The Java platform has supported multithreaded programming with the Thread class and Runnable interface since Java 1.0.

A thread is a unit of program execution that runs independently from other threads.Java makes it easy to define and work with multiple threads of execution within a program.

One of the commonly used classes in java Thread program is the java.lang.Thread which represents a thread and defines methods for setting and querying thread properties and for starting the execution of a thread.

Two ways to define a thread.

• One is to subclass Thread, override the run( ) method and then instantiate your Thread subclass.
• The other is to define a class that implements the Runnable method and then pass an instance of this Runnable object to the Thread() constructor.

An example by Implementing the Runnable Interface

/**
 * Created by IntelliJ IDEA.
 * User: xu guanglin
 * Date: 2007-9-11
 * Time: 22:11:28
 * Copyright 2005-2007 Java Development.
 * All Rights Reserved.
 * ===================================================
 * http://www.javadevelopment.org/
 * ===================================================
 * The above paragraph must be included in full.
 */
class Counter implements Runnable {
    private int currentValue;
    private Thread thread;

    public Counter(String threadName) {
        currentValue = 0;
        thread = new Thread(this, threadName);   // Create a new thread.
        System.out.println(thread);
        thread.start();                          // Start the thread.
    }

    public int getValue() {
        return currentValue;
    }

    public void run() {                          //  Thread entry point
        try {
            while (currentValue < 5) {
                System.out.println(thread.getName() + ": " + (currentValue++));
                Thread.sleep(250);               //   Current thread sleeps.
            }
        } catch (InterruptedException e) {
            System.out.println(thread.getName() + " interrupted.");
        }
        System.out.println("Exit from thread: " + thread.getName());
    }
}

 /**
 * Created by IntelliJ IDEA.
 * User: xu guanglin
 * Date: 2007-9-11
 * Time: 22:18:23
 * Copyright 2005-2007 Java Development.
 * All Rights Reserved.
 * ===================================================
 * http://www.javadevelopment.org/
 * ===================================================
 * The above paragraph must be included in full.
 */
public class ThreadDemo {
    public static void main(String[] args) {
        Counter counterA = new Counter("Counter A"); //  Create a thread.

        try {
            int val;
            do {
                val = counterA.getValue();       // Access the counter value.
                System.out.println("Counter value read by main thread: " + val);
                Thread.sleep(1000);              //  Current thread sleeps.
            } while (val < 5);
        } catch (InterruptedException e) {
            System.out.println("main thread interrupted.");
        }

        System.out.println("Exit from main() method.");
    }
}