Package com.google.common.collect

Class ForwardingObject

  • Direct Known Subclasses:
    ForwardingCache, ForwardingCollection, ForwardingExecutorService, ForwardingFuture, ForwardingIterator, ForwardingMap, ForwardingMapEntry, ForwardingMultimap, ForwardingTable
    @GwtCompatible
public abstract class ForwardingObject
extends java.lang.Object
    An abstract base class for implementing the decorator pattern. The delegate() method must be overridden to return the instance being decorated.

    This class does not forward the hashCode and equals methods through to the backing object, but relies on Object's implementation. This is necessary to preserve the symmetry of equals. Custom definitions of equality are usually based on an interface, such as Set or List, so that the implementation of equals can cast the object being tested for equality to the custom interface. ForwardingObject implements no such custom interfaces directly; they are implemented only in subclasses. Therefore, forwarding 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 Collections.unmodifiableCollection(java.util.Collection<? extends T>). Use an interface-specific subclass of ForwardingObject, such as ForwardingList, to preserve equality behavior, or override equals directly.

    The toString method is forwarded to the delegate. Although this class does not implement Serializable, a serializable subclass may be created since this class has a parameter-less constructor.

    Since:
    2.0
    Author:
    Mike Bostock

    • Constructor Summary

      Constructors 
      Modifier Constructor Description
      protected ForwardingObject()
      Constructor for use by subclasses.

    • Method Summary

      All Methods Instance Methods Abstract Methods Concrete Methods 
      Modifier and Type Method Description
      protected abstract java.lang.Object delegate()
      Returns the backing delegate instance that methods are forwarded to.
      java.lang.String toString()
      Returns the string representation generated by the delegate's toString method.

      • Methods inherited from class java.lang.Object

        clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

    • Constructor Detail

      • ForwardingObject

        protected ForwardingObject()
        Constructor for use by subclasses.

    • Method Detail

      • delegate

        protected abstract java.lang.Object delegate()
        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 ForwardingSet.delegate(). Concrete subclasses override this method to supply the instance being decorated.

      • toString

        public java.lang.String toString()
        Returns the string representation generated by the delegate's toString method.
        Overrides:
        toString in class java.lang.Object