code: /*
   * Copyright (c) 2016-present, RxJava Contributors.
   *
   * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
   * compliance with the License. You may obtain a copy of the License at
   *
   * http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software distributed under the License is
   * distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
   * the License for the specific language governing permissions and limitations under the License.
   */
  
  package io.reactivex.rxjava3.internal.jdk8;
  
  import java.util.concurrent.CompletionStage;
  import java.util.function.BiConsumer;
  
  import io.reactivex.rxjava3.core.*;
  import io.reactivex.rxjava3.disposables.Disposable;
  import io.reactivex.rxjava3.internal.jdk8.FlowableFromCompletionStage.BiConsumerAtomicReference;
  
  /**
   * Wrap a CompletionStage and signal its outcome.
   * @param <T> the element type
   * @since 3.0.0
   */
  public final class SingleFromCompletionStage<T> extends Single<T> {
  
      final CompletionStage<T> stage;
  
      public SingleFromCompletionStage(CompletionStage<T> stage) {
          this.stage = stage;
      }
  
      @Override
      protected void subscribeActual(SingleObserver<? super T> observer) {
          // We need an indirection because one can't detach from a whenComplete
          // and cancellation should not hold onto the stage.
          BiConsumerAtomicReference<T> whenReference = new BiConsumerAtomicReference<>();
          CompletionStageHandler<T> handler = new CompletionStageHandler<>(observer, whenReference);
          whenReference.lazySet(handler);
  
          observer.onSubscribe(handler);
          stage.whenComplete(whenReference);
      }
  
      static final class CompletionStageHandler<T>
      implements Disposable, BiConsumer<T, Throwable> {
  
          final SingleObserver<? super T> downstream;
  
          final BiConsumerAtomicReference<T> whenReference;
  
          CompletionStageHandler(SingleObserver<? super T> downstream, BiConsumerAtomicReference<T> whenReference) {
              this.downstream = downstream;
              this.whenReference = whenReference;
          }
  
          @Override
          public void accept(T item, Throwable error) {
              if (error != null) {
                  downstream.onError(error);
              }
              else if (item != null) {
                  downstream.onSuccess(item);
              } else {
                  downstream.onError(new NullPointerException("The CompletionStage terminated with null."));
              }
          }
  
          @Override
          public void dispose() {
              whenReference.set(null);
          }
  
          @Override
          public boolean isDisposed() {
              return whenReference.get() == null;
          }
      }
  }
  