Mumbai University > Information Technology > Sem 3 > Object Oriented Programming Methodology

Marks: 5 M

Year: May 2015

THREAD CREATION

There are two ways to create thread in java;

• Implement the Runnable interface (java.lang.Runnable)
• By Extending the Thread class (java.lang.Thread)

IMPLEMENTING THE RUNNABLE INTERFACE

The Runnable Interface Signature

public interface Runnable {

    void run();
}

One way to create a thread in java is to implement the Runnable Interface and then instantiate an object of the class. We need to override the run() method into our class which is the only method that needs to be implemented. The run() method contains the logic of the thread.

The procedure for creating threads based on the Runnable interface is as follows:

1. A class implements the Runnable interface, providing the run() method that will be executed by the thread. An object of this class is a Runnable object.
2. An object of Thread class is created by passing a Runnable object as argument to the Thread constructor. The Thread object now has a Runnable object that implements the run() method.
3. The start() method is invoked on the Thread object created in the previous step. The start() method returns immediately after a thread has been spawned.

4. The thread ends when the run() method ends, either by normal completion or by throwing an uncaught exception. Below is a program that illustrates instantiation and running of threads using the runnable interface instead of extending the Thread class. To start the thread you need to invoke the start() method on your object.

   class RunnableThread implements Runnable {

   Thread runner;
   public RunnableThread() {
   }
   public RunnableThread(String threadName) {
       runner = new Thread(this, threadName); //Create a new thread.
       System.out.println(runner.getName());
       runner.start(); //Start the thread.
   }
   public void run() {
       //Display info about this particular thread
       System.out.println(Thread.currentThread());
   }
   }
   
   public class RunnableExample {
   
   public static void main(String[] args) {
       Thread thread1 = new Thread(new RunnableThread(), "thread1");
       Thread thread2 = new Thread(new RunnableThread(), "thread2");
       RunnableThread thread3 = new RunnableThread("thread3");
       //Start the threads
       thread1.start();
       thread2.start();
       try {
           //delay for one second
           Thread.currentThread().sleep(1000);
       } catch (InterruptedException e) {
       }
       //Display info about the main thread
       System.out.println(Thread.currentThread());
   }
   }
   

This approach of creating a thread by implementing the Runnable Interface must be used whenever the class being used to instantiate the thread object is required to extend some other class.

EXTENDING THREAD CLASS

The procedure for creating threads based on extending the Thread is as follows:

1. A class extending the Thread class overrides the run() method from the Thread class to define the code executed by the thread.
2. This subclass may call a Thread constructor explicitly in its constructors to initialize the thread, using the super() call.
3. The start() method inherited from the Thread class is invoked on the object of the class to make the thread eligible for running.

Below is a program that illustrates instantiation and running of threads by extending the Thread class instead of implementing the Runnable interface. To start the thread you need to invoke the start()method on your object.

class XThread extends Thread {

    XThread() {
    }
    XThread(String threadName) {
        super(threadName); // Initialize thread.
        System.out.println(this);
        start();
    }
    public void run() {
        //Display info about this particular thread
        System.out.println(Thread.currentThread().getName());
    }
}

public class ThreadExample {

    public static void main(String[] args) {
        Thread thread1 = new Thread(new XThread(), "thread1");
        Thread thread2 = new Thread(new XThread(), "thread2");
        //      The below 2 threads are assigned default names
        Thread thread3 = new XThread();
        Thread thread4 = new XThread();
        Thread thread5 = new XThread("thread5");
        //Start the threads
        thread1.start();
        thread2.start();
        thread3.start();
        thread4.start();
        try {
    //The sleep() method is invoked on the main thread to cause a one second delay.
            Thread.currentThread().sleep(1000);
        } catch (InterruptedException e) {
        }
        //Display info about the main thread
        System.out.println(Thread.currentThread());
    }
}