final Runnable runnable;
      final Executor executor;

      RunnableExecutorPair(Runnable runnable, Executor executor) {
        this.runnable = runnable;
        this.executor = executor;
      }

      void execute() {
        try {
          executor.execute(runnable);
        } catch (RuntimeException e) {
          log.log(
              Level.SEVERE,
              "RuntimeException while executing runnable "

            runCalled.getAndIncrement();
          }
        },
        directExecutor());
    Runnable execute =
        new Runnable() {
          @Override
          public void run() {
            list.execute();
          }
        };
    Thread thread1 = new Thread(execute);
    Thread thread2 = new Thread(execute);
    thread1.start();
    thread2.start();
    assertEquals(0, runCalled.get());
    okayToRun.countDown();

import java.util.Deque;
import java.util.concurrent.Executor;
import java.util.concurrent.RejectedExecutionException;
import java.util.logging.Level;
import javax.annotation.CheckForNull;

/**
 * Executor ensuring that all Runnables submitted are executed in order, using the provided
 * Executor, and sequentially such that no two will ever be running at the same time.
 *
 * <p>Tasks submitted to {@link #execute(Runnable)} are executed in FIFO order.
 *

   * (unless the executor has been shutdown).
   *
   * <p>The returned executor is backed by the executor returned by {@link
   * MoreExecutors#newDirectExecutorService} and subject to the same constraints.
   *
   * <p>Although all tasks are immediately executed in the thread that submitted the task, this

      new IteratorOperation() {
        @Override
        public @Nullable Object execute(Iterator<?> iterator) {
          iterator.remove();
          return null;
        }
      };

  private static final IteratorOperation NEXT_METHOD =
      new IteratorOperation() {
        @Override
        public @Nullable Object execute(Iterator<?> iterator) {
          return iterator.next();
        }
      };

    assertFalse(task.isCancelled());

    // Start the task to put it in the RUNNING state.  Have to use a separate
    // thread because the task will block on the task latch after unblocking
    // the run latch.
    exec.execute(task);
    runLatch.await();
    assertEquals(1, listenerLatch.getCount());
    assertFalse(task.isDone());
    assertFalse(task.isCancelled());

    // Finish the task by unblocking the task latch.  Then wait for the

 * guaranteed to execute. There is no guarantee, however, that listeners will be executed in the
 * order that they are added.
 *
 * <p>Exceptions thrown by a listener will be propagated up to the executor. Any exception thrown
 * during {@code Executor.execute} (e.g., a {@code RejectedExecutionException} or an exception

   * propagated. If, on the other hand, the time limit is reached, we attempt to abort the call to
   * the target, and throw a {@link TimeoutException} to the caller.
   *
   * @param callable the Callable to execute
   * @param timeoutDuration with timeoutUnit, the maximum length of time to wait
   * @param timeoutUnit with timeoutDuration, the maximum length of time to wait
   * @return the result returned by the Callable

              }
            }
          };
      executor.execute(errorTask);
      service.execute(barrierTask); // submit directly to the service
      // the barrier task runs after the error task so we know that the error has been observed by
      // SequentialExecutor by the time the barrier is satisfied
      barrier.await(1, TimeUnit.SECONDS);
      executor.execute(barrierTask);

    Preconditions.checkNotNull(unit, "unit must not be null!");
    return delegate.invokeAny(tasks, timeout, unit);
  }

  @Override
  public void execute(Runnable command) {
    Preconditions.checkNotNull(command, "command must not be null!");
    delegate.execute(command);
  }

  @Override
  public ListenableScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit) {