@CanIgnoreReturnValue
  public Caffeine<K, V> executor(Executor executor) {
    requireState(this.executor == null, "executor was already set to %s", this.executor);
    this.executor = requireNonNull(executor);
    return this;
  }
@Test
  public void executor() {
    var builder = Caffeine.newBuilder().executor(directExecutor());
    assertThat(builder.getExecutor()).isSameInstanceAs(directExecutor());
    assertThat(builder.build()).isNotNull();
  }