspace • Projects
- 7. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 7 of 54 • Perspectives • Views The primary unit of organization in Eclipse is the workspace. A workspace contains all of your projects. A perspective is a way of looking at each project (hence the name), and within a perspective are one or more views. The Java perspective Figure 2 shows the Java perspective, which is the default perspective for Eclipse. You should see this perspective when you start Eclipse. Figure 2. Eclipse Java perspective The Java perspective contains the tools that you need to begin writing Java applications. Each tabbed window shown in Figure 2 is a view for the Java perspective. Package Explorer and Outline are two particularly useful views.
- 8. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 8 of 54 The Eclipse environment is highly configurable. Each view is dockable, so you can move it around in the Java perspective and place it where you want it. For now, though, stick with the default perspective and view setup. Create a project Follow these steps to create a new Java project: 1. Click File > New > Java Project... to start the New Java Project wizard, shown in Figure 3. Figure 3. New Java Project wizard 2. Enter Tutorial as the project name and click Finish. 3. If you want to modify the default project settings, click Next (recommended only if you have experience with the Eclipse IDE). 4. Click Finish to accept the project setup and create the project.
- 9. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 9 of 54 You have now created a new Eclipse Java project and source folder. Your development environment is ready for action. However, an understanding of the OOP paradigm — covered in this tutorial's next two sections — is essential. If you are familiar with OOP concepts and principles, you might want to skip to the "Getting started with the Java language" section. Object-oriented programming concepts The Java language is (mostly) object oriented. If you haven't used an object-oriented language before, OOP concepts might seem strange at first. This section is a short introduction to OOP language concepts, using structured programming as a point of contrast. What is an object? Structured programming languages like C and COBOL follow a different programming paradigm from object-oriented ones. The structured-programming paradigm is highly data oriented: You have data structures, and then program instructions act on that data. Object-oriented languages such as the Java language combine data and program instructions into objects. An object is a self-contained entity that contains attributes and behavior, and nothing more. Instead of having a data structure with fields (attributes) and passing that structure around to all of the program logic that acts on it (behavior), in an object-oriented language, data and program logic are combined. This combination can occur at vastly different levels of granularity, from fine-grained objects such as a Number, to coarse-grained objects, such as a FundsTransfer service in a large banking application. Parent and child objects A parent object is one that serves as the structural basis for deriving more-complex child objects. A child object looks like its parent but is more specialized. With the object-oriented paradigm, you can reuse the common attributes and behavior of the parent object, adding to its child objects attributes and behavior that differ. (You learn more about inheritance in the next section of this tutorial.) Object communication and coordination Objects talk to other objects by sending messages (method calls in Java parlance). Furthermore, in an object-oriented application, program code coordinates the activities among objects to perform tasks within the context of the given application domain. Object summary A well-written object: • Has crisp boundaries • Performs a finite set of activities • Knows only about its data and any other objects that it needs to accomplish its activities In essence, an object is a discrete entity that has only the necessary dependencies on other objects to perform its tasks. Now, you see what an object looks like.
- 10. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 10 of 54 The Person object I start with an example that is based on a common application-development scenario: an individual being represented by a Person object. Going back to the definition of an object, you know that an object has two primary elements: attributes and behavior. I show how these apply to the Person object. Attributes What attributes can a person have? Some common ones include: • Name • Age • Height • Weight • Eye color • Gender You can probably think of more (and you can always add more attributes later), but this list is a good start. Behavior An actual person can do all sorts of things, but object behaviors usually relate to some kind of application context. In a business-application context, for instance, you might want to ask your Person object, "What is your age?" In response, Person would tell you the value of its Age attribute. More-complex logic can be hidden inside of the Person object — for example, calculating a person's Body Mass Index (BMI) for a health application — but for now, suppose that Person has the behavior of answering these questions: • What is your name? • What is your age? • What is your height? • What is your weight? • What is your eye color? • What is your gender? State and string State is an important concept in OOP. An object's state is represented at any moment in time by the value of its attributes. In the case of Person, its state is defined by attributes such as name, age, height, and weight. If you wanted to present a list of several of those attributes, you might do so using a String class, which I talk more about later in the tutorial. Using the concepts of state and string together, you can say to Person, "Tell me who you are by giving me a listing (or String) of your attributes."
- 11. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 11 of 54 Principles of OOP If you come from a structured-programming background, the OOP value proposition might not be clear yet. After all, the attributes of a person and any logic to retrieve (and convert) those values can be written in C or COBOL. This section clarifies the benefits of the OOP paradigm by explaining its defining principles: encapsulation, inheritance, and polymorphism. Encapsulation Recall that an object is above all discrete, or self-contained. This characteristic is the principle of encapsulation at work. Hiding is another term that is sometimes used to express the self- contained, protected nature of objects. Regardless of terminology, what's important is that the object maintains a boundary between its state and behavior and the outside world. Like objects in the real world, objects used in computer programming have various types of relationships with different categories of objects in the applications that use them. On the Java platform, you can use access modifiers (which I introduce later in the tutorial) to vary the nature of object relationships from public to private. Public access is wide open, whereas private access means the object's attributes are accessible only within the object itself. The public/private boundary enforces the object-oriented principle of encapsulation. On the Java platform, you can vary the strength of that boundary on an object-by-object basis, depending on a system of trust. Encapsulation is a powerful feature of the Java language. Inheritance In structured programming, it's common to copy a structure, give it a new name, and add or modify the attributes that make the new entity (such as an Account record) different from its original source. Over time, this approach generates a great deal of duplicated code, which can create maintenance issues. OOP introduces the concept of inheritance, whereby specialized classes — without additional code — can "copy" the attributes and behavior of the source classes that they specialize. If some of those attributes or behaviors need to change, you override them. The only source code you change is the code needed for creating specialized classes. As you know from the "Object- oriented programming concepts" section, the source object is called the parent, and the new specialization is called the child. Inheritance at work Suppose you are writing a human-resources application and want to use the Person class as the basis (called the super class) for a new class called Employee. Being the child of Person, Employee would have all of the attributes of a Person class, along with additional ones, such as: • Taxpayer identification number • Employee number • Salary
- 12. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 12 of 54 Inheritance makes it easy to create the new Employee class without needing to copy all of the Person code manually. You'll see plenty of examples of inheritance in Java programming later in the tutorial, especially in Part 2. Polymorphism Polymorphism is a harder concept to grasp than encapsulation and inheritance. In essence, it means that objects that belong to the same branch of a hierarchy, when sent the same message (that is, when told to do the same thing), can manifest that behavior differently. To understand how polymorphism applies to a business-application context, return to the Person example. Remember telling Person to format its attributes into a String? Polymorphism makes it possible for Person to represent its attributes in a variety of ways depending on the type of Person it is. Polymorphism is one of the more complex concepts you'll encounter in OOP on the Java platform and is beyond the scope of an introductory tutorial. Getting started with the Java language The Java language: Not purely object-oriented With the Java language, you can create first-class objects, but not everything in the language is an object. Two qualities differentiate the Java language from purely object-oriented languages such as Smalltalk. First, the Java language is a mixture of objects and primitive types. Second, with Java, you can write code that exposes the inner workings of one object to any other object that uses it. The Java language does give you the tools necessary to follow sound OOP principles and produce sound object-oriented code. Because Java is not purely object-oriented, you must exercise some discipline in how you write code — the language doesn't force you to do the right thing, so you must do it yourself. (This tutorial's last section, "Writing good Java code," provides tips.) It would be impossible to introduce the entire Java language syntax in a single tutorial. The remainder of Part 1 focuses on the basics of the language, leaving you with enough knowledge and practice to write simple programs. OOP is all about objects, so this section starts with two topics specifically related to how the Java language handles them: reserved words and the structure of a Java object. Reserved words Like any programming language, the Java language designates certain words that the compiler recognizes as special. For that reason, you're not allowed to use them for naming your Java constructs. The list of reserved words is surprisingly short: • abstract • assert • boolean
