001    /*
002     * Copyright (C) 2007 Google Inc.
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.collect;
018    
019    import com.google.common.annotations.Beta;
020    import com.google.common.annotations.GwtCompatible;
021    
022    import java.util.NoSuchElementException;
023    import java.util.Queue;
024    
025    /**
026     * A queue which forwards all its method calls to another queue. Subclasses
027     * should override one or more methods to modify the behavior of the backing
028     * queue as desired per the <a
029     * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
030     *
031     * <p><b>Warning:</b> The methods of {@code ForwardingQueue} forward
032     * <b>indiscriminately</b> to the methods of the delegate. For example,
033     * overriding {@link #add} alone <b>will not</b> change the behavior of {@link
034     * #offer} which can lead to unexpected behavior. In this case, you should
035     * override {@code offer} as well, either providing your own implementation, or
036     * delegating to the provided {@code standardOffer} method.
037     *
038     * <p>The {@code standard} methods are not guaranteed to be thread-safe, even
039     * when all of the methods that they depend on are thread-safe.
040     *
041     * @author Mike Bostock
042     * @author Louis Wasserman
043     * @since 2 (imported from Google Collections Library)
044     */
045    @GwtCompatible
046    public abstract class ForwardingQueue<E> extends ForwardingCollection<E>
047        implements Queue<E> {
048    
049      /** Constructor for use by subclasses. */
050      protected ForwardingQueue() {}
051    
052      @Override protected abstract Queue<E> delegate();
053    
054      public boolean offer(E o) {
055        return delegate().offer(o);
056      }
057    
058      public E poll() {
059        return delegate().poll();
060      }
061    
062      public E remove() {
063        return delegate().remove();
064      }
065    
066      public E peek() {
067        return delegate().peek();
068      }
069    
070      public E element() {
071        return delegate().element();
072      }
073    
074      /**
075       * A sensible definition of {@link #offer} in terms of {@link #add}. If you
076       * override {@link #add}, you may wish to override {@link #offer} to forward
077       * to this implementation.
078       * 
079       * @since 7
080       */
081      @Beta protected boolean standardOffer(E e) {
082        try {
083          return add(e);
084        } catch (IllegalStateException caught) {
085          return false;
086        }
087      }
088    
089      /**
090       * A sensible definition of {@link #peek} in terms of {@link #element}. If you
091       * override {@link #element}, you may wish to override {@link #peek} to
092       * forward to this implementation.
093       * 
094       * @since 7
095       */
096      @Beta protected E standardPeek() {
097        try {
098          return element();
099        } catch (NoSuchElementException caught) {
100          return null;
101        }
102      }
103    
104      /**
105       * A sensible definition of {@link #poll} in terms of {@link #remove}. If you
106       * override {@link #remove}, you may wish to override {@link #poll} to forward
107       * to this implementation.
108       * 
109       * @since 7
110       */
111      @Beta protected E standardPoll() {
112        try {
113          return remove();
114        } catch (NoSuchElementException caught) {
115          return null;
116        }
117      }
118    }