Provides default implementations of {@link ExecutorService} execution methods. This class implements the {@code submit}, {@code invokeAny} and {@code invokeAll} methods using a {@link RunnableFuture} returned by {@code newTaskFor}, which defaults to the {@link FutureTask} class provided in this package. For example, the implementation of {@code submit(Runnable)} creates an associated {@code RunnableFuture} that is executed and returned. Subclasses may override the {@code newTaskFor} methods to return {@code RunnableFuture} implementations other than {@code FutureTask}.