001    /*
002     * Copyright (C) 2007 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.collect;
018    
019    import com.google.common.annotations.GwtCompatible;
020    
021    import java.io.Serializable;
022    
023    /**
024     * An abstract base class for implementing the <a
025     * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
026     * The {@link #delegate()} method must be overridden to return the instance
027     * being decorated.
028     *
029     * <p>This class does <i>not</i> forward the {@code hashCode} and {@code equals}
030     * methods through to the backing object, but relies on {@code Object}'s
031     * implementation. This is necessary to preserve the symmetry of {@code equals}.
032     * Custom definitions of equality are usually based on an interface, such as
033     * {@code Set} or {@code List}, so that the implementation of {@code equals} can
034     * cast the object being tested for equality to the custom interface. {@code
035     * ForwardingObject} implements no such custom interfaces directly; they
036     * are implemented only in subclasses. Therefore, forwarding {@code equals}
037     * would break symmetry, as the forwarding object might consider itself equal to
038     * the object being tested, but the reverse could not be true. This behavior is
039     * consistent with the JDK's collection wrappers, such as
040     * {@link java.util.Collections#unmodifiableCollection}. Use an
041     * interface-specific subclass of {@code ForwardingObject}, such as {@link
042     * ForwardingList}, to preserve equality behavior, or override {@code equals}
043     * directly.
044     *
045     * <p>The {@code toString} method is forwarded to the delegate. Although this
046     * class does not implement {@link Serializable}, a serializable subclass may be
047     * created since this class has a parameter-less constructor.
048     *
049     * @author Mike Bostock
050     * @since 2.0 (imported from Google Collections Library)
051     */
052    @GwtCompatible
053    public abstract class ForwardingObject {
054    
055      /** Constructor for use by subclasses. */
056      protected ForwardingObject() {}
057    
058      /**
059       * Returns the backing delegate instance that methods are forwarded to.
060       * Abstract subclasses generally override this method with an abstract method
061       * that has a more specific return type, such as {@link
062       * ForwardingSet#delegate}. Concrete subclasses override this method to supply
063       * the instance being decorated.
064       */
065      protected abstract Object delegate();
066    
067      /**
068       * Returns the string representation generated by the delegate's
069       * {@code toString} method.
070       */
071      @Override public String toString() {
072        return delegate().toString();
073      }
074    
075      /* No equals or hashCode. See class comments for details. */
076    }