Extend Thread vs implement Runnable

There are two ways to create your own thread type: subclass java.lang.Thread class, or implementing java.lang.Runnable and pass it to Thread constructor or java.util.concurrent.ThreadFactory. What is the difference, and which one is better?
1, The practical reason is, a Java class can have only one superclass. So if your thread class extends java.lang.Thread, it cannot inherit from any other classes. This limits how you can reuse your application logic.
2, From a design point of view, there should be a clean separation between how a task is identified and defined, between how it is executed. The former is the responsibility of a Runnalbe impl, and the latter is job of the Thread class.
3, A Runnable instance can be passed to other libraries that accept task submission, e.g., java.util.concurrent.Executors. A Thread subclass inherits all the overhead of thread management and is hard to reuse.
4, Their instances also have different lifecycle. Once a thread is started and completed its work, it's subject to garbage collection. An instance of Runnalbe task can be resubmitted or retried multiple times, though usually new tasks are instantiated for each submission to ease state management.

Implementing the Runnable Interface

The Clock applet shown below displays the current time and updates its display every second. You can scroll this page and perform other tasks while the clock continues to update because the code that updates the clock's display runs within its own thread.
The Clock applet uses a different technique than SimpleThread for providing the run method for its thread. Instead of subclassing Thread, Clock implements the Runnable interface (and therefore implements the run method defined in it). Clock then creates a thread with itself as the Thread's target. When created in this way, the Thread gets its run method from its target. The code that accomplishes this is shown in bold here:

import java.awt.Graphics;
import java.util.*;
import java.text.DateFormat;
import java.applet.Applet;

public class Clock extends Applet implements Runnable {
    private Thread clockThread = null;
    public void start() {
        if (clockThread == null) {
            clockThread = new Thread(this, "Clock");
            clockThread.start();
        }
    }
    public void run() {
        Thread myThread = Thread.currentThread();
        while (clockThread == myThread) {
            repaint();
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e){
            // the VM doesn't want us to sleep anymore,
            // so get back to work
            }
        }
    }
    public void paint(Graphics g) {
        // get the time and convert it to a date
        Calendar cal = Calendar.getInstance();
        Date date = cal.getTime();
        // format it and display it
        DateFormat dateFormatter = DateFormat.getTimeInstance();
        g.drawString(dateFormatter.format(date), 5, 10);
    }
    // overrides Applet's stop method, not Thread's
    public void stop() {
        clockThread = null;
    }
}

The Clock applet's run method loops until the browser asks it to stop. During each iteration of the loop, the clock repaints its display. The paint method figures out what time it is, formats it in a localized way, and displays it. You'll see more of the Clock applet in The Life Cycle of a Thread which uses it to teach you about the life of a thread.

Deciding to Use the Runnable Interface

You have now seen two ways to provide the run method for a Java thread:

1. Subclass the Thread class defined in the java.lang package and override the run method.
   
   

   ---

   
   Example: See the SimpleThread class described in Subclassing Thread and Overriding run.
   

   ---

   
   
   
2. Provide a class that implements the Runnable interface (also defined in the java.lang package) and therefore implements the run method. In this case, a Runnable object provides the run method to the thread.
   
   

   ---

   
   Example: See the Clock applet just shown.
   

   ---

   
   

There are good reasons for choosing either of these options over the other. However, for most cases, including that of the Clock applet, the following rule of thumb will guide you to the best option.

---

Rule of Thumb: If your class must subclass some other class (the most common example being Applet), you should use Runnable as described in option #2.

---

To run in a Java-enabled browser, the Clock class has to be a subclass of the Applet class. Also, the Clock applet needs a thread so that it can continuously update its display without taking over the process in which it is running. (Some browsers might create a new thread for each applet so as to prevent a misbehaved applet from taking over the main browser thread. However, you should not count on this when writing your applets; your applets should create their own threads when doing computer-intensive work.) But since the Java language does not support multiple class inheritance, the Clock class cannot be a subclass of both Thread and Applet. Thus the Clock class must use the Runnable interface to provide its threaded behavior.

Download

Download the source code from here. Also see Clock applet here (soon to be added).

Customizing a Thread's run Method

The run method gives a thread something to do. Its code implements the thread's running behavior. It can do anything that can be encoded in Java statements: compute a list of prime's, sort some data, perform some animation. The Thread class implements a generic thread that, by default, does nothing. That is, the implementation of its run method is empty. This is not particularly useful, so the Thread class defines API that lets a Runnable object provide a more interesting run method for a thread.
There are two techniques for providing a run method for a thread:

• Subclassing Thread and Overriding run
• Implementing the Runnable Interface