- 13. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 13 of 54 • break • byte • case • catch • char • class • const • continue • default • do • double • else • enum • extends • final • finally • float • for • goto • if • implements • import • instanceof • int • interface • long • native • new • package • private • protected • public • return • short • static • strictfp • super • switch • synchronized • this • throw • throws • transient • try • void
- 14. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 14 of 54 • volatile • while Note that true, false, and null are technically not reserved words. Although they are literals, I included them in this list because you can't use them to name Java constructs. One advantage of programming with an IDE is that it can use syntax coloring for reserved words, as I show later in this tutorial. Structure of a Java class A class is a blueprint for a discrete entity (object) that contains attributes and behavior. The class defines the object's basic structure, and at runtime, your application creates an instance of the object. An object has a crisp boundary and a state, and it can do things when correctly asked. Every object-oriented language has rules about how to define a class. In the Java language, classes are defined as shown in Listing 1: Listing 1. Class definition package packageName; import ClassNameToImport; accessSpecifier class ClassName { accessSpecifier dataType variableName [= initialValue]; accessSpecifier ClassName([argumentList]) { constructorStatement(s) } accessSpecifier returnType methodName ([argumentList]) { methodStatement(s) } // This is a comment /* This is a comment too */ /* This is a multiline comment */ } Listing 1 contains various types of constructs, which I've differentiated with font formatting. The constructs shown in bold (which you'll find in the list of reserved words) are literals; in any object definition, they must be exactly what they are in the listing. The names that I've given the other constructs describe the concepts they represent. I explain all of the constructs in detail in the rest of this section. Note: In Listing 1 and some other code examples in this section, square brackets indicate that the constructs within them are not required. The brackets themselves (unlike { and }) are not part of the Java syntax. Comments in code Notice that Listing 1 also includes some comment lines: // This is a comment /* This is a comment too */ /* This is a multiline comment */
- 15. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 15 of 54 With just about every programming language, programmers can add comments to help document the code. Java syntax allows for both single-line and multiline comments. A single-line comment must be contained on one line, although you can use adjacent single-line comments to form a block. A multiline comment begins with /*, must be terminated with */, and can span any number of lines. You learn more about comments when you get to this tutorial's "Writing good Java code" section. Packaging classes With the Java language, you can choose the names for your classes, such as Account, Person, or LizardMan. At times, you might end up using the same name to express two slightly different concepts. This situation is called a name collision, and it happens frequently. The Java language uses packages to resolve these conflicts. A Java package is a mechanism for providing a namespace— an area inside of which names are unique, but outside of which they might not be. To identify a construct uniquely, you must fully qualify it by including its namespace. Packages also give you a nice way to build more-complex applications with discrete units of functionality. Package definition To define a package, you use the package keyword followed by a legal package name, terminated with a semicolon. Often package names are separated by dots and follow this de facto standard scheme: package orgType.orgName.appName.compName; This package definition breaks down like so: • orgType is the organization type, such as com, org, or net. • orgName is the name of the organization's domain, such as makotojava, oracle, or ibm. • appName is the name of the application, abbreviated. • compName is the name of the component. The Java language doesn't force you to follow this package convention. In fact, you don't need to specify a package at all, in which case all of your classes must have unique names and will reside in the default package. As a best practice, I recommend that you define all of your Java classes in packages named as I've described here. You follow that convention throughout this tutorial. Import statements Eclipse simplifies imports When you write code in the Eclipse editor, you can type the name of a class you want to use, followed by Ctrl+Shift+O. Eclipse figures out which imports you need and adds them
- 16. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 16 of 54 automatically. If Eclipse finds two classes with the same name, it displays a dialog box asking you which class you want to add imports for. Up next in the class definition (referring back to Listing 1) is the import statement. An import statement tells the Java compiler where to find classes that you reference inside of your code. Any nontrivial class uses other classes for some functionality, and the import statement is how you tell the Java compiler about them. An import statement usually looks like this: import ClassNameToImport; You specify the import keyword, followed by the class that you want to import, followed by a semicolon. The class name should be fully qualified, meaning that it should include its package. To import all classes within a package, you can put .* after the package name. For example, this statement imports every class in the com.makotojava package: import com.makotojava.*; Importing an entire package can make your code less readable, however, so I recommend that you import only the classes that you need, using their fully qualified names. Class declaration To define an object in the Java language, you must declare a class. Again, think of a class as a template for an object, like a cookie cutter. Listing 1 includes this class declaration: accessSpecifier class ClassName { accessSpecifier dataType variableName [= initialValue]; accessSpecifier ClassName([argumentList]) { constructorStatement(s) } accessSpecifier returnType methodName([argumentList]) { methodStatement(s) } } A class's accessSpecifier can have several values, but usually it is public. You look at other values of accessSpecifier soon. Class-naming conventions You can name classes pretty much however you want, but the convention is to use camel case: Start with an uppercase letter, capitalize the first letter of each concatenated word, and make all the other letters lowercase. Class names should contain only letters and numbers. Sticking to these guidelines ensures that your code is more accessible to other developers who are following the same conventions. Classes can have two types of members: variables and methods.
- 17. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 17 of 54 Variables The values of a given class's variables distinguish each instance of that class and define its state. These values are often referred to as instance variables. A variable has: • An accessSpecifier • A dataType • A variableName • Optionally, an initialValue The possible accessSpecifier values are: Public variables It's never a good idea to use public variables, but in extremely rare cases it can be necessary, so the option exists. The Java platform doesn't constrain your use cases, so it's up to you to be disciplined about using good coding conventions, even if tempted to do otherwise. • public: Any object in any package can see the variable. (Don't ever use this value; see the Public variables sidebar.) • protected: Any object defined in the same package, or a subclass (defined in any package), can see the variable. • No specifier (also called friendly or package private access): Only objects whose classes are defined in the same package can see the variable. • private: Only the class containing the variable can see it. A variable's dataType depends on what the variable is — it might be a primitive type or another class type (again, more about this later). The variableName is up to you, but by convention, variable names use the camel case convention I described in "Class-naming conventions," except that they begin with a lowercase letter. (This style is sometimes called lower camel case.) Don't worry about the initialValue for now; just know that you can initialize an instance variable when you declare it. (Otherwise, the compiler generates a default for you that is set when the class is instantiated.) Example: Class definition for Person Before moving on to methods, here's an example that summarizes what you've learned so far. Listing 2 is a class definition for Person. Listing 2. Basic class definition for Person package com.makotojava.intro; public class Person { private String name; private int age; private int height; private int weight; private String eyeColor; private String gender; }
- 18. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 18 of 54 The basic class definition for Person isn't useful at this point, because it defines only its attributes (and private ones at that). To be more interesting, the Person class needs behavior — and that means methods. Methods A class's methods define its behavior. Methods fall into two main categories: constructors or all other methods, of which there are many types. A constructor method is used only to create an instance of a class. Other types of methods can be used for virtually any application behavior. The class definition back in Listing 1 shows the way to define the structure of a method, which includes elements like: • accessSpecifier • returnType • methodName • argumentList The combination of these structural elements in a method's definition is called the method's signature. Next, you look in more detail at the two types of methods, starting with constructors. Constructor methods You use constructors to specify how to instantiate a class. Listing 1 shows the constructor- declaration syntax in abstract form; here it is again: accessSpecifier ClassName([argumentList]) { constructorStatement(s) } Constructors are optional If you don't provide a constructor, the compiler provides one for you, called the default (or no- argument) constructor. However, if you provide a constructor other than a no-argument (or no-arg) constructor, the compiler does not automatically generate one for you. A constructor's accessSpecifier is the same as for variables. The name of the constructor must match the name of the class. So if you call your class Person, the name of the constructor must also be Person. For any constructor other than the default constructor, you pass an argumentList, which is one or more of: argumentType argumentName
