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.base.Preconditions;
020    
021    import java.util.concurrent.Executor;
022    
023    /**
024     * A {@link ListenableFuture} which forwards all its method calls to another
025     * future. Subclasses should override one or more methods to modify the behavior
026     * of the backing future as desired per the <a
027     * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
028     *
029     * <p>Most subclasses can just use {@link SimpleForwardingListenableFuture}.
030     *
031     * @param <V> The result type returned by this Future's {@code get} method
032     * 
033     * @author Shardul Deo
034     * @since 4.0
035     */
036    public abstract class ForwardingListenableFuture<V> extends ForwardingFuture<V>
037        implements ListenableFuture<V> {
038    
039      /** Constructor for use by subclasses. */
040      protected ForwardingListenableFuture() {}
041    
042      @Override
043      protected abstract ListenableFuture<V> delegate();
044    
045      @Override
046      public void addListener(Runnable listener, Executor exec) {
047        delegate().addListener(listener, exec);
048      }
049    
050      /*
051       * TODO(cpovirk): Use standard Javadoc form for SimpleForwarding* class and
052       * constructor
053       */
054      /**
055       * A simplified version of {@link ForwardingListenableFuture} where subclasses
056       * can pass in an already constructed {@link ListenableFuture} 
057       * as the delegate.
058       * 
059       * @since 9.0
060       */
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    }