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• ForwardingObject.java

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org.glassfish.jersey.internal.guava.ForwardingObject Maven / Gradle / Ivy

/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.glassfish.jersey.internal.guava;

import java.io.Serializable;

/**
 * An abstract base class for implementing the decorator pattern.
 * The {@link #delegate()} method must be overridden to return the instance
 * being decorated.
 * 

 * 
This class does not forward the {@code hashCode} and {@code equals}
 * methods through to the backing object, but relies on {@code Object}'s
 * implementation. This is necessary to preserve the symmetry of {@code equals}.
 * Custom definitions of equality are usually based on an interface, such as
 * {@code Set} or {@code List}, so that the implementation of {@code equals} can
 * cast the object being tested for equality to the custom interface. {@code
 * ForwardingObject} implements no such custom interfaces directly; they
 * are implemented only in subclasses. Therefore, forwarding {@code equals}
 * would break symmetry, as the forwarding object might consider itself equal to
 * the object being tested, but the reverse could not be true. This behavior is
 * consistent with the JDK's collection wrappers, such as
 * {@link java.util.Collections#unmodifiableCollection}. Use an
 * interface-specific subclass of {@code ForwardingObject}, such as {@link
 * ForwardingList}, to preserve equality behavior, or override {@code equals}
 * directly.
 * 

 * 
The {@code toString} method is forwarded to the delegate. Although this
 * class does not implement {@link Serializable}, a serializable subclass may be
 * created since this class has a parameter-less constructor.
 *
 * @author Mike Bostock
 * @since 2.0 (imported from Google Collections Library)
 */
abstract class ForwardingObject {

    /**
     * Constructor for use by subclasses.
     */
    ForwardingObject() {
    }

    /**
     * Returns the backing delegate instance that methods are forwarded to.
     * Abstract subclasses generally override this method with an abstract method
     * that has a more specific return type, such as {@link
     * ForwardingSet#delegate}. Concrete subclasses override this method to supply
     * the instance being decorated.
     */
    protected abstract Object delegate();

    /**
     * Returns the string representation generated by the delegate's
     * {@code toString} method.
     */
    @Override
    public String toString() {
        return delegate().toString();
    }

  /* No equals or hashCode. See class comments for details. */
}