- 19. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 19 of 54 Arguments in an argumentList are separated by commas, and no two arguments can have the same name. argumentType is either a primitive type or another class type (the same as with variable types). Class definition with a constructor Now, you see what happens when you add the capability to create a Person object in two ways: by using a no-arg constructor and by initializing a partial list of attributes. Listing 3 shows how to create constructors and also how to use argumentList: Listing 3. Person class definition with a constructor package com.makotojava.intro; public class Person { private String name; private int age; private int height; private int weight; private String eyeColor; private String gender; public Person() { // Nothing to do... } public Person(String name, int age, int height, int weight String eyeColor, String gender) { this.name = name; this.age = age; this.height = height; this.weight = weight; this.eyeColor = eyeColor; this.gender = gender; } } Note the use of the this keyword in making the variable assignments in Listing 3. The this keyword is Java shorthand for "this object," and you must use it when you reference two variables with the same name. In this case, age is both a constructor parameter and a class variable, so the this keyword helps the compiler to disambiguate the reference. The Person object is getting more interesting, but it needs more behavior. And for that, you need more methods. Other methods A constructor is a particular kind of method with a particular function. Similarly, many other types of methods perform particular functions in Java programs. Exploration of other method types begins in this section and continues throughout the tutorial. Back in Listing 1, I showed you how to declare a method: accessSpecifier returnType methodName ([argumentList]) { methodStatement(s) }
- 20. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 20 of 54 Other methods look much like constructors, with a couple of exceptions. First, you can name other methods whatever you like (though, of course, certain rules apply). I recommend the following conventions: • Start with a lowercase letter. • Avoid numbers unless they are absolutely necessary. • Use only alphabetic characters. Second, unlike constructors, other methods have an optional return type. Person's other methods Armed with this basic information, you can see in Listing 4 what happens when you add a few more methods to the Person object. (I've omitted constructors for brevity.) Listing 4. Person with a few new methods package com.makotojava.intro; public class Person { private String name; private int age; private int height; private int weight; private String eyeColor; private String gender; public String getName() { return name; } public void setName(String value) { name = value; } // Other getter/setter combinations... } Notice the comment in Listing 4 about "getter/setter combinations." You'll work more with getters and setters later in the tutorial. For now, all you need to know is that a getter is a method for retrieving the value of an attribute, and a setter is a method for modifying that value. Listing 4 shows only one getter/setter combination (for the Name attribute), but you can define more in a similar fashion. Note in Listing 4 that if a method doesn't return a value, you must tell the compiler by specifying the void return type in its signature. Static and instance methods Generally, two types of (nonconstructor) methods are used: instance methods and static methods. Instance methods are dependent on the state of a specific object instance for their behavior. Static methods are also sometimes called class methods, because their behavior is not dependent on any single object's state. A static method's behavior happens at the class level. Static methods are used largely for utility; you can think of them as being global methods (à la C) while keeping the code for the method with the class that defines it. For example, throughout this tutorial, you use the JDK Logger class to output information to the console. To create a Logger class instance, you don't instantiate a Logger class; instead, you invoke a static method called getLogger().
- 21. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 21 of 54 The syntax for invoking a static method on a class is different from the syntax used to invoke a method on an object. You also use the name of the class that contains the static method, as shown in this invocation: Logger l = Logger.getLogger("NewLogger"); In this example, Logger is the name of the class, and getLogger(...) is the name of the method. So to invoke a static method, you don't need an object instance, just the name of the class. Your first Java class It's time to pull together what you've learned in the previous sections and start writing some code. This section walks you through declaring a class and adding variables and methods to it using the Eclipse Package Explorer. You learn how to use the Logger class to keep an eye on your application's behavior, and also how to use a main() method as a test harness. Creating a package If you're not already there, get to the Package Explorer view (in the Java perspective) in Eclipse through Window > Perspective > Open Perspective. You're going to get set up to create your first Java class. The first step is to create a place for the class to live. Packages are namespace constructs, and they also conveniently map directly to the file system's directory structure. Rather than use the default package (almost always a bad idea), you create one specifically for the code you are writing. Click File > New > Package to start the Java Package wizard, shown in Figure 4: Figure 4. The Eclipse Java Package wizard
- 22. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 22 of 54 Type com.makotojava.intro into the Name text box and click Finish. You'll see the new package created in the Package Explorer. Declaring the class You can create a class from the Package Explorer in more than one way, but the easiest way is to right-click the package you just created and choose New > Class.... The New Class dialog box opens. In the Name text box, type Person and then click Finish. The new class is displayed in your edit window. I recommend closing a few of the views (Problems, Javadoc, and others) that open by default in the Java Perspective the first time you open it to make it easier to see your source code. (Eclipse remembers that you don't want to see those views the next time you open Eclipse and go to the Java perspective.) Figure 5 shows a workspace with the essential views open. Figure 5. A well-ordered workspace
- 23. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 23 of 54 Eclipse generates a shell class for you and includes the package statement at the top. You just need to flesh out the class now. You can configure how Eclipse generates new classes through Window > Preferences > Java > Code Style > Code Templates. For simplicity, go with Eclipse's out-of-the-box code generation. In Figure 5, notice the asterisk (*) next to the new source-code file name, indicating that I've made a modification. And notice that the code is unsaved. Next, notice that I made a mistake when declaring the Name attribute: I declared Name's type to be Strin. The compiler could not find a reference to such a class and flagged it as a compile error (that's the wavy red line underneath Strin). Of course, I can fix my mistake by adding a g to the end of Strin. This is a small demonstration of the power of using an IDE instead of command-line tools for software development. Go ahead and correct the error by changing the type to String. Adding class variables In Listing 3, you began to flesh out the Person class, but I didn't explain much of the syntax. Now, I formally define how to add class variables. Recall that a variable has an accessSpecifier, a dataType, a variableName, and, optionally, an initialValue. Earlier, you looked briefly at how to define the accessSpecifier and variableName. Now, you see the dataType that a variable can have. A dataType can be either a primitive type or a reference to another object. For example, notice that Age is an int (a primitive type) and that Name is a String (an object). The JDK comes packed full of useful classes like java.lang.String, and those in the java.lang package do not need to be imported (a shorthand courtesy of the Java compiler). But whether the dataType is a JDK class such as String or a user-defined class, the syntax is essentially the same. Table 1 shows the eight primitive data types you're likely to see on a regular basis, including the default values that primitives take on if you do not explicitly initialize a member variable's value. Table 1. Primitive data types Type Size Default value Range of values boolean n/a false true or false byte 8 bits 0 -128 to 127 char 16 bits (unsigned) u0000' u0000' to uffff' or 0 to 65535 short 16 bits 0 -32768 to 32767 int 32 bits 0 -2147483648 to 2147483647 long 64 bits 0 -9223372036854775808 to 9223372036854775807 float 32 bits 0.0 1.17549435e-38 to 3.4028235e +38 double 64 bits 0.0 4.9e-324 to 1.7976931348623157e+308
- 24. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 24 of 54 Built-in logging Before going further into coding, you need to know how your programs tell you what they are doing. The Java platform includes the java.util.logging package, a built-in logging mechanism for gathering program information in a readable form. Loggers are named entities that you create through a static method call to the Logger class: import java.util.logging.Logger; //... Logger l = Logger.getLogger(getClass().getName()); When calling the getLogger() method, you pass it a String. For now, just get in the habit of passing the name of the class that the code you're writing is located in. From any regular (that is, nonstatic) method, the preceding code always references the name of the class and passes that to the Logger. If you are making a Logger call inside of a static method, reference the name of the class you're inside of: Logger l = Logger.getLogger(Person.class.getName()); In this example, the code you're inside of is the Person class, so you reference a special literal called class that retrieves the Class object (more on this later) and gets its Name attribute. This tutorial's "Writing good Java code" section includes a tip on how not to do logging. Before we get into the meat of testing, first go into the Eclipse source-code editor for Person and add this code just after public class Person { from Listing 3 so that it looks like this: package com.makotojava.intro; public class Person { private String name; private int age; private int height; private int weight; private String eyeColor; private String gender; } Eclipse has a handy code generator to generate getters and setters (among other things). To try out the code generator, put your mouse caret on the Person class definition (that is, on the word Person in the class definition) and click Source > Generate Getters and Setters.... When the dialog box opens, click Select All, as shown in Figure 6.
