    001    /*
    002     * Copyright (C) 2010 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.base;
    018
    019    import com.google.common.annotations.Beta;
    020    import com.google.common.annotations.GwtCompatible;
    021
    022    import java.io.Serializable;
    023
    024    /**
    025     * Contains static factory methods for creating {@code Equivalence} instances.
    026     *
    027     * <p>All methods return serializable instances.
    028     *
    029     * @author Bob Lee
    030     * @author Kurt Alfred Kluever
    031     * @author Gregory Kick
    032     * @since 4.0
    033     */
    034    @Beta
    035    @GwtCompatible
    036    public final class Equivalences {
    037      private Equivalences() {}
    038
    039      /**
    040       * Returns an equivalence that delegates to {@link Object#equals} and {@link Object#hashCode}.
    041       * {@link Equivalence#equivalent} returns {@code true} if both values are null, or if neither
    042       * value is null and {@link Object#equals} returns {@code true}. {@link Equivalence#hash} returns
    043       * {@code 0} if passed a null value.
    044       *
    045       * @since 8.0 (present null-friendly behavior)
    046       * @since 4.0 (otherwise)
    047       */
    048      public static Equivalence<Object> equals() {
    049        return Equals.INSTANCE;
    050      }
    051
    052      /**
    053       * Returns an equivalence that uses {@code ==} to compare values and {@link
    054       * System#identityHashCode(Object)} to compute the hash code.  {@link Equivalence#equivalent}
    055       * returns {@code true} if {@code a == b}, including in the case that a and b are both null.
    056       */
    057      public static Equivalence<Object> identity() {
    058        return Identity.INSTANCE;
    059      }
    060
    061      /**
    062       * Returns an equivalence over iterables based on the equivalence of their elements.  More
    063       * specifically, two iterables are considered equivalent if they both contain the same number of
    064       * elements, and each pair of corresponding elements is equivalent according to
    065       * {@code elementEquivalence}.  Null iterables are equivalent to one another.
    066       *
    067       * @since 9.0
    068       * @deprecated use {@link Equivalence#pairwise}, which behaves exactly the same. <b>This method is
    069       *     scheduled for deletion from Guava in Guava release 11.0.</b>
    070       */
    071      @Deprecated
    072      @GwtCompatible(serializable = true)
    073      public
    074      static <T> Equivalence<Iterable<T>> pairwise(Equivalence<? super T> elementEquivalence) {
    075        /*
    076         * Ideally, the returned equivalence would support {@code Iterable<? extends T>}.  However, the
    077         * need for this is so rare that it's not worth making callers deal with the ugly wildcard.
    078         */
    079        return new PairwiseEquivalence<T>(elementEquivalence);
    080      }
    081
    082      private static final class Equals extends Equivalence<Object>
    083          implements Serializable {
    084
    085        static final Equals INSTANCE = new Equals();
    086
    087        @Override protected boolean doEquivalent(Object a, Object b) {
    088          return a.equals(b);
    089        }
    090        @Override public int doHash(Object o) {
    091          return o.hashCode();
    092        }
    093
    094        private Object readResolve() {
    095          return INSTANCE;
    096        }
    097        private static final long serialVersionUID = 1;
    098      }
    099
    100      private static final class Identity extends Equivalence<Object>
    101          implements Serializable {
    102
    103        static final Identity INSTANCE = new Identity();
    104
    105        @Override protected boolean doEquivalent(Object a, Object b) {
    106          return false;
    107        }
    108
    109        @Override protected int doHash(Object o) {
    110          return System.identityHashCode(o);
    111        }
    112
    113        private Object readResolve() {
    114          return INSTANCE;
    115        }
    116        private static final long serialVersionUID = 1;
    117      }
    118    }