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Java – Introduction

Java was developed by James Gosling, Patrick Naughton, Chris Warth, Ed Frank, and Mike Sheridan at Sun Microsystems in 1991.
This language was initially called “Oak”
It was renamed “Java” in 1995.
Why the Java Icon like this?

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Java - General

Java is:

platform independent programming language
similar to C++ in syntax

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Java Development Kit

javac - The Java Compiler
java - The Java Interpreter
jdb - The Java Debugger
appletviewer -Tool to run the applets

javap - to print the Java bytecodes
javaprof - Java profiler
javadoc - documentation generator
javah - creates C header files

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Java - General

Java has some interesting features:

automatic type checking,
automatic garbage collection,
simplifies pointers; no directly accessible pointer to memory,
simplified network access,
multi-threading!

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How it works…!

Compile-time Environment

Java

Bytecodes

move locally

or through

network

Java

Source

(.java)

Java

Compiler

Java

Bytecode

(.class )

Java

Interpreter

Just in Time

Compiler

Runtime System

Class Loader

Bytecode

Verifier

Java Class

Libraries

Operating System

Hardware

Java

Virtual

machine

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How it works…!

Java is independent only for one reason:

Only depends on the Java Virtual Machine (JVM),
code is compiled to bytecode, which is interpreted by the resident JVM,
JIT (just in time) compilers attempt to increase speed.

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Object-Oriented

Java supports OOP

Polymorphism
Inheritance
Encapsulation

Java programs contain nothing but definitions and instantiations of classes

Everything is encapsulated in a class!

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Java Advantages

Portable - Write Once, Run Anywhere
Security has been well thought through
Robust memory management
Designed for network programming
Multi-threaded (multiple simultaneous tasks)
Dynamic & extensible (loads of libraries)

Classes stored in separate files
Loaded only when needed

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Basic Java Syntax

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Primitive Types and Variables

boolean, char, byte, short, int, long, float, double etc.
These basic (or primitive) types are the only types that are not objects (due to performance issues).
This means that you don’t use the new operator to create a primitive variable.
Declaring primitive variables:

float s;

char a;

Byte c;

int a, index = 2;

double g = 1.2, brightness

boolean v = false;

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Initialisation

If no value is assigned prior to use, then the compiler will give an error
Java sets primitive variables to zero or false in the case of a boolean variable
All object references are initially set to null
An array of anything is an object

Set to null on declaration
Elements to zero false or null on creation

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Declarations

int index = 1.2; // compiler error

boolean retOk = 1; // compiler error

double fiveFourths = 5 / 4; // no error!

float ratio = 5.8f; // correct

double fiveFourths = 5.0 / 4.0; // correct

1.2f is a float value accurate to 7 decimal places.
1.2 is a double value accurate to 15 decimal places.

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Assignment

All Java assignments are right associative

int a = 1, b = 2, c = 5

a = b = c

System.out.print(

“a= “ + a + “b= “ + b + “c= “ + c)

What is the value of a, b & c
Done right to left: a = (b = c);

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public class examplpro {
public static void main(String[ ] args) {
int a = 1, b = 2, c = 5;

System.out.println("a= " + a + "; b= " + b + "; c= " + c);

a = (b = c);
System.out.println("a= " + a + "; b= " + b + "; c= " + c);
}

}

O/P
a= 1; b= 2; c= 5
a= 5; b= 5; c= 5

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Basic Mathematical Operators

* / % + - are the mathematical operators
* / % have a higher precedence than + or -

double myVal = a + b % d – c * d / b;

Is the same as:

double myVal = (a + (b % d)) –

((c * d) / b);

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Flow of Control

Java executes one statement after the other in the order they are written
Many Java statements are flow control statements:

Alternation: if, if else, switch

Looping: for, while, do while

Escapes: break, continue, return

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If – The Conditional Statement

The if statement evaluates an expression and if that evaluation is true then the specified action is taken

if ( x < 10 ) x = 10;

If the value of x is less than 10, make x equal to 10
It could have been written:

if ( x < 10 )

x = 10;

Or, alternatively:

if ( x < 10 ) { x = 10; }

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Relational Operators

== Equal (careful)

!= Not equal

>= Greater than or equal

<= Less than or equal

> Greater than

< Less than

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If… else

The if … else statement evaluates an expression and performs one action if that evaluation is true or a different action if it is false.

if (x != oldx) {

System.out.print(“x was changed”);

}

else {

System.out.print(“x is unchanged”);

}

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Nested if … else

if ( myVal > 100 ) {

if ( remainderOn == true) {

myVal = mVal % 100;

}

else {

myVal = myVal / 100.0;

}

else

{

System.out.print(“myVal is in range”);

}

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else if

Useful for choosing between alternatives:

if ( n == 1 ) {

// execute code block #1

}

else if ( j == 2 ) {

// execute code block #2

}

else {

// if all previous tests have failed, execute code block #3

}

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A Warning…

WRONG!

if( i == j )

if ( j == k )

System.out.print(

“i equals k”);

else

System.out.print(

“i is not equal to j”);

CORRECT!

if( i == j ) {

if ( j == k )

System.out.print(

“i equals k”);

}

else

System.out.print(“i is not equal to j”); // Correct!

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The switch Statement

switch ( n ) {

case 1:

// execute code block #1

break;

case 2:

// execute code block #2

break;

default:

// if all previous tests fail then //execute code block #4

break;

}

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The for loop

Loop n times

for ( i = 0; i < n; n++ ) {

// this code body will execute n times

// ifrom 0 to n-1

}

Nested for:

for ( j = 0; j < 10; j++ ) {

for ( i = 0; i < 20; i++ ){

// this code body will execute 200 times

}

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while loops

while(response == 1) {

System.out.print( “ID =” + userID[n]);

n++;

response = readInt( “Enter “);

}

What is the minimum number of times the loop is executed?

What is the maximum number of times?

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do {… } while loops

do {

System.out.print( “ID =” + userID[n] );

n++;

response = readInt( “Enter ” );

}while (response == 1);

What is the minimum number of times the loop is executed?

What is the maximum number of times?

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Arrays

Am array is a list of similar things
An array has a fixed:

name
type
length

These must be declared when the array is created.
Arrays sizes cannot be changed during the execution of the code

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myArray has room for 8 elements

the elements are accessed by their index
in Java, array indices start at 0

myArray =

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Declaring Arrays

int myArray[];

declares myArray to be an array of integers

myArray = new int[8];

sets up 8 integer-sized spaces in memory, labelled myArray[0] to myArray[7]

int myArray[] = new int[8];

combines the two statements in one line

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Assigning Values

refer to the array elements by index to store values in them.

myArray[0] = 3;

myArray[1] = 6;

myArray[2] = 3; ...

can create and initialise in one step:

int myArray[] = {3, 6, 3, 1, 6, 3, 4, 1};

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Iterating Through Arrays

for loops are useful when dealing with arrays:

for (int i = 0; i < myArray.length; i++) {

myArray[i] = getsomevalue();

}

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Declaring the Array

1. Declare the array

private Student studentList[];

this declares studentList

2 .Create the array

studentList = new Student[10];

this sets up 10 spaces in memory that can hold references to Student objects

3. Create Student objects and add them to the array: studentList[0] = new Student("Cathy", "Computing");