- 25. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 25 of 54 Figure 6. Eclipse generating getters and setters For the insertion point, choose Last member and click OK. Now, add a constructor to Person by typing the code from Listing 5 into your source window just below the top part of the class definition (the line immediately beneath public class Person ()). Listing 5. Person constructor public Person(String name, int age, int height, int weight, String eyeColor, String gender) { this.name = name; this.age = age; this.height = height; this.weight = weight; this.eyeColor = eyeColor; this.gender = gender; } Make sure that you have no wavy lines indicating compile errors. Using main() as a test harness main() is a special method that you can include in any class so that the JRE can execute its code. A class is not required to have a main() method — in fact, most never will — and a class can have at most one main() method. main() is a handy method to have because it gives you a quick test harness for the class. In enterprise development, you would use test libraries such as JUnit, but using main() as your test harness can be a quick-and-dirty way to create a test harness.
- 26. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 26 of 54 Generate a JUnit test case Now you generate a JUnit test case where you instantiate a Person, using the constructor in Listing 5, and then print the state of the object to the console. In this sense, the "test" makes sure that the order of the attributes on the constructor call are correct (that is, that they are set to the correct attributes). In the Package Explorer, right-click your Person class and then click New > JUnit Test Case. The first page of the New JUnit Test Case wizard opens, as shown in Figure 7. Figure 7. Creating a JUnit test case Accept the defaults by clicking Next. You see the Test Methods dialog box, shown in Figure 8.
- 27. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 27 of 54 Figure 8. Select methods for the wizard to generate test cases In this dialog box, you select the method or methods that you want the wizard to build tests for. In this case, select just the constructor, as shown in Figure 8. Click Finish, and Eclipse generates the JUnit test case. Next, open PersonTest, go into the testPerson() method, and make it look like Listing 6.
- 28. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 28 of 54 Listing 6. The testPerson() method @Test public void testPerson() { Person p = new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE"); Logger l = Logger.getLogger(Person.class.getName()); l.info("Name: " + p.getName()); l.info("Age:" + p.getAge()); l.info("Height (cm):" + p.getHeight()); l.info("Weight (kg):" + p.getWeight()); l.info("Eye Color:" + p.getEyeColor()); l.info("Gender:" + p.getGender()); assertEquals("Joe Q Author", p.getName()); assertEquals(42, p.getAge()); assertEquals(173, p.getHeight()); assertEquals(82, p.getWeight()); assertEquals("Brown", p.getEyeColor()); assertEquals("MALE", p.getGender()); } Don't worry about the Logger class for now. Just enter the code as you see it in Listing 6. You're now ready to run your first Java program (and JUnit test case). Executing code in Eclipse To run a Java application from inside Eclipse, select the class you want to run, then click the Run icon (which is green and has a small triangular arrow pointing to the right). Eclipse is smart enough to recognize that the class you want to run is a JUnit test case, and so it launches JUnit. Now, sit back and watch it go. Figure 9 shows what happens.
- 29. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 29 of 54 Figure 9. See Person run The Console view opens automatically and shows Logger output. The JUnit view also opens to show the results of the test. Adding behavior to a Java class Person is looking good so far, but it can use some additional behavior to make it more interesting. Creating behavior means adding methods. This section looks more closely at accessor methods— namely, the getters and setters you've already seen in action. Accessor methods To encapsulate a class's data from other objects, you declare its variables to be private and then provide accessor methods. As you have seen, a getter is an accessor method for retrieving the value of an attribute; a setter is an accessor method for modifying that value. The naming of accessors follows a strict convention known as the JavaBeans pattern, whereby any attribute Foo has a getter called getFoo() and a setter called setFoo().
- 30. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 30 of 54 The JavaBeans pattern is so common that support for it is built into the Eclipse IDE. You've even already seen it in action — when you generated getters and setters for Person in the preceding section. Accessors follow these guidelines: • The attribute itself is always declared with private access. • The access specifier for getters and setters is public. • Getters don't take any parameters and return a value whose type is the same as the attribute it accesses. • Settings only take one parameter, of the type of the attribute, and do not return a value. Declaring accessors By far the easiest way to declare accessors is to let Eclipse do it for you, as shown in Figure 6. But you should also know how to hand-code a getter-and-setter pair. Suppose you have an attribute, Foo, whose type is java.lang.String. A complete declaration for it (following the accessor guidelines) would be: private String foo; public String getFoo() { return foo; } public void setFoo(String value) { foo = value; } You might notice right away that the parameter value passed to the setter is named differently than if it had been generated by Eclipse. The naming follows my own convention, which I recommend to other developers. On the rare occasion that I do hand-code a setter, I always use the name value as the parameter value to the setter. This eye-catcher reminds me that I've hand-coded the setter. Because I usually allow Eclipse to generate getters and setters for me, when I don't, there's a good reason. Using value as the setter's parameter value reminds me that this setter is special. (Code comments also do that.) Calling methods Invoking, or calling, methods is easy. You saw in Listing 6 how to invoke the various getters of Person to return their values. Now, I formalize the mechanics of making method calls. Method invocation with and without parameters To invoke a method on an object, you need a reference to that object. Method-invocation syntax comprises the object reference, a literal dot, the method name, and any parameters that need to be passed: objectReference.someMethod(); objectReference.someOtherMethod(parameter); Here is a method invocation without parameters:
- 31. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 31 of 54 Person p = /*obtain somehow */; p.getName(); And here's a method invocation with parameters (accessing the Name attribute of Person): Person p = new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE"); Remember that constructors are methods, too. And you can separate the parameters with spaces and newlines. The Java compiler doesn't care. These next two method invocations are identical: new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE"); new Person("Joe Q Author",// Name 42, // Age 173, // Height in cm 82, // Weight in kg "Brown",// Eye Color "MALE");// Gender Notice how the comments in the second constructor invocation leave behind readability for the next developer. At a glance that person can tell what each parameter is for. Nested method invocation Method invocations can also be nested: Logger l = Logger.getLogger(Person.class.getName()); l.info("Name: " + p.getName()); Here you are passing the return value of Person.class.getName() to the getLogger() method. Remember that the getLogger() method call is a static method call, so its syntax differs slightly. (You don't need a Logger reference to make the invocation; instead, you use the name of the class as the left side of the invocation.) That's really all there is to method invocation. Strings and operators The tutorial has so far introduced several variables of type String, but without much explanation. You learn more about strings in this section, and also find out when and how to use operators. Strings Handling strings in C is labor-intensive because they're null-terminated arrays of 8-bit characters that you must manipulate. In the Java language, strings are first-class objects of type String, with methods that help you manipulate them. (The closest Java code gets to the C world with regard to strings is the char primitive data type, which can hold a single Unicode character, such as a.) You've already seen how to instantiate a String object and set its value (back in Listing 4), but you have several other ways to do that. Here are a couple of ways to create a String instance with a value of hello:
- 32. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 32 of 54 String greeting = "hello"; greeting = new String("hello"); Because Strings are first-class objects in the Java language, you can use new to instantiate them. Setting a variable of type String has the same result, because the Java language creates a String object to hold the literal, then assigns that object to the instance variable. Concatenating strings You can do many things with String, and the class has many helpful methods. Without even using a method, you've already done something interesting with two Strings by concatenating, or combining, them: l.info("Name: " + p.getName()); The plus (+) sign is shorthand for concatenating Strings in the Java language. (You incur a performance penalty for doing this type of concatenation inside a loop, but for now, you don't need to worry about that.) Concatenation example Now, you can try concatenating Strings inside of the Person class. At this point, you have a name instance variable, but it would be nice to have a firstName and lastName. You can then concatenate them when another object requests Person's full name. The first thing you need to do is add the new instance variables (at the same location in the source code where name is currently defined): //private String name; private String firstName; private String lastName; You don't need name anymore; you've replaced it with firstName and lastName. Chaining method calls Now you can generate getters and setters for firstName and lastName (as shown back in Figure 6), remove the setName() method, and change getName() to look like this: public String getName() { return firstName.concat(" ").concat(lastName); } This code illustrates chaining of method calls. Chaining is a technique commonly used with immutable objects like String, where a modification to an immutable object always returns the modification (but doesn't change the original). You then operate on the returned, changed value.
