Introduction what is java

Introduction to Java

CS 331

Introduction
• Present the syntax of Java
• Introduce the Java API
• Demonstrate how to build
– stand-alone Java programs
– Java applets, which run within browsers e.g.
Netscape
• Example programs

Why Java?
• It’s the current “hot” language
• It’s almost entirely object-oriented
• It has a vast library of predefined objects
and operations
• It’s more platform independent
– this makes it great for Web programming
• It’s more secure
• It isn’t C++

Applets, Servlets and
Applications
• An applet is designed to be embedded in a
Web page, and run by a browser
• Applets run in a sandbox with numerous
restrictions; for example, they can’t read
files and then use the network
• A servlet is designed to be run by a web
server
• An application is a conventional program

Building Standalone JAVA
Programs (on UNIX)
• Prepare the file foo.java using an editor
• Invoke the compiler: javac foo.java
• This creates foo.class
• Run the java interpreter: java foo

Java Virtual Machine
• The .class files generated by the compiler are
not executable binaries
– so Java combines compilation and interpretation
• Instead, they contain “byte-codes” to be
executed by the Java Virtual Machine
– other languages have done this, e.g. UCSD Pascal
• This approach provides platform
independence, and greater security

HelloWorld (standalone)
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World!");
}
}

• Note that String is built in
• println is a member function for the
System.out class

Comments are almost like C++
• /* This kind of comment can span multiple lines
*/
• // This kind is to the end of the line
• /**
* This kind of comment is a special
* ‘javadoc’ style comment
*/

Primitive data types are like C
• Main data types are int, double,
boolean, char
• Also have byte, short, long, float
• boolean has values true and false
• Declarations look like C, for example,
– double x, y;
– int count = 0;

Expressions are like C
• Assignment statements mostly look like those in C; you
can use =, +=, *= etc.
• Arithmetic uses the familiar + - * / %
• Java also has ++ and --
• Java has boolean operators && || !
• Java has comparisons < <= == != >= >
• Java does not have pointers or pointer arithmetic

Control statements are like C
• if (x < y) smaller = x;
• if (x < y){ smaller=x;sum += x;}
else { smaller = y; sum += y; }
• while (x < y) { y = y - x; }
• do { y = y - x; } while (x < y)
• for (int i = 0; i < max; i++)
sum += i;
• BUT: conditions must be boolean !

Control statements II
switch (n + 1) {
case 0: m = n - 1; break;
case 1: m = n + 1;
case 3: m = m * n; break;
default: m = -n; break;
}
• Java also introduces the try statement,
about which more later

Java isn't C!
• In C, almost everything is in functions
• In Java, almost everything is in classes
• There is often only one class per file
• There must be only one public class per file
• The file name must be the same as the name
of that public class, but with a .java
extension

Java program layout
• A typical Java file looks like:
import java.awt.*;
import java.util.*;
public class SomethingOrOther {
// object definitions go here
. . .
}
This must be in a file named SomethingOrOther.java !

What is a class?
• Early languages had only arrays
– all elements had to be of the same type
• Then languages introduced structures (called
records, or structs)
– allowed different data types to be grouped
• Then Abstract Data Types (ADTs) became
popular
– grouped operations along with the data

So, what is a class?
• A class consists of
– a collection of fields, or variables, very much
like the named fields of a struct
– all the operations (called methods) that can be
performed on those fields
– can be instantiated
• A class describes objects and operations
defined on those objects

Name conventions
• Java is case-sensitive; maxval, maxVal, and
MaxVal are three different names
• Class names begin with a capital letter
• All other names begin with a lowercase letter
• Subsequent words are capitalized: theBigOne
• Underscores are not used in names
• These are very strong conventions!

The class hierarchy
• Classes are arranged in a hierarchy
• The root, or topmost, class is Object
• Every class but Object has at least one
superclass
• A class may have subclasses
• Each class inherits all the fields and methods
of its (possibly numerous) superclasses

An example of a class
class Person {
String name;
int age;
void birthday ( ) {
age++;
System.out.println (name + ' is
now ' + age);
}
}

Another example of a class

class Driver extends Person {
long driversLicenseNumber;
Date expirationDate;
}

Creating and using an object
• Person john;
john = new Person ( );
john.name = "John Smith";
john.age = 37;
• Person mary = new Person ( );
mary.name = "Mary Brown";
mary.age = 33;
mary.birthday ( );

An array is an object
• Person mary = new Person ( );
• int myArray[ ] = new int[5];
– or:
• int myArray[ ] = {1, 4, 9, 16,
25};
• String languages [ ] =
{"Prolog", "Java"};

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