001    /*
002     * Copyright (C) 2009 The Guava Authors
003     *
004     * Licensed under the Apache License, Version 2.0 (the "License");
005     * you may not use this file except in compliance with the License.
006     * You may obtain a copy of the License at
007     *
008     * http://www.apache.org/licenses/LICENSE-2.0
009     *
010     * Unless required by applicable law or agreed to in writing, software
011     * distributed under the License is distributed on an "AS IS" BASIS,
012     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013     * See the License for the specific language governing permissions and
014     * limitations under the License.
015     */
016    
017    package com.google.common.util.concurrent;
018    
019    import com.google.common.annotations.Beta;
020    import com.google.common.base.Preconditions;
021    
022    import java.util.concurrent.Executor;
023    
024    /**
025     * A {@link ListenableFuture} which forwards all its method calls to another
026     * future. Subclasses should override one or more methods to modify the behavior
027     * of the backing future as desired per the <a
028     * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
029     *
030     * <p>Most subclasses can just use {@link SimpleForwardingListenableFuture}.
031     *
032     * @param <V> The result type returned by this Future's {@code get} method
033     * 
034     * @author Shardul Deo
035     * @since 4
036     */
037    @Beta
038    public abstract class ForwardingListenableFuture<V> extends ForwardingFuture<V>
039        implements ListenableFuture<V> {
040    
041      /** Constructor for use by subclasses. */
042      protected ForwardingListenableFuture() {}
043    
044      @Override
045      protected abstract ListenableFuture<V> delegate();
046    
047      @Override
048      public void addListener(Runnable listener, Executor exec) {
049        delegate().addListener(listener, exec);
050      }
051    
052      // TODO(cpovirk): Use Standard Javadoc form for SimpleForwarding*
053      /**
054       * A simplified version of {@link ForwardingListenableFuture} where subclasses
055       * can pass in an already constructed {@link ListenableFuture} 
056       * as the delegate.
057       * 
058       * @since 9
059       */
060      @Beta
061      public abstract static class SimpleForwardingListenableFuture<V>
062          extends ForwardingListenableFuture<V> {
063        private final ListenableFuture<V> delegate;
064    
065        protected SimpleForwardingListenableFuture(ListenableFuture<V> delegate) {
066          this.delegate = Preconditions.checkNotNull(delegate);
067        }
068    
069        @Override
070        protected final ListenableFuture<V> delegate() {
071          return delegate;
072        }
073      }
074    }