AI-generated Key Takeaways
- 
          The Executorinterface provides a way to decouple task submission from the mechanics of how each task will be run, often using thread pools instead of explicitly creating threads.
- 
          Executorimplementations have flexibility in how they execute tasks, including running them immediately, in a new thread, or in a pooled thread.
- 
          ExecutorServiceextendsExecutorwith more advanced features, such as managing termination and tracking task progress withFutureobjects.
- 
          Several implementations like ThreadPoolExecutorandScheduledThreadPoolExecutorare available in thejava.util.concurrentpackage, along with factory methods inExecutorsfor easier creation.
- 
          Memory consistency is guaranteed: actions in a thread before submitting a task happen-before its execution begins. 
| Known Indirect Subclasses | 
An object that executes submitted Runnable tasks. This
 interface provides a way of decoupling task submission from the
 mechanics of how each task will be run, including details of thread
 use, scheduling, etc.  An Executor is normally used
 instead of explicitly creating threads. For example, rather than
 invoking new Thread(new RunnableTask()).start() for each
 of a set of tasks, you might use:
 
 Executor executor = anExecutor();
 executor.execute(new RunnableTask1());
 executor.execute(new RunnableTask2());
 ...Executor interface does not strictly require
 that execution be asynchronous. In the simplest case, an executor
 can run the submitted task immediately in the caller's thread:
  class DirectExecutor implements Executor {
   public void execute(Runnable r) {
     r.run();
   }
 } class ThreadPerTaskExecutor implements Executor {
   public void execute(Runnable r) {
     new Thread(r).start();
   }
 }Executor implementations impose some sort of
 limitation on how and when tasks are scheduled.  The executor below
 serializes the submission of tasks to a second executor,
 illustrating a composite executor.
  class SerialExecutor implements Executor {
   final Queue<Runnable> tasks = new ArrayDeque<>();
   final Executor executor;
   Runnable active;
   SerialExecutor(Executor executor) {
     this.executor = executor;
   }
   public synchronized void execute(final Runnable r) {
     tasks.add(new Runnable() {
       public void run() {
         try {
           r.run();
         } finally {
           scheduleNext();
         }
       }
     });
     if (active == null) {
       scheduleNext();
     }
   }
   protected synchronized void scheduleNext() {
     if ((active = tasks.poll()) != null) {
       executor.execute(active);
     }
   }
 }Executor implementations provided in this package
 implement ExecutorService, which is a more extensive
 interface.  The ThreadPoolExecutor class provides an
 extensible thread pool implementation. The Executors class
 provides convenient factory methods for these Executors.
 Memory consistency effects: Actions in a thread prior to
 submitting a Runnable object to an Executor
 happen-before
 its execution begins, perhaps in another thread.
Public Method Summary
| abstract void | 
Public Methods
public abstract void execute (Runnable command)
Executes the given command at some time in the future.  The command
 may execute in a new thread, in a pooled thread, or in the calling
 thread, at the discretion of the Executor implementation.
Parameters
| command | the runnable task | 
|---|
Throws
| RejectedExecutionException | if this task cannot be accepted for execution | 
|---|---|
| NullPointerException | if command is null | 
