Basics Of Threads by ravindu

What is a thread?

There are two ways to create a thread:

1. By extending Thread class
2. By implementing Runnable interface.

Thread class provide constructors and methods to create and perform operations on a thread .Thread class extends Object class and implements Runnable interface.

Creating a thread

Thread t = new Thread();

By extending Thread class

The only disadvantage of this way is you can only inherit one class .

Class Downloder extends Threds{

Public void run(){ // always the executing code should be inside the run method

Filedownloder fd= new Filedownloder();

Fd.downlode();

}

}

To invoke the run method we should start the thread

Thread t = new Thread();

t.start(); we should call the start method in order to call the run()

By implementing Runnable interface.

Here the problem of inheritance is overcome.

Class downloader implement runnable{

}

Public void run(){

Filedownloder fd =new Filedownloder(){

Fd.downloder();

}

}

Invoking the run method

downloder d= new downloader();

Thread t=new Thread(d);

t.start();

Abstract Class

The most important cases in java abstraction  

• Abstract classes may or may not contain abstract methods ie., methods with out body ( public void get(); )
• But, if a class have at least one abstract method, then the class must be declared abstract.
• If a class is declared abstract it cannot be instantiated (an object cannot be created ).
• To use an abstract class you have to inherit it from another class, provide implementations to the abstract methods in it.
• If you inherit an abstract class you have to provide implementations to all the abstract methods in it.

Final keyword

Final Keyword tutorial Report By:-Ravindu Perera 2014259

Case 1

How to stop a variable from changing?

public class Tutorial {

public static void main(String[] args) {

Test test = new Test();

System.out.println(test.p1);

}

}

class Test {

public final double p1 = 3.14;

Test() {

p1 = 2.14;//due to the finall keyword the intialised value cannot change

}

}

here because the variable made final so we cannot change the variable it is made a constant.

Case 2

How to stop a method from being overriden when it is inherited?

public class finalMethod {

final void data() {

System.out.println("I have 1 GB");

}

}

class tre extends finalMethod {

void data() {//here the method is not being allowed to ovrride

System.out.println("I have 5GB");

}

public static void main(String []args){

}

}

because the method uses the keyword final the extened class will be not allow to overrride the method as final keyword stops it.

Case 3

How to prevent a class from being inherited?

public final class finalMethod {

final void data() {

System.out.println("I have 1 GB");

}

}

class tre extends finalMethod {//the final keyword prevents inheritance

void data() {

System.out.println("I have 5GB");

}

public static void main(String []args){

}

}

in this code the final keyword is being used to prevent the superclass being inherited to the subclass so an error message is being shown.