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 javax.annotation.Nullable;
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
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 (present null-friendly behavior)
046       * @since 4 (otherwise)
047       */
048      public static Equivalence<Object> equals() {
049        return Impl.EQUALS;
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 Impl.IDENTITY;
059      }
060    
061      private enum Impl implements Equivalence<Object> {
062        EQUALS {
063          @Override
064          public boolean equivalent(@Nullable Object a, @Nullable Object b) {
065            // TODO(kevinb): use Objects.equal() after testing issue is worked out.
066            return (a == null) ? (b == null) : a.equals(b);
067          }
068    
069          @Override
070          public int hash(@Nullable Object o) {
071            return (o == null) ? 0 : o.hashCode();
072          }
073        },
074        IDENTITY {
075          @Override
076          public boolean equivalent(@Nullable Object a, @Nullable Object b) {
077            return a == b;
078          }
079    
080          @Override
081          public int hash(@Nullable Object o) {
082            return System.identityHashCode(o);
083          }
084        },
085      }
086    
087      /**
088       * Returns an equivalence over iterables based on the equivalence of their elements.  More
089       * specifically, two iterables are considered equivalent if they both contain the same number of
090       * elements, and each pair of corresponding elements is equivalent according to
091       * {@code elementEquivalence}.  Null iterables are equivalent to one another.
092       *
093       * @since 9
094       */
095      @GwtCompatible(serializable = true)
096      public static <T> Equivalence<Iterable<T>> pairwise(Equivalence<? super T> elementEquivalence) {
097        /*
098         * Ideally, the returned equivalence would support {@code Iterable<? extends T>}.  However, the
099         * need for this is so rare that it's not worth making callers deal with the ugly wildcard.
100         */
101        return new PairwiseEquivalence<T>(elementEquivalence);
102      }
103    }