- 33. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 33 of 54 Operators As you might expect, the Java language can do arithmetic, and you've already seen how to assign variables. Now, I give you a brief look at some of the Java language operators you need as your skills improve. The Java language uses two types of operators: • Unary: Only one operand is needed. • Binary: Two operands are needed. The Java language's arithmetic operators are summarized in Table 2. Table 2. Java language's arithmetic operators Operator Usage Description + a + b Adds a and b + +a Promotes a to int if it's a byte, short, or char - a - b Subtracts b from a - -a Arithmetically negates a * a * b Multiplies a and b / a / b Divides a by b % a % b Returns the remainder of dividing a by b (the modulus operator) ++ a++ Increments a by 1; computes the value of a before incrementing ++ ++a Increments a by 1; computes the value of a after incrementing -- a-- Decrements a by 1; computes the value of a before decrementing -- --a Decrements a by 1; computes the value of a after decrementing += a += b Shorthand for a = a + b -= a -= b Shorthand for a = a - b *= a *= b Shorthand for a = a * b %= a %= b Shorthand for a = a % b Additional operators In addition to the operators in Table 2, you've seen several other symbols that are called operators in the Java language, including: • Period (.), which qualifies names of packages and invokes methods • Parentheses (()), which delimit a comma-separated list of parameters to a method • new, which (when followed by a constructor name) instantiates an object The Java language syntax also includes several operators that are used specifically for conditional programming — that is, programs that respond differently based on different input. You look at those in the next section.
- 34. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 34 of 54 Conditional operators and control statements In this section, you learn about the various statements and operators you can use to tell your Java programs how you want them to act based on different input. Relational and conditional operators The Java language gives you operators and control statements that you can use to make decisions in your code. Most often, a decision in code starts with a Boolean expression (that is, one that evaluates to either true or false). Such expressions use relational operators, which compare one operand or expression to another, and conditional operators. Table 3 lists the relational and conditional operators of the Java language. Table 3. Relational and conditional operators Operator Usage Returns true if... > a > b a is greater than b >= a >= b a is greater than or equal to b < a < b a is less than b <= a <= b a is less than or equal to b == a == b a is equal to b != a != b a is not equal to b && a && b a and b are both true, conditionally evaluates b (if a is false, b is not evaluated) || a || b a or b is true, conditionally evaluates b (if a is true, b is not evaluated) ! !a a is false & a & b a and b are both true, always evaluates b | a | b a or b is true, always evaluates b ^ a ^ b a and b are different The if statement Now that you have a bunch of operators, it's time to use them. This code shows what happens when you add some logic to the Person object's getHeight() accessor: public int getHeight() { int ret = height; // If locale of the machine this code is running on is U.S., if (Locale.getDefault().equals(Locale.US)) ret /= 2.54;// convert from cm to inches return ret; } If the current locale is in the United States (where the metric system is not in use), it might make sense to convert the internal value of height (in centimeters) to inches. This (somewhat contrived) example illustrates the use of the if statement, which evaluates a Boolean expression inside parentheses. If that expression evaluates to true, it executes the next statement.
- 35. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 35 of 54 In this case, you only need to execute one statement if the Locale of the machine the code is running on is Locale.US. If you need to execute more than one statement, you can use curly braces to form a compound statement. A compound statement groups many statements into one — and compound statements can also contain other compound statements. Variable scope Every variable in a Java application has scope, or localized namespace, where you can access it by name within the code. Outside that space the variable is out of scope, and you get a compile error if you try to access it. Scope levels in the Java language are defined by where a variable is declared, as shown in Listing 7. Listing 7. Variable scope public class SomeClass { private String someClassVariable; public void someMethod(String someParameter) { String someLocalVariable = "Hello"; if (true) { String someOtherLocalVariable = "Howdy"; } someClassVariable = someParameter; // legal someLocalVariable = someClassVariable; // also legal someOtherLocalVariable = someLocalVariable;// Variable out of scope! } public void someOtherMethod() { someLocalVariable = "Hello there";// That variable is out of scope! } } Within SomeClass, someClassVariable is accessible by all instance (that is, nonstatic) methods. Within someMethod, someParameter is visible, but outside of that method it is not, and the same is true for someLocalVariable. Within the if block, someOtherLocalVariable is declared, and outside of that if block it is out of scope. For this reason, we say that Java has block scope, because blocks (delimited by { and }) define the scope boundaries. Scope has many rules, but Listing 7 shows the most common ones. Take a few minutes to familiarize yourself with them. The else statement Sometimes in a program's control flow, you want to take action only if a particular expression fails to evaluate to true. That's when else comes in handy: public int getHeight() { int ret; if (gender.equals("MALE")) ret = height + 2; else { ret = height; Logger.getLogger("Person").info("Being honest about height..."); } return ret; }
- 36. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 36 of 54 The else statement works the same way as if, in that it executes only the next statement it runs across. In this case, two statements are grouped into a compound statement (notice the curly braces), which the program then executes. You can also use else to perform an additional if check: if (conditional) { // Block 1 } else if (conditional2) { // Block 2 } else if (conditional3) { // Block 3 } else { // Block 4 } // End If conditional evaluates to true, then Block 1 is executed and the program jumps to the next statement after the final curly brace (which is indicated by // End). If conditional does not evaluate to true, then conditional2 is evaluated. If conditional2 is true, then Block 2 is executed, and the program jumps to the next statement after the final curly brace. If conditional2 is not true, then the program moves on to conditional3, and so on. Only if all three conditionals fail is Block 4 executed. The ternary operator The Java language provides a handy operator for doing simple if / else statement checks. Its syntax is: (conditional) ? statementIfTrue : statementIfFalse; If conditional evaluates to true, then statementIfTrue is executed; otherwise, statementIfFalse is executed. Compound statements are not allowed for either statement. The ternary operator comes in handy when you know that you need to execute one statement as the result of the conditional evaluating to true, and another if it does not. Ternary operators are most often used to initialize a variable (such as a return value), like so: public int getHeight() { return (gender.equals("MALE")) ? (height + 2) : height; } The parentheses following the question mark aren't strictly required, but they do make the code more readable. Loops In addition to being able to apply conditions to your programs and see different outcomes based on various if/then scenarios, you sometimes want your code to do the same thing over and over again until the job is done. In this section, learn about two constructs used to iterate over code or execute it more than once: for loops and while loops.
- 37. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 37 of 54 What is a loop? A loop is a programming construct that executes repeatedly while some condition (or set of conditions) is met. For instance, you might ask a program to read all records until the end of a file, or loop over all the elements of an array, processing each one. (You learn about arrays in this tutorial's "Java Collections" section.) for loops The basic loop construct in the Java language is the for statement, which you can use to iterate over a range of values to determine how many times to execute a loop. The abstract syntax for a for loop is: for (initialization; loopWhileTrue; executeAtBottomOfEachLoop) { statementsToExecute } At the beginning of the loop, the initialization statement is executed (multiple initialization statements can be separated by commas). Provided that loopWhileTrue (a Java conditional expression that must evaluate to either true or false) is true, the loop executes. At the bottom of the loop, executeAtBottomOfEachLoop executes. Example of a for loop If you wanted to change a main() method to execute three times, you can use a for loop, as shown in Listing 8. Listing 8. A for loop public static void main(String[] args) { Logger l = Logger.getLogger(Person.class.getName()); for (int aa = 0; aa < 3; aa++) { Person p = new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE"); l.info("Loop executing iteration# " + aa); l.info("Name: " + p.getName()); l.info("Age:" + p.getAge()); l.info("Height (cm):" + p.getHeight()); l.info("Weight (kg):" + p.getWeight()); l.info("Eye Color:" + p.getEyeColor()); l.info("Gender:" + p.getGender()); } } The local variable aa is initialized to zero at the beginning of Listing 8. This statement executes only once, when the loop is initialized. The loop then continues three times, and each time aa is incremented by one. As you'll see later, an alternate for loop syntax is available for looping over constructs that implement the Iterable interface (such as arrays and other Java utility classes). For now, just note the use of the for loop syntax in Listing 8. while loops The syntax for a while loop is:
- 38. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 38 of 54 while (condition) { statementsToExecute } As you might suspect, while condition evaluates to true, so the loop executes. At the top of each iteration (that is, before any statements execute), the condition is evaluated. If the condition evaluates to true, the loop executes. So it's possible that a while loop will never execute if its conditional expression is not true at least once. Look again at the for loop in Listing 8. For comparison, Listing 9 uses a while loop to obtain the same result. Listing 9. A while loop public static void main(String[] args) { Logger l = Logger.getLogger(Person.class.getName()); int aa = 0; while (aa < 3) { Person p = new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE"); l.info("Loop executing iteration# " + aa); l.info("Name: " + p.getName()); l.info("Age:" + p.getAge()); l.info("Height (cm):" + p.getHeight()); l.info("Weight (kg):" + p.getWeight()); l.info("Eye Color:" + p.getEyeColor()); l.info("Gender:" + p.getGender()); aa++; } } As you can see, a while loop requires a bit more housekeeping than a for loop. You must initialize the aa variable and also remember to increment it at the bottom of the loop. do...while loops If you want a loop that always executes once and then checks its conditional expression, try using a do...while loop, as shown in Listing 10. Listing 10. A do...while loop int aa = 0; do { Person p = new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE"); l.info("Loop executing iteration# " + aa); l.info("Name: " + p.getName()); l.info("Age:" + p.getAge()); l.info("Height (cm):" + p.getHeight()); l.info("Weight (kg):" + p.getWeight()); l.info("Eye Color:" + p.getEyeColor()); l.info("Gender:" + p.getGender()); aa++; } while (aa < 3); The conditional expression (aa < 3) is not checked until the end of the loop.
