| /* |
| * Copyright (C) 2010 The Guava Authors |
| * |
| * Licensed under the Apache License, Version 2.0 (the "License"); |
| * you may not use this file except in compliance with the License. |
| * You may obtain a copy of the License at |
| * |
| * http://www.apache.org/licenses/LICENSE-2.0 |
| * |
| * Unless required by applicable law or agreed to in writing, software |
| * distributed under the License is distributed on an "AS IS" BASIS, |
| * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| * See the License for the specific language governing permissions and |
| * limitations under the License. |
| */ |
| |
| package com.google.common.base; |
| |
| import static com.google.common.base.Preconditions.checkNotNull; |
| |
| import com.google.common.annotations.Beta; |
| import com.google.common.annotations.GwtCompatible; |
| |
| import java.io.Serializable; |
| |
| import javax.annotation.Nullable; |
| |
| /** |
| * A strategy for determining whether two instances are considered equivalent. Examples of |
| * equivalences are the {@linkplain #identity() identity equivalence} and {@linkplain #equals equals |
| * equivalence}. |
| * |
| * @author Bob Lee |
| * @author Ben Yu |
| * @author Gregory Kick |
| * @since 10.0 (<a href="http://code.google.com/p/guava-libraries/wiki/Compatibility" |
| * >mostly source-compatible</a> since 4.0) |
| */ |
| @GwtCompatible |
| public abstract class Equivalence<T> { |
| /** |
| * Constructor for use by subclasses. |
| */ |
| protected Equivalence() {} |
| |
| /** |
| * Returns {@code true} if the given objects are considered equivalent. |
| * |
| * <p>The {@code equivalent} method implements an equivalence relation on object references: |
| * |
| * <ul> |
| * <li>It is <i>reflexive</i>: for any reference {@code x}, including null, {@code |
| * equivalent(x, x)} returns {@code true}. |
| * <li>It is <i>symmetric</i>: for any references {@code x} and {@code y}, {@code |
| * equivalent(x, y) == equivalent(y, x)}. |
| * <li>It is <i>transitive</i>: for any references {@code x}, {@code y}, and {@code z}, if |
| * {@code equivalent(x, y)} returns {@code true} and {@code equivalent(y, z)} returns {@code |
| * true}, then {@code equivalent(x, z)} returns {@code true}. |
| * <li>It is <i>consistent</i>: for any references {@code x} and {@code y}, multiple invocations |
| * of {@code equivalent(x, y)} consistently return {@code true} or consistently return {@code |
| * false} (provided that neither {@code x} nor {@code y} is modified). |
| * </ul> |
| */ |
| public final boolean equivalent(@Nullable T a, @Nullable T b) { |
| if (a == b) { |
| return true; |
| } |
| if (a == null || b == null) { |
| return false; |
| } |
| return doEquivalent(a, b); |
| } |
| |
| /** |
| * Returns {@code true} if {@code a} and {@code b} are considered equivalent. |
| * |
| * <p>Called by {@link #equivalent}. {@code a} and {@code b} are not the same |
| * object and are not nulls. |
| * |
| * @since 10.0 (previously, subclasses would override equivalent()) |
| */ |
| protected abstract boolean doEquivalent(T a, T b); |
| |
| /** |
| * Returns a hash code for {@code t}. |
| * |
| * <p>The {@code hash} has the following properties: |
| * <ul> |
| * <li>It is <i>consistent</i>: for any reference {@code x}, multiple invocations of |
| * {@code hash(x}} consistently return the same value provided {@code x} remains unchanged |
| * according to the definition of the equivalence. The hash need not remain consistent from |
| * one execution of an application to another execution of the same application. |
| * <li>It is <i>distributable across equivalence</i>: for any references {@code x} and {@code y}, |
| * if {@code equivalent(x, y)}, then {@code hash(x) == hash(y)}. It is <i>not</i> necessary |
| * that the hash be distributable across <i>inequivalence</i>. If {@code equivalence(x, y)} |
| * is false, {@code hash(x) == hash(y)} may still be true. |
| * <li>{@code hash(null)} is {@code 0}. |
| * </ul> |
| */ |
| public final int hash(@Nullable T t) { |
| if (t == null) { |
| return 0; |
| } |
| return doHash(t); |
| } |
| |
| /** |
| * Returns a hash code for non-null object {@code t}. |
| * |
| * <p>Called by {@link #hash}. |
| * |
| * @since 10.0 (previously, subclasses would override hash()) |
| */ |
| protected abstract int doHash(T t); |
| |
| /** |
| * Returns a new equivalence relation for {@code F} which evaluates equivalence by first applying |
| * {@code function} to the argument, then evaluating using {@code this}. That is, for any pair of |
| * non-null objects {@code x} and {@code y}, {@code |
| * equivalence.onResultOf(function).equivalent(a, b)} is true if and only if {@code |
| * equivalence.equivalent(function.apply(a), function.apply(b))} is true. |
| * |
| * <p>For example: |
| * |
| * <pre> {@code |
| * Equivalence<Person> SAME_AGE = Equivalence.equals().onResultOf(GET_PERSON_AGE);}</pre> |
| * |
| * <p>{@code function} will never be invoked with a null value. |
| * |
| * <p>Note that {@code function} must be consistent according to {@code this} equivalence |
| * relation. That is, invoking {@link Function#apply} multiple times for a given value must return |
| * equivalent results. |
| * For example, {@code Equivalence.identity().onResultOf(Functions.toStringFunction())} is broken |
| * because it's not guaranteed that {@link Object#toString}) always returns the same string |
| * instance. |
| * |
| * @since 10.0 |
| */ |
| public final <F> Equivalence<F> onResultOf(Function<F, ? extends T> function) { |
| return new FunctionalEquivalence<F, T>(function, this); |
| } |
| |
| /** |
| * Returns a wrapper of {@code reference} that implements |
| * {@link Wrapper#equals(Object) Object.equals()} such that |
| * {@code wrap(a).equals(wrap(b))} if and only if {@code equivalent(a, b)}. |
| * |
| * @since 10.0 |
| */ |
| public final <S extends T> Wrapper<S> wrap(@Nullable S reference) { |
| return new Wrapper<S>(this, reference); |
| } |
| |
| /** |
| * Wraps an object so that {@link #equals(Object)} and {@link #hashCode()} delegate to an |
| * {@link Equivalence}. |
| * |
| * <p>For example, given an {@link Equivalence} for {@link String strings} named {@code equiv} |
| * that tests equivalence using their lengths: |
| * |
| * <pre> {@code |
| * equiv.wrap("a").equals(equiv.wrap("b")) // true |
| * equiv.wrap("a").equals(equiv.wrap("hello")) // false}</pre> |
| * |
| * <p>Note in particular that an equivalence wrapper is never equal to the object it wraps. |
| * |
| * <pre> {@code |
| * equiv.wrap(obj).equals(obj) // always false}</pre> |
| * |
| * @since 10.0 |
| */ |
| public static final class Wrapper<T> implements Serializable { |
| private final Equivalence<? super T> equivalence; |
| @Nullable private final T reference; |
| |
| private Wrapper(Equivalence<? super T> equivalence, @Nullable T reference) { |
| this.equivalence = checkNotNull(equivalence); |
| this.reference = reference; |
| } |
| |
| /** Returns the (possibly null) reference wrapped by this instance. */ |
| @Nullable public T get() { |
| return reference; |
| } |
| |
| /** |
| * Returns {@code true} if {@link Equivalence#equivalent(Object, Object)} applied to the wrapped |
| * references is {@code true} and both wrappers use the {@link Object#equals(Object) same} |
| * equivalence. |
| */ |
| @Override public boolean equals(@Nullable Object obj) { |
| if (obj == this) { |
| return true; |
| } |
| if (obj instanceof Wrapper) { |
| Wrapper<?> that = (Wrapper<?>) obj; // note: not necessarily a Wrapper<T> |
| |
| if (this.equivalence.equals(that.equivalence)) { |
| /* |
| * We'll accept that as sufficient "proof" that either equivalence should be able to |
| * handle either reference, so it's safe to circumvent compile-time type checking. |
| */ |
| @SuppressWarnings("unchecked") |
| Equivalence<Object> equivalence = (Equivalence<Object>) this.equivalence; |
| return equivalence.equivalent(this.reference, that.reference); |
| } |
| } |
| return false; |
| } |
| |
| /** |
| * Returns the result of {@link Equivalence#hash(Object)} applied to the wrapped reference. |
| */ |
| @Override public int hashCode() { |
| return equivalence.hash(reference); |
| } |
| |
| /** |
| * Returns a string representation for this equivalence wrapper. The form of this string |
| * representation is not specified. |
| */ |
| @Override public String toString() { |
| return equivalence + ".wrap(" + reference + ")"; |
| } |
| |
| private static final long serialVersionUID = 0; |
| } |
| |
| /** |
| * Returns an equivalence over iterables based on the equivalence of their elements. More |
| * specifically, two iterables are considered equivalent if they both contain the same number of |
| * elements, and each pair of corresponding elements is equivalent according to |
| * {@code this}. Null iterables are equivalent to one another. |
| * |
| * <p>Note that this method performs a similar function for equivalences as {@link |
| * com.google.common.collect.Ordering#lexicographical} does for orderings. |
| * |
| * @since 10.0 |
| */ |
| @GwtCompatible(serializable = true) |
| public final <S extends T> Equivalence<Iterable<S>> pairwise() { |
| // Ideally, the returned equivalence would support Iterable<? extends T>. However, |
| // the need for this is so rare that it's not worth making callers deal with the ugly wildcard. |
| return new PairwiseEquivalence<S>(this); |
| } |
| |
| /** |
| * Returns a predicate that evaluates to true if and only if the input is |
| * equivalent to {@code target} according to this equivalence relation. |
| * |
| * @since 10.0 |
| */ |
| @Beta |
| public final Predicate<T> equivalentTo(@Nullable T target) { |
| return new EquivalentToPredicate<T>(this, target); |
| } |
| |
| private static final class EquivalentToPredicate<T> implements Predicate<T>, Serializable { |
| |
| private final Equivalence<T> equivalence; |
| @Nullable private final T target; |
| |
| EquivalentToPredicate(Equivalence<T> equivalence, @Nullable T target) { |
| this.equivalence = checkNotNull(equivalence); |
| this.target = target; |
| } |
| |
| @Override public boolean apply(@Nullable T input) { |
| return equivalence.equivalent(input, target); |
| } |
| |
| @Override public boolean equals(@Nullable Object obj) { |
| if (this == obj) { |
| