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
017package com.google.common.base;
018
019import com.google.common.annotations.Beta;
020import com.google.common.annotations.GwtCompatible;
021
022import 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
036public 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}