- 39. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 39 of 54 Loop branching At times, you need to bail out of a loop before the conditional expression evaluates to false. This situation can occur if you are searching an array of Strings for a particular value, and once you find it, you don't care about the other elements of the array. For the times when you want to bail, the Java language provides the break statement, shown in Listing 11. Listing 11. A break statement public static void main(String[] args) { Logger l = Logger.getLogger(Person.class.getName()); int aa = 0; while (aa < 3) { if (aa == 1) break; Person p = new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE"); l.info("Loop executing iteration# " + aa); l.info("Name: " + p.getName()); l.info("Age:" + p.getAge()); l.info("Height (cm):" + p.getHeight()); l.info("Weight (kg):" + p.getWeight()); l.info("Eye Color:" + p.getEyeColor()); l.info("Gender:" + p.getGender()); aa++; } } The break statement takes you to the next executable statement outside of the loop in which it's located. Loop continuation In the (simplistic) example in Listing 11, you only want to execute the loop once and bail. You can also skip a single iteration of a loop but continue executing the loop. For that purpose, you need the continue statement, shown in Listing 12. Listing 12. A continue statement public static void main(String[] args) { Logger l = Logger.getLogger(Person.class.getName()); int aa = 0; while (aa < 3) { if (aa == 1) continue; else aa++; Person p = new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE"); l.info("Loop executing iteration# " + aa); l.info("Name: " + p.getName()); l.info("Age:" + p.getAge()); l.info("Height (cm):" + p.getHeight()); l.info("Weight (kg):" + p.getWeight()); l.info("Eye Color:" + p.getEyeColor()); l.info("Gender:" + p.getGender()); } }
- 40. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 40 of 54 In Listing 12, you skip the second iteration of a loop but continue to the third. continue comes in handy when you are, say, processing records and come across a record you definitely don't want to process. You can skip that record and move on to the next one. Java Collections Most real-world applications deal with collections of things like files, variables, records from files, or database result sets. The Java language has a sophisticated Collections Framework that enables you to create and manage collections of objects of various types. This section won't teach you everything about Java Collections, but it does introduce the most commonly used collection classes and get you started with using them. Arrays Most programming languages include the concept of an array to hold a collection of things, and the Java language is no exception. An array is nothing more than a collection of elements of the same type. Note: The square brackets in this section's code examples are part of the required syntax for Java Collections, not indicators of optional elements. You can declare an array in one of two ways: • Create it with a certain size, which is fixed for the life of the array. • Create it with a certain set of initial values. The size of this set determines the size of the array — it is exactly large enough to hold all of those values, and its size is fixed for the life of the array. Declaring an array In general, you declare an array like this: new elementType [arraySize] You can create an integer array of elements in two ways. This statement creates an array that has space for five elements but is empty: // creates an empty array of 5 elements: int[] integers = new int[5]; This statement creates the array and initializes it all at once: // creates an array of 5 elements with values: int[] integers = new int[] { 1, 2, 3, 4, 5 }; The initial values go between the curly braces and are separated by commas. Another way to create an array is to create it and then code a loop to initialize it:
- 41. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 41 of 54 int[] integers = new int[5]; for (int aa = 0; aa < integers.length; aa++) { integers[aa] = aa+1; } The preceding code declares an integer array of five elements. If you try to put more than five elements in the array, the Java runtime will throw an exception. You'll learn about exceptions and how to handle them in Part 2. Loading an array To load the array, you loop through the integers from 1 through the length of the array (which you get by calling .length on the array — more about that in a minute). In this case, you stop when you hit 5. Once the array is loaded, you can access it as before: Logger l = Logger.getLogger("Test"); for (int aa = 0; aa < integers.length; aa++) { l.info("This little integer's value is: " + integers[aa]); } This newer (available since JDK 5) syntax also works: Logger l = Logger.getLogger("Test"); for (int i : integers) { l.info("This little integer's value is: " + i); } I find the newer syntax simpler to work with, and I use it throughout this section. The element index Think of an array as a series of buckets, and into each bucket goes an element of a certain type. Access to each bucket is gained using an index: element = arrayName [elementIndex]; To access an element, you need the reference to the array (its name) and the index where the element that you want resides. The length method A handy method, as you've already seen, is length. It's a built-in method, so its syntax doesn't include the usual parentheses. Just type the word length, and it will return — as you would expect — the size of the array. Arrays in the Java language are zero-based. So, for some array named array, the first element in the array always resides at array[0], and the last resides at array[array.length - 1].
- 42. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 42 of 54 An array of objects You've seen how arrays can hold primitive types, but it's worth mentioning that they can also hold objects. In that sense, the array is the Java language's most utilitarian collection. Creating an array of java.lang.Integer objects isn't much different from creating an array of primitive types. Once again, you have two ways to do it: // creates an empty array of 5 elements: Integer[] integers = new Integer[5]; // creates an array of 5 elements with values: Integer[] integers = new Integer[] { Integer.valueOf(1), Integer.valueOf(2) Integer.valueOf(3) Integer.valueOf(4) Integer.valueOf(5)); Boxing and unboxing Every primitive type in the Java language has a JDK counterpart class, which you can see in Table 4. Table 4. Primitives and JDK counterparts Primitive JDK counterpart boolean java.lang.Boolean byte java.lang.Byte char java.lang.Character short java.lang.Short int java.lang.Integer long java.lang.Long float java.lang.Float double java.lang.Double Each JDK class provides methods to parse and convert from its internal representation to a corresponding primitive type. For example, this code converts the decimal value 238 to an Integer: int value = 238; Integer boxedValue = Integer.valueOf(value); This technique is known as boxing, because you're putting the primitive into a wrapper, or box. Similarly, to convert the Integer representation back to its int counterpart, you unbox it: Integer boxedValue = Integer.valueOf(238); int intValue = boxedValue.intValue();
- 43. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 43 of 54 Autoboxing and auto-unboxing Strictly speaking, you don't need to box and unbox primitives explicitly. Instead, you can use the Java language's autoboxing and auto-unboxing features: int intValue = 238; Integer boxedValue = intValue; // intValue = boxedValue; I recommend that you avoid autoboxing and auto-unboxing, however, because it can lead to code- readability issues. The code in the boxing and unboxing snippets is more obvious, and thus more readable, than the autoboxed code; I believe that's worth the extra effort. Parsing and converting boxed types You've seen how to obtain a boxed type, but what about parsing a String that you suspect has a boxed type into its correct box? The JDK wrapper classes have methods for that, too: String characterNumeric = "238"; Integer convertedValue = Integer.parseInt(characterNumeric); You can also convert the contents of a JDK wrapper type to a String: Integer boxedValue = Integer.valueOf(238); String characterNumeric = boxedValue.toString(); Note that when you use the concatenation operator in a String expression (you've already seen this in calls to Logger), the primitive type is autoboxed, and wrapper types automatically have toString() invoked on them. Pretty handy. Lists A List is a collection construct that is by definition an ordered collection, also known as a sequence. Because a List is ordered, you have complete control over where in the List items go. A Java List collection can only hold objects, and it defines a strict contract about how it behaves. List is an interface, so you can't instantiate it directly. You'll work with its most commonly used implementation, ArrayList. There are two ways to make the declaration. First, using the explicit syntax: List<String> listOfStrings = new ArrayList<String>(); Second, using the "diamond" operator, introduced in JDK 7: List<String> listOfStrings = new ArrayList<>(); Notice the type of the object in the ArrayList instantiation is not specified. This is the case because the type of the class on the right side of the expression must match that of the left side. Throughout the remainder of this tutorial, I use both types, because you are likely to see both usages in practice.
