code: public void execute(Runnable command) {
          if (command == null)
              throw new NullPointerException();
          /*
           * Proceed in 3 steps:
           *
           * 1. If fewer than corePoolSize threads are running, try to
           * start a new thread with the given command as its first
           * task.  The call to addWorker atomically checks runState and
           * workerCount, and so prevents false alarms that would add
           * threads when it shouldn't, by returning false.
           *
           * 2. If a task can be successfully queued, then we still need
           * to double-check whether we should have added a thread
           * (because existing ones died since last checking) or that
           * the pool shut down since entry into this method. So we
           * recheck state and if necessary roll back the enqueuing if
           * stopped, or start a new thread if there are none.
           *
           * 3. If we cannot queue task, then we try to add a new
           * thread.  If it fails, we know we are shut down or saturated
           * and so reject the task.
           */
          int c = ctl.get();
          if (workerCountOf(c) < corePoolSize) {
              if (addWorker(command, true))
                  return;
              c = ctl.get();
          }
          if (isRunning(c) && workQueue.offer(command)) {
              int recheck = ctl.get();
              if (! isRunning(recheck) && remove(command))
                  reject(command);
              else if (workerCountOf(recheck) == 0)
                  addWorker(null, false);
          }
          else if (!addWorker(command, false))
              reject(command);
      }
docs: Executes the given task sometime in the future.  The task
  may execute in a new thread or in an existing pooled thread.
  
  If the task cannot be submitted for execution, either because this
  executor has been shutdown or because its capacity has been reached,
  the task is handled by the current {@code RejectedExecutionHandler}.
  
  @param command the task to execute
  @throws RejectedExecutionException at discretion of
  {@code RejectedExecutionHandler}, if the task
  cannot be accepted for execution
  @throws NullPointerException if {@code command} is null