If your class is extending the Thread class then it becomes a single thread which inherits the properties Thread class, so it'll be heavy. (When extending Thread class each of the threads creates unique object and associate with it, but when implementing Runnable, it shares the same object to multiple Threads).
If your class is Implementing the Runnable interface then you only override the run() .So this instance creates a separate Thread and every individual Thread runs separately but not as a single heavy Thread in your program.
Another thing, Since Java does not support multiple inheritance, if you implement the Runnable you'll avoid problems of multiple extending, so if you implement Runnable interface you can extend any class that you are required other than Thread class.
