}
  }

  private static final class Listener {
    final Runnable command;
    final Executor executor;

    Listener(Runnable command, Executor executor) {
      this.command = checkNotNull(command);
      this.executor = checkNotNull(executor);
    }

    void execute() {
      try {
        executor.execute(command);
      } catch (RuntimeException e) {
        log.log(
            Level.SEVERE,

    @CheckForNull // null only for TOMBSTONE
    final Executor executor;

    // writes to next are made visible by subsequent CAS's on the listeners field
    @CheckForNull Listener next;

    Listener(Runnable task, Executor executor) {
      this.task = task;
      this.executor = executor;
    }

    Listener() {
      this.task = null;
      this.executor = null;
    }
  }