Thread Creation

In Java, an object of the Thread class can represent a thread. Thread can be implemented through any one of two ways:

Extending the java.lang.Thread Class
Implementing the java.lang.Runnable Interface

I. Extending the java.lang.Thread Class

For creating a thread a class have to extend the Thread Class. For creating a thread by this procedure you have to follow these steps:

Extend the java.lang.Thread Class.

Override the run( ) method in the subclass from the Thread class to define the code executed by the thread.

Create an instance of this subclass. This subclass may call a Thread class constructor by subclass constructor.

Invoke the start( ) method on the instance of the class to make the thread eligible for running.

The following program demonstrates a single thread creation extending the "Thread" Class:


class MyThread extends Thread{

String s=null; MyThread(String s1){ s=s1; start(); } public void run(){ System.out.println(s); } } public class RunThread{ public static void main(String args[]){ MyThread m1=new MyThread("Thread started...."); } }

Output of the Program is :

C:\j2se6\thread>javac RunThread.java

C:\j2se6\thread>java RunThread
Thread started....

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II. Implementing the java.lang.Runnable Interface

The procedure for creating threads by implementing the Runnable Interface is as follows:

A Class implements the Runnable Interface, override the run() method to define the code executed by thread. An object of this class is Runnable Object.
Create an object of Thread Class by passing a Runnable object as argument.
Invoke the start( ) method on the instance of the Thread class.

The following program demonstrates the thread creation implenting the Runnable interface:


class MyThread1 implements Runnable{

  Thread t;

 
String s=null;

MyThread1(String s1){ s=s1; t=new Thread(this); t.start(); } public void run(){ System.out.println(s); } } public class RunableThread{ public static void main(String args[]){ MyThread1 m1=new MyThread1("Thread started...."); } }

However, this program returns the output same as of the output generated through the previous program.

Output of the Program is:

C:\j2se6\thread>javac RunableThread.java

C:\j2se6\thread>java RunableThread
Thread started....

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There are two reasons for implementing a Runnable interface preferable to extending the Thread Class:

If you extend the Thread Class, that means that subclass cannot extend any other Class, but if you implement Runnable interface then you can do this.
The class implementing the Runnable interface can avoid the full overhead of Thread class which can be excessive.

join() & isAlive() methods:

The following program demonstrates the join() & isAlive() methods:


class DemoAlive extends Thread {

  int value;



  public DemoAlive(String str){

  super(str); 

  value=0;

  start();  

  }

    

 public void run(){  

  try{

  while (value < 5){  

  System.out.println(getName() + ": " + (value++));

 Thread.sleep(250);  

  }

  } catch (Exception e) {}

  System.out.println("Exit from thread: " + getName());

  }

}

    

public class DemoJoin{  

    

      public static void main(String[] args){

  DemoAlive da = new DemoAlive("Thread a");

  DemoAlive db = new DemoAlive("Thread b");

  try{

  System.out.println("Wait for the child threads to finish.");

  da.join();



  if (!da.isAlive())

  System.out.println("Thread A not alive.");

    

  db.join();

    

  if (!db.isAlive())

  System.out.println("Thread B not alive.");

  } catch (Exception e) { }

  System.out.println("Exit from Main Thread.");

  }

}

Output of this program is:

C:\j2se6\thread>javac DemoJoin.java

C:\j2se6\thread>java DemoJoin
Wait for the child threads to finish.
Thread a: 0
Thread b: 0
Thread a: 1
Thread b: 1
Thread a: 2
Thread b: 2
Thread a: 3
Thread b: 3
Thread a: 4
Thread b: 4
Exit from thread: Thread a
Thread A not alive.
Exit from thread: Thread b
Thread B not alive.
Exit from Main Thread.

C:\j2se6\thread>

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