Thread Body

All the action takes place in the thread body, which is the thread's run method. After a thread has been created and initialized, the runtime system calls its run method. The code in the run method implements the behavior for which the thread was created. It's the thread's raison d'etre.
Often, a thread's run method is a loop. For example, an animation thread might loop through and display a series of images. Sometimes a thread's run method performs an operation that takes a long time, for example, downloading and playing a sound or a JPEG movie.
You can choose one of two ways to provide a customized run method for a Java thread:

Subclass the Thread class defined in the java.lang package and override the run method.
Example: The SimpleThread class described in A Simple Thread Example.

Provide a class that implements the Runnable interface, also defined in the java.lang package. Now, when you instantiate a thread (either directly from the Thread class, or from a subclass of Thread), give the new thread a handle to an instance of your Runnable class. This Runnable object provides the run method to the thread.
Example: The Clock applet you see on the following page.

There are good reasons for choosing either of the two options described above over the other. However, for most cases, 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.

Threads of Control