- 44. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 44 of 54 Note that I have assigned the ArrayList object to a variable of type List. With Java programming, you can assign a variable of one type to another, provided the variable being assigned to is a superclass or interface implemented by the variable being assigned from. You can look more at how variable assignments are affected in Part 2 in the "Inheritance" section. Formal type The <Object> in the preceding code snippet is called the formal type. <Object> tells the compiler that this List contains a collection of type Object, which means you can pretty much put whatever you like in the List. If you want to tighten up the constraints on what can or cannot go into the List, you can define the formal type differently: List<Person> listOfPersons = new ArrayList<Person>(); Now your List can only hold Person instances. Using Lists Using Lists is super easy, like Java collections in general. Here are some of the things you do with Lists: • Put something in the List. • Ask the List how big it currently is. • Get something out of the List. Now, you can try some of these things out. You've already seen how to create an instance of List by instantiating its ArrayList implementation type, so you can start from there. To put something in a List, call the add() method: List<Integer> listOfIntegers = new ArrayList<>(); listOfIntegers.add(Integer.valueOf(238)); The add() method adds the element to the end of the List. To ask the List how big it is, call size(): List<Integer> listOfIntegers = new ArrayList<>(); listOfIntegers.add(Integer.valueOf(238)); Logger l = Logger.getLogger("Test"); l.info("Current List size: " + listOfIntegers.size()); To retrieve an item from the List, call get() and pass it the index of the item you want: List<Integer> listOfIntegers = new ArrayList<>(); listOfIntegers.add(Integer.valueOf(238)); Logger l = Logger.getLogger("Test"); l.info("Item at index 0 is: " listOfIntegers.get(0));
- 45. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 45 of 54 In a real-world application, a List would contain records, or business objects, and you would possibly want to look over them all as part of your processing. How do you do that in a generic fashion? You want to iterate over the collection, which you can do because List implements the java.lang.Iterable interface. (You learn about interfaces in Part 2.) Iterable If a collection implements java.lang.Iterable, it's called an iterable collection. You can start at one end and walk through the collection item-by-item until you run out of items. You've already seen the special syntax for iterating over collections that implement the Iterable interface, in the "Loops" section. Here it is again: for (objectType varName : collectionReference) { // Start using objectType (via varName) right away... } Iterating over a List That previous example was abstract; now, here's a more realistic one: List<Integer> listOfIntegers = obtainSomehow(); Logger l = Logger.getLogger("Test"); for (Integer i : listOfIntegers) { l.info("Integer value is : " + i); } That little code snippet does the same thing as this longer one: List<Integer> listOfIntegers = obtainSomehow(); Logger l = Logger.getLogger("Test"); for (int aa = 0; aa < listOfIntegers.size(); aa++) { Integer I = listOfIntegers.get(aa); l.info("Integer value is : " + i); } The first snippet uses shorthand syntax: There is no index variable (aa in this case) to initialize, and no call to the List 's get() method. Because List extends java.util.Collection, which implements Iterable, you can use the shorthand syntax to iterate over any List. Sets A Set is a collections construct that by definition contains unique elements — that is, no duplicates. Whereas a List can contain the same object hundreds of times, a Set can only contain a given instance once. A Java Set collection can only hold objects, and it defines a strict contract about how it behaves. Because Set is an interface, you can't instantiate it directly, so here is one of my favorite implementations: HashSet. HashSet is easy to use and is similar to List.
- 46. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 46 of 54 Here are some things you do with a Set: • Put something in the Set. • Ask the Set how big it currently is. • Get something out of the Set. Using Sets A Set's distinguishing attribute is that it guarantees uniqueness among its elements but doesn't care about the order of the elements. Consider the following code: Set<Integer> setOfIntegers = new HashSet<Integer>(); setOfIntegers.add(Integer.valueOf(10)); setOfIntegers.add(Integer.valueOf(11)); setOfIntegers.add(Integer.valueOf(10)); for (Integer i : setOfIntegers) { l.info("Integer value is: " + i); } You might expect that the Set would have three elements in it, but it only has two because the Integer object that contains the value 10 is added only once. Keep this behavior in mind when iterating over a Set, like so: Set<Integer> setOfIntegers = new HashSet(); setOfIntegers.add(Integer.valueOf(10)); setOfIntegers.add(Integer.valueOf(20)); setOfIntegers.add(Integer.valueOf(30)); setOfIntegers.add(Integer.valueOf(40)); setOfIntegers.add(Integer.valueOf(50)); Logger l = Logger.getLogger("Test"); for (Integer i : setOfIntegers) { l.info("Integer value is : " + i); } Chances are the objects print out in a different order from the order you added them in, because a Set guarantees uniqueness, not order. You can see this if you paste the previous code into the main() method of your Person class and run it. Maps A Map is a handy collection construct that you can use to associate one object (the key) with another (the value). As you might imagine, the key to the Map must be unique, and it's used to retrieve the value at a later time. A Java Map collection can only hold objects, and it defines a strict contract about how it behaves. Because Map is an interface, you can't instantiate it directly, so here is one of my favorite implementations: HashMap. Here are some of the things you do with Maps: • Put something in the Map. • Get something out of the Map.
- 47. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 47 of 54 • Get a Set of keys to the Map— for iterating over it. Using Maps To put something into a Map, you need to have an object that represents its key and an object that represents its value: public Map<String, Integer> createMapOfIntegers() { Map<String, Integer> mapOfIntegers = new HashMap<>(); mapOfIntegers.put("1", Integer.valueOf(1)); mapOfIntegers.put("2", Integer.valueOf(2)); mapOfIntegers.put("3", Integer.valueOf(3)); //... mapOfIntegers.put("168", Integer.valueOf(168)); } In this example, Map contains Integer s, keyed by a String, which happens to be their String representation. To retrieve a particular Integer value, you need its String representation: mapOfIntegers = createMapOfIntegers(); Integer oneHundred68 = mapOfIntegers.get("168"); Using Set with Map On occasion, you might find yourself with a reference to a Map, and you want to walk over its entire set of contents. In this case, you need a Set of the keys to the Map: Set<String> keys = mapOfIntegers.keySet(); Logger l = Logger.getLogger("Test"); for (String key : keys) { Integer value = mapOfIntegers.get(key); l.info("Value keyed by '" + key + "' is '" + value + "'"); } Note that the toString() method of the Integer retrieved from the Map is automatically called when used in the Logger call. Map doesn't return a List of its keys because the Map is keyed, and each key is unique. Uniqueness is the distinguishing characteristic of a Set. Archiving Java code Now that you've learned a bit about writing Java applications, you might be wondering how to package them up so that other developers can use them, or how to import other developers' code into your applications. This section shows you how. JARs The JDK ships with a tool called JAR, which stands for Java Archive. You use this tool to create JAR files. After you package your code into a JAR file, other developers can drop the JAR file into their projects and configure their projects to use your code. Creating a JAR file in Eclipse is easy. In your workspace, right-click the com.makotojava.intro package and click File > Export. You see the dialog box shown in Figure 10. Choose Java > JAR file and click Next.
- 48. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 48 of 54 Figure 10. Export dialog box When the next dialog box opens, browse to the location where you want to store your JAR file and name the file whatever you like. The .jar extension is the default, which I recommend using. Click Finish. You see your JAR file in the location you selected. You can use the classes in it from your code if you put the JAR in your build path in Eclipse. Doing that is easy, too, as you see next. Using third-party applications As you grow more comfortable with writing Java applications, you might want to use more and more third-party applications to support your code. As great as the JDK is, it does not do everything you need for writing great Java code. The Java open source community provides many libraries to help shore up these gaps. For the sake of example, suppose that you want to use Commons Lang, a JDK replacement library for manipulating the core Java classes. The classes provided by Commons Lang help you manipulate arrays, create random numbers, and perform string manipulation. Let's assume you've already downloaded Commons Lang, which is stored in a JAR file. To use the classes, your first step is to create a lib directory in your project and drop the JAR file into it: 1. Right-click the Intro root folder in Project Explorer view. 2. Click New > Folder and call the folder lib. 3. Click Finish. The new folder shows up at the same level as src. Now copy the Commons Lang JAR file into your new lib directory. For this example, the file is called commons-lang3.3.4.jar. (It's common in naming a JAR file to include the version number, in this case 3.4.)
- 49. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 49 of 54 Now all you need to do is tell Eclipse to include the classes in the commons-lang3.3.4.jar file into your project: 1. Right-click the Intro project in your workspace; then click Properties. 2. In the Properties dialog box, click the Libraries tab, as shown in Figure 11: Figure 11. Properties > Java Build Path 3. Click the Add External JARs button. Browse to the project's lib directory, click the commons- lang3.3.4.jar file, and click OK. Once the code (that is, the class files) in the JAR file have been processed by Eclipse, they are available to reference (import) from your Java code. Notice in Project Explorer that there is a new Folder called Referenced Libraries that contains the commons-lang3.3.4.jar file. Writing good Java code You've got enough Java syntax under your belt to write basic Java programs, which means that the first half of this tutorial is about to conclude. This final section lays out a few best practices that can help you write cleaner, more maintainable Java code. Keep classes small You created a few classes in this tutorial. After generating getter/setter pairs for even the small number (by the standards of a real-world Java class) of attributes, the Person class has 150 lines of code. This is a small class. It's not uncommon to see classes with 50 or 100 methods and a thousand lines of source (or more). The key point for methods is to keep just the ones you need. If
- 50. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 50 of 54 you need several helper methods that do essentially the same thing but take different parameters (such as the printAudit() method), that's a fine choice. Just be sure to limit the list of methods to what you need, and no more. In general, classes represent some conceptual entity in your application, and their size should reflect only the functionality to do whatever that entity needs to do. They should stay tightly focused to do a small number of things and do them well. Name methods carefully A good coding pattern when it comes to method names is the intention-revealing method-names pattern. This pattern is easiest to understand with a simple example. Which of the following method names is easier to decipher at a glance? • a() • computeInterest() The answer should be obvious, yet for some reason, programmers have a tendency to give methods (and variables, for that matter) small, abbreviated names. Certainly, a ridiculously long name can be inconvenient, but a name that conveys what a method does needn't be ridiculously long. Six months after you write a bunch of code, you might not remember what you meant to do with a method called compInt(), but it's obvious that a method called computeInterest(), well, probably computes interest. Keep methods small Small methods are just as preferable as small classes, and for similar reasons. One idiom I try to follow is to keep the size of a method to one page as I look at it on my screen. This makes my application classes more maintainable. If a method grows beyond one page, I refactor it. Refactoring is changing the design of existing code without changing its results. Eclipse has a wonderful set of refactoring tools. Usually a long method contains subgroups of functionality bunched together. Take this functionality and move it to another method (naming it accordingly) and pass in parameters as needed. Limit each method to a single job. I've found that a method doing only one thing well doesn't usually take more than about 30 lines of code. Use comments Please, use comments. The people who follow along behind you (or even you, yourself, six months down the road) will thank you. You might have heard the old adage Well-written code is self-documenting, so who needs comments? I'll give you two reasons why this adage is false: • Most code is not well written. • Your code probably isn't as well written as you'd like to think. So, comment your code. Period.
- 51. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 51 of 54 Use a consistent style Coding style is a matter of personal preference, but I advise you to use standard Java syntax for braces: public static void main(String[] args) { } Don't use this style: public static void main(String[] args) { } Or this one: public static void main(String[] args) { } Why? Well, it's standard, so most code you run across (as in, code you didn't write, but might be paid to maintain) will most likely be written that way. Having said that, Eclipse does allow you to define code styles and format your code any way you like. The main thing is that you pick a style and stick with it. Use built-in logging Before Java 1.4 introduced built-in logging, the canonical way to find out what your program was doing was to make a system call like this one: public void someMethod() { // Do some stuff... // Now tell all about it System.out.println("Telling you all about it:"); // Etc... } The Java language's built-in logging facility (refer back to "Your first Java object") is a better alternative. I never use System.out.println() in my code, and I suggest you don't use it either. Another alternative is the commonly used log4j replacement library, part of the Apache umbrella project. In the footsteps of Fowler The best book in the industry (in my opinion, and I'm not alone) is Refactoring: Improving the Design of Existing Code by Martin Fowler et al. This book is even fun to read. The authors talk about "code smells" that beg for refactoring, and they go into great detail about the various techniques for fixing them. Refactoring and the ability to write test-first code are the most important skills for new programmers to learn. If everybody were good at both, it would revolutionize the industry. If you
- 52. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 52 of 54 become good at both, you will ultimately produce cleaner code and more-functional applications than many of your peers. Conclusion to Part 1 In this tutorial, you learned about object-oriented programming, discovered Java syntax that you can use to create useful objects, and familiarized yourself with an IDE that helps you control your development environment. You know how to create and run Java objects that can do a good number of things, including doing different things based on different input. You also know how to JAR up your applications for other developers to use in their programs, and you've got some basic best Java programming practices under your belt. What's next In the second half of this tutorial, you begin learning about some of the more advanced constructs of Java programming, although the overall discussion is still introductory in scope. Java programming topics covered in that tutorial include: • Exception handling • Inheritance and abstraction • Interfaces • Nested classes • Regular expressions • Generics • Enum types • I/O • Serialization Read "Introduction to Java programming, Part 2: Constructs for real-world applications."
- 53. ibm.com/developerWorks/ developerWorks® Introduction to Java programming, Part 1: Java language basics Page 53 of 54 Resources • developerWorks Premium provides an all-access pass to powerful tools, curated technical library from Safari Books Online, conference discounts and proceedings, SoftLayer and Bluemix credits, and more. • Java technology home page: The official Java site has links to all things related to the Java platform, including the Java language specification and Java API documentation. • Java 8: Learn more about JDK 8 and the tools that come with it. • Refactoring: Improving the Design of Existing Code (Martin Fowler et al., Addison- Wesley, 1999): This book is an excellent resource for learning how to write cleaner, more maintainable code. • New to Java technology: Check out this compendium of developerWorks resources for beginning Java developers. • 5 things you didn't know about ...: This developerWorks series provides short introductions to lesser-known (but often introductory) Java programming tips and lore. • The Java Tutorials: Get a comprehensive introduction to the Java language. • Log4J (J. Steven Perry, O'Reilly Media, 2009): Read more about Log4j, a JDK replacement logging library. • JDK 8: Download JDK 8 from Oracle. • Eclipse: Download the Eclipse IDE for Java Developers. • IBM developer kits: IBM provides several Java developer kits for use on popular platforms. • Apache Commons Lang: Download Commons Lang from Apache. • Apache log4j 2: Download log4j 2 from Apache.
- 54. developerWorks® ibm.com/developerWorks/ Introduction to Java programming, Part 1: Java language basics Page 54 of 54 About the author J Steven Perry Steve Perry is a software developer, architect, and general Java nut who has been developing software professionally since 1991. His professional interests range from the inner workings of the JVM to UML modeling and everything in-between. Steve has a passion for writing and mentoring; he is the author of Java Management Extensions (O'Reilly), Log4j (O'Reilly), and the IBM developerWorks articles "Joda-Time" and OpenID for Java Web applications." In his spare time, he hangs out with his kids, plays ice hockey, and practices yoga. © Copyright IBM Corporation 2010, 2015 (www.ibm.com/legal/copytrade.shtml) Trademarks (www.ibm.com/developerworks/ibm/trademarks/)