Programming Tutorials

How to use 'implements Runnable' in Java

By: Emiley J in Java Tutorials on 2007-09-08  

The easiest way to create a thread is to create a class that implements the Runnable interface. Runnable abstracts a unit of executable code. You can construct a thread on any object that implements Runnable. To implement Runnable, a class need only implement a single method called run(), which is declared like this:

public void run()

Inside run(), you will define the code that constitutes the new thread. It is important to understand that run() can call other methods, use other classes, and declare variables, just like the main thread can. The only difference is that run() establishes the entry point for another, concurrent thread of execution within your program. This thread will end when run() returns.

After you create a class that implements Runnable, you will instantiate an object of type Thread from within that class. Thread defines several constructors. The one that we will use is shown here:

Thread(Runnable threadOb, String threadName)

In this constructor, threadOb is an instance of a class that implements the Runnable interface. This defines where execution of the thread will begin. The name of the new thread is specified by threadName.

After the new thread is created, it will not start running until you call its start() method, which is declared within Thread. In essence, start() executes a call to run(). The start() method is shown here:

void start()

Here is an example that creates a new thread using Runnable interface and starts it running:

// Create a second thread.
class NewThread implements Runnable {
    Thread t;

    NewThread() {
        // Create a new, second thread
        t = new Thread(this, "Demo Thread");
        System.out.println("Child thread: " + t);
        t.start(); // Start the thread
    }

    // This is the entry point for the second thread.
    public void run() {
        try {
            for (int i = 5; i > 0; i--) {
                System.out.println("Child Thread: " + i);
                Thread.sleep(500);
            }
        } catch (InterruptedException e) {
            System.out.println("Child interrupted.");
        }
        System.out.println("Exiting child thread.");
    }
}

class ThreadDemo {
    public static void main(String args[]) {
        new NewThread(); // create a new thread
        try {
            for (int i = 5; i > 0; i--) {
                System.out.println("Main Thread: " + i);
                Thread.sleep(1000);
            }
        } catch (InterruptedException e) {
            System.out.println("Main thread interrupted.");
        }
        System.out.println("Main thread exiting.");
    }
}

Inside NewThread's constructor, a new Thread object is created by the following statement:

t = new Thread(this, "Demo Thread");

Passing this as the first argument indicates that you want the new thread to call the run() method on this object. Next, start() is called, which starts the thread of execution beginning at the run() method. This causes the child thread's for loop to begin. After calling start(), NewThread's constructor returns to main(). When the main thread resumes, it enters its for loop. Both threads continue running, sharing the CPU, until their loops finish. The output produced by this program is as follows:

Child thread: Thread[Demo Thread,5,main]
Main Thread: 5
Child Thread: 5
Child Thread: 4
Main Thread: 4
Child Thread: 3
Child Thread: 2
Main Thread: 3
Child Thread: 1
Exiting child thread.
Main Thread: 2
Main Thread: 1
Main thread exiting.

As mentioned earlier, in a multithreaded program, the main thread must be the last thread to finish running. If the main thread finishes before a child thread has completed, then the Java run-time system may "hang." The preceding program ensures that the main thread finishes last, because the main thread sleeps for 1,000 milliseconds between iterations, but the child thread sleeps for only 500 milliseconds. This causes the child thread to terminate earlier than the main thread. Shortly, you will see a better way to ensure that the main thread finishes last.