    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 static com.google.common.base.Preconditions.checkNotNull;
    020
    021    import com.google.common.annotations.Beta;
    022    import com.google.common.annotations.GwtCompatible;
    023
    024    import java.io.Serializable;
    025
    026    import javax.annotation.Nullable;
    027
    028    /**
    029     * A strategy for determining whether two instances are considered equivalent. Examples of
    030     * equivalences are the {@link Equivalences#identity() identity equivalence} and {@link
    031     * Equivalences#equals equals equivalence}.
    032     *
    033     * @author Bob Lee
    034     * @author Ben Yu
    035     * @author Gregory Kick
    036     * @since 10.0 (<a href="http://code.google.com/p/guava-libraries/wiki/Compatibility"
    037     *        >mostly source-compatible</a> since 4.0)
    038     */
    039    @Beta
    040    @GwtCompatible
    041    public abstract class Equivalence<T> {
    042      /**
    043       * Constructor for use by subclasses.
    044       */
    045      protected Equivalence() {}
    046
    047      /**
    048       * Returns {@code true} if the given objects are considered equivalent.
    049       *
    050       * <p>The {@code equivalent} method implements an equivalence relation on object references:
    051       *
    052       * <ul>
    053       * <li>It is <i>reflexive</i>: for any reference {@code x}, including null, {@code
    054       *     equivalent(x, x)} returns {@code true}.
    055       * <li>It is <i>symmetric</i>: for any references {@code x} and {@code y}, {@code
    056       *     equivalent(x, y) == equivalent(y, x)}.
    057       * <li>It is <i>transitive</i>: for any references {@code x}, {@code y}, and {@code z}, if
    058       *     {@code equivalent(x, y)} returns {@code true} and {@code equivalent(y, z)} returns {@code
    059       *     true}, then {@code equivalent(x, z)} returns {@code true}.
    060       * <li>It is <i>consistent</i>: for any references {@code x} and {@code y}, multiple invocations
    061       *     of {@code equivalent(x, y)} consistently return {@code true} or consistently return {@code
    062       *     false} (provided that neither {@code x} nor {@code y} is modified).
    063       * </ul>
    064       */
    065      public final boolean equivalent(@Nullable T a, @Nullable T b) {
    066        if (a == b) {
    067          return true;
    068        }
    069        if (a == null || b == null) {
    070          return false;
    071        }
    072        return doEquivalent(a, b);
    073      }
    074
    075      /**
    076       * Returns {@code true} if {@code a} and {@code b} are considered equivalent.
    077       *
    078       * <p>Called by {@link #equivalent}. {@code a} and {@code b} are not the same
    079       * object and are not nulls.
    080       *
    081       * @since 10.0 (previously, subclasses would override equivalent())
    082       */
    083      protected abstract boolean doEquivalent(T a, T b);
    084
    085      /**
    086       * Returns a hash code for {@code t}.
    087       *
    088       * <p>The {@code hash} has the following properties:
    089       * <ul>
    090       * <li>It is <i>consistent</i>: for any reference {@code x}, multiple invocations of
    091       *     {@code hash(x}} consistently return the same value provided {@code x} remains unchanged
    092       *     according to the definition of the equivalence. The hash need not remain consistent from
    093       *     one execution of an application to another execution of the same application.
    094       * <li>It is <i>distributable accross equivalence</i>: for any references {@code x} and {@code y},
    095       *     if {@code equivalent(x, y)}, then {@code hash(x) == hash(y)}. It is <i>not</i> necessary
    096       *     that the hash be distributable accorss <i>inequivalence</i>. If {@code equivalence(x, y)}
    097       *     is false, {@code hash(x) == hash(y)} may still be true.
    098       * <li>{@code hash(null)} is {@code 0}.
    099       * </ul>
    100       */
    101      public final int hash(@Nullable T t) {
    102        if (t == null) {
    103          return 0;
    104        }
    105        return doHash(t);
    106      }
    107
    108      /**
    109       * Returns a hash code for non-null object {@code t}.
    110       *
    111       * <p>Called by {@link #hash}.
    112       *
    113       * @since 10.0 (previously, subclasses would override hash())
    114       */
    115      protected abstract int doHash(T t);
    116
    117      /**
    118       * Returns a new equivalence relation for {@code F} which evaluates equivalence by first applying
    119       * {@code function} to the argument, then evaluating using {@code this}. That is, for any pair of
    120       * non-null objects {@code x} and {@code y}, {@code
    121       * equivalence.onResultOf(function).equivalent(a, b)} is true if and only if {@code
    122       * equivalence.equivalent(function.apply(a), function.apply(b))} is true.
    123       *
    124       * <p>For example: <pre>   {@code
    125       *
    126       *    Equivalence<Person> SAME_AGE = Equivalences.equals().onResultOf(GET_PERSON_AGE);
    127       * }</pre>
    128       *
    129       * <p>{@code function} will never be invoked with a null value.
    130       *
    131       * <p>Note that {@code function} must be consistent according to {@code this} equivalence
    132       * relation. That is, invoking {@link Function#apply} multiple times for a given value must return
    133       * equivalent results.
    134       * For example, {@code Equivalences.identity().onResultOf(Functions.toStringFunction())} is broken
    135       * because it's not guaranteed that {@link Object#toString}) always returns the same string
    136       * instance.
    137       *
    138       * @since 10.0
    139       */
    140      public final <F> Equivalence<F> onResultOf(Function<F, ? extends T> function) {
    141        return new FunctionalEquivalence<F, T>(function, this);
    142      }
    143
    144      /**
    145       * Returns a wrapper of {@code reference} that implements
    146       * {@link Wrapper#equals(Object) Object.equals()} such that
    147       * {@code wrap(this, a).equals(wrap(this, b))} if and only if {@code this.equivalent(a, b)}.
    148       *
    149       * @since 10.0
    150       */
    151      public final <S extends T> Wrapper<S> wrap(@Nullable S reference) {
    152        return new Wrapper<S>(this, reference);
    153      }
    154
    155      /**
    156       * Wraps an object so that {@link #equals(Object)} and {@link #hashCode()} delegate to an
    157       * {@link Equivalence}.
    158       *
    159       * <p>For example, given an {@link Equivalence} for {@link String strings} named {@code equiv}
    160       * that tests equivalence using their lengths:
    161       *
    162       * <pre>   {@code
    163       *   equiv.wrap("a").equals(equiv.wrap("b")) // true
    164       *   equiv.wrap("a").equals(equiv.wrap("hello")) // false
    165       * }</pre>
    166       *
    167       * <p>Note in particular that an equivalence wrapper is never equal to the object it wraps.
    168       *
    169       * <pre>   {@code
    170       *   equiv.wrap(obj).equals(obj) // always false
    171       * }</pre>
    172       *
    173       * @since 10.0
    174       */
    175      @Beta
    176      public static final class Wrapper<T> implements Serializable {
    177        private final Equivalence<? super T> equivalence;
    178        @Nullable private final T reference;
    179
    180        private Wrapper(Equivalence<? super T> equivalence, @Nullable T reference) {
    181          this.equivalence = checkNotNull(equivalence);
    182          this.reference = reference;
    183        }
    184
    185        /** Returns the (possibly null) reference wrapped by this instance. */
    186        @Nullable public T get() {
    187          return reference;
    188        }
    189
    190        /**
    191         * Returns {@code true} if {@link Equivalence#equivalent(Object, Object)} applied to the wrapped
    192         * references is {@code true} and both wrappers use the {@link Object#equals(Object) same}
    193         * equivalence.
    194         */
    195        @Override public boolean equals(@Nullable Object obj) {
    196          if (obj == this) {
    197            return true;
    198          } else if (obj instanceof Wrapper) {
    199            Wrapper<?> that = (Wrapper<?>) obj;
    200            /*
    201             * We cast to Equivalence<Object> here because we can't check the type of the reference held
    202             * by the other wrapper.  But, by checking that the Equivalences are equal, we know that
    203             * whatever type it is, it is assignable to the type handled by this wrapper's equivalence.
    204             */
    205            @SuppressWarnings("unchecked")
    206            Equivalence<Object> equivalence = (Equivalence<Object>) this.equivalence;
    207            return equivalence.equals(that.equivalence)
    208                && equivalence.equivalent(this.reference, that.reference);
    209          } else {
    210            return false;
    211          }
    212        }
    213
    214        /**
    215         * Returns the result of {@link Equivalence#hash(Object)} applied to the the wrapped reference.
    216         */
    217        @Override public int hashCode() {
    218          return equivalence.hash(reference);
    219        }
    220
    221        /**
    222         * Returns a string representation for this equivalence wrapper. The form of this string
    223         * representation is not specified.
    224         */
    225        @Override public String toString() {
    226          return equivalence + ".wrap(" + reference + ")";
    227        }
    228
    229        private static final long serialVersionUID = 0;
    230      }
    231
    232      /**
    233       * Returns an equivalence over iterables based on the equivalence of their elements.  More
    234       * specifically, two iterables are considered equivalent if they both contain the same number of
    235       * elements, and each pair of corresponding elements is equivalent according to
    236       * {@code this}.  Null iterables are equivalent to one another.
    237       *
    238       * <p>Note that this method performs a similar function for equivalences as {@link
    239       * com.google.common.collect.Ordering#lexicographical} does for orderings.
    240       *
    241       * @since 10.0
    242       */
    243      @GwtCompatible(serializable = true)
    244      public final <S extends T> Equivalence<Iterable<S>> pairwise() {
    245        // Ideally, the returned equivalence would support Iterable<? extends T>. However,
    246        // the need for this is so rare that it's not worth making callers deal with the ugly wildcard.
    247        return new PairwiseEquivalence<S>(this);
    248      }
    249
    250      /**
    251       * Returns a predicate that evaluates to true if and only if the input is
    252       * equivalent to {@code target} according to this equivalence relation.
    253       *
    254       * @since 10.0
    255       */
    256      public final Predicate<T> equivalentTo(@Nullable T target) {
    257        return new EquivalentToPredicate<T>(this, target);
    258      }
    259
    260      private static final class EquivalentToPredicate<T> implements Predicate<T>, Serializable {
    261
    262        private final Equivalence<T> equivalence;
    263        @Nullable private final T target;
    264
    265        EquivalentToPredicate(Equivalence<T> equivalence, @Nullable T target) {
    266          this.equivalence = checkNotNull(equivalence);
    267          this.target = target;
    268        }
    269
    270        @Override public boolean apply(@Nullable T input) {
    271          return equivalence.equivalent(input, target);
    272        }
    273
    274        @Override public boolean equals(@Nullable Object obj) {
    275          if (this == obj) {
    276            return true;
    277          }
    278          if (obj instanceof EquivalentToPredicate) {
    279            EquivalentToPredicate<?> that = (EquivalentToPredicate<?>) obj;
    280            return equivalence.equals(that.equivalence)
    281                && Objects.equal(target, that.target);
    282          }
    283          return false;
    284        }
    285
    286        @Override public int hashCode() {
    287          return Objects.hashCode(equivalence, target);
    288        }
    289
    290        @Override public String toString() {
    291          return equivalence + ".equivalentTo(" + target + ")";
    292        }
    293
    294        private static final long serialVersionUID = 0;
    295      }
    296    }