001/*
002 * Copyright (C) 2009 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015package com.google.common.util.concurrent;
016
017import com.google.common.annotations.GwtCompatible;
018import com.google.common.base.Preconditions;
019import java.util.concurrent.Executor;
020import org.jspecify.annotations.Nullable;
021
022/**
023 * A {@link ListenableFuture} which forwards all its method calls to another future. Subclasses
024 * should override one or more methods to modify the behavior of the backing future as desired per
025 * the <a href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
026 *
027 * <p><b>Warning:</b> The methods of {@code ForwardingListenableFuture} forward
028 * <b>indiscriminately</b> to the methods of the delegate. For example, overriding {@link #get(long,
029 * TimeUnit)} alone <b>will not</b> change the behavior of {@link #get()}, which can lead to
030 * unexpected behavior. In this case, you should override {@code get()} as well.
031 *
032 * <p><b>{@code default} method warning:</b> This class does <i>not</i> forward calls to {@code
033 * default} methods. Instead, it inherits their default implementations. When those implementations
034 * invoke methods, they invoke methods on the {@code ForwardingListenableFuture}.
035 *
036 * <p>Most subclasses can just use {@link SimpleForwardingListenableFuture}.
037 *
038 * @author Shardul Deo
039 * @since 4.0
040 */
041@GwtCompatible
042public abstract class ForwardingListenableFuture<V extends @Nullable Object>
043    extends ForwardingFuture<V> implements ListenableFuture<V> {
044
045  /** Constructor for use by subclasses. */
046  protected ForwardingListenableFuture() {}
047
048  @Override
049  protected abstract ListenableFuture<? extends V> delegate();
050
051  @Override
052  public void addListener(Runnable listener, Executor exec) {
053    delegate().addListener(listener, exec);
054  }
055
056  // TODO(cpovirk): Use standard Javadoc form for SimpleForwarding* class and constructor
057  /**
058   * A simplified version of {@link ForwardingListenableFuture} where subclasses can pass in an
059   * already constructed {@link ListenableFuture} as the delegate.
060   *
061   * @since 9.0
062   */
063  public abstract static class SimpleForwardingListenableFuture<V extends @Nullable Object>
064      extends ForwardingListenableFuture<V> {
065    private final ListenableFuture<V> delegate;
066
067    /** Constructor for use by subclasses. */
068    protected SimpleForwardingListenableFuture(ListenableFuture<V> delegate) {
069      this.delegate = Preconditions.checkNotNull(delegate);
070    }
071
072    @Override
073    protected final ListenableFuture<V> delegate() {
074      return delegate;
075    }
076  }
077}