Extension example. Here is a sketch of a class that customizes ThreadPoolExecutor to use a CustomTask class instead of the default FutureTask:

 
 public class CustomThreadPoolExecutor extends ThreadPoolExecutor {

   static class CustomTask<V> implements RunnableFuture<V> { ... }

   protected <V> RunnableFuture<V> newTaskFor(Callable<V> c) {
       return new CustomTask<V>(c);
   }
   protected <V> RunnableFuture<V> newTaskFor(Runnable r, V v) {
       return new CustomTask<V>(r, v);
   }
   // ... add constructors, etc.
 }