basic java

Basic Java

Java is:

–     platform independent programming language

–     similar to C++ in syntax

–     similar to Smalltalk in mental paradigm

n Pros:  also ubiquitous to net

n Cons: interpreted, and still under development (moving target)

n Java has some interesting features:

n automatic type checking,

n automatic garbage collection,

n simplifies pointers; no directly accessible pointer to memory,

n simplified network access,

n multi-threading

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.

 

Pointer denial – reduces chances of virulent programs corrupting host,

–     Applets even more restricted –

–     May not

–     run local executables,

–     Read or write to local file system,

–     Communicate with any server other than the originating server.

 

Java supports OOD:-

–     Polymorphism

–     Inheritance

–     Encapsulation

n Java programs contain nothing but definitions and instantiations of classes

–     Everything is encapsulated in a class!

Java Advantages:-

n Portable – Write Once, Run Anywhere

n Security has been well thought through

n Robust memory management

n Designed for network programming

n Multi-threaded (multiple simultaneous tasks)

n Dynamic & extensible (loads of libraries)

–      Classes stored in separate files

–      Loaded only when needed

Basic Java Syntax

Primitive Types and Variables

n boolean, char, byte, short, int, long, float, double etc.

n These basic (or primitive) types are the only types that are not objects (due to performance issues).

n This means that you don’t use the new operator to create a primitive variable.

n Declaring primitive variables:

float initVal;

int retVal, index = 2;

double gamma = 1.2, brightness

boolean valueOk = false;

Initialisation:-

n If no value is assigned prior to use, then the compiler will give an error

n Java sets primitive variables to zero or false in the case of a boolean variable

n All object references are initially set to null

n An array of anything is an object

–     Set to null on declaration

–     Elements to zero false or null on creation

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

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

n 1.2 is a double value accurate to 15 decimal places.

 

Flow of Control:-

n Java executes one statement after the other in the order they are written

n Many Java statements are flow control statements:

Alternation:     if, if else, switch

Looping:           for, while, do while

Escapes:           break, continue, return

 

If – The Conditional Statement

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

if ( x < 10 ) x = 10;

n If the value of x is less than 10, make x equal to 10

n It could have been written:

if ( x < 10 )

x = 10;

n Or, alternatively:

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

 

Relational Operators

==   Equal (careful)

!=         Not equal

>=   Greater than or equal

<=   Less than or equal

>          Greater than

<          Less than

If… else

n 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”);

}

 

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”);

}

 

else if

n 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

}

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;

}

 for loop

n Loop n times

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

      // this code body will execute n times

      // ifrom  0 to n-1

}

n Nested for:

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

      for ( i = 0; i < 20; i++ ){

            // this code body will execute 200 times

      }

}

     

while loops

while(response == 1) {

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

      n++;

      response = readInt( “Enter “);

}

do {… } while loops

do {

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

      n++;

      response = readInt( “Enter ” );

}while (response == 1);

 

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