Comp102 lec 3

Java
• Programming Language
• Developed by: James Gosling at Sun
Microsystems in 1991 , referred as OAK
• Released in 1995 as core component of Sun
Microsystems Java Platform
• Derives much of its syntax from C and C++
• Applications are compiled to bytecode that can
run on any JVM( Java Virtual Machine) regardless
of computer architecture

Based on material from: http://en.wikipedia.org/wiki/Java_%28programming_language%29

Java Virtual Machine
• A virtual machine (VM) is a software implementation of a
machine that executes programs like a physical machine1
• JVM enables a set of computer software programs and
data structures to use a virtual machine model for the
execution of other computer programs and scripts.2
• The model used by a JVM accepts a form of computer
intermediate language commonly referred to as Java
bytecode.2
• Java bytecode instructions are analogous to machine
code, but are intended to be interpreted by a virtual
machine

Based on material from:
1. http://en.wikipedia.org/wiki/Virtual_machine
2.http://en.wikipedia.org/wiki/Java_Virtual_Machine

Java is:
• General-purpose
• Concurrent
• class-based and
• Object-oriented,
• Is specifically designed to have as few implementation
dependencies as possible.
• It is intended to let application developers "write once,
run anywhere".
• Java is considered by many as one of the most influential
programming languages of the 20th century, and widely
used from application software to web application

Based on material from: http://en.wikipedia.org/wiki/Java_%28programming_language%29

Principles
• There were five primary goals in the creation
of the Java language:
– It should be "simple, object oriented, and
familiar".
– It should be "robust and secure".
– It should be "architecture neutral and portable".
– It should execute with "high performance".
– It should be "interpreted, threaded, and
dynamic".

Design Goals of the JavaTM Programming Language

Object Oriented
• A technique or way to model real world
problems
• Many interacting objects exist
• Objects interact via messages
• Class Vs Object
– A class is a user defined data type
– Object is any instance of class

Object
• The fundamental idea behind object-oriented
language is to combine into a single unit both
– data and
– the functions that operate on that data.
– Such a unit is called an object
• Five basic properties
– Identity
– Attributes
– Behavior
– Interface
– Life Span

Java Program
• A java program is defined by a public class that takes
the form:
public class program-name
{
optional variable declarations and methods
public static void main(String[] args)
{
statements
}
optional variable declarations and methods
}

To compile Java Code
• javac program-name.java
• For this to work, the javac must be in your shell's path
or you must explicitly specify the path to the program
(such as c:j2sebinjavac program-name.java).
• If the compilation is successful, javac will quietly end
and return you to a command prompt.
• If you look in the directory, there will now be a
program-name.class file.
• This file is the compiled version of your program.
Once your program is in this form, its ready to run.
• If not, or you receive an error message, check for
typographical errors in your source code.

To RUN(Execute) Java Program
• java Program-name

Java Comments
• The Java programming language supports three kinds of
comments:
• /* text */
The compiler ignores everything from /* to */.
• /** documentation */
This indicates a documentation comment (doc comment,
for short). The compiler ignores this kind of comment, just
like it ignores comments that use /* and */. The JDK
javadoc tool uses doc comments when preparing
automatically generated documentation.
• // text
The compiler ignores everything from // to the end of the
line.

Java Data and Variables
• 8 primitive types
– byte
– short
– int
– long
– float
– double
– char
– boolean

Integer Primitive Data Types
Type Size Range
byte 8 bits -128 to +127
-32,768 to
short 16 bits
+32,767
(about)-2 billion
int 32 bits
to +2 billion
(about)-10E18 to
long 64 bits
+10E18

Floating Point Primitive DataTypes

Method to Declare variable
• Syntax
– DataType VariableName;
– Datatype VariableName = InitialValue;
• Examples
– int yr = 2010;
– double value = 89.12 ;

Variable
• Variables only exist within the structure in which
they are defined.
• For example, if a variable is created within a
method, it cannot be accessed outside the method.
• In addition, a different method can create a variable
of the same name which will not conflict with the
other variable.
• A java variable can be thought of as a little box
made up of one or more bytes that can hold a value
of a particular data type

Example
class example
{
public static void main ( String[] args )
{
long x = 123; //a declaration of a variable named x with a datatype of long
System.out.println("The variable x has: " + x );
}
}

Java Command Line Arguments
• Arguments are passed as a String array to the
main method of a class.
• The first element (element 0) is the first
argument passed not the name of the class

Comp102 lec 3

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Comp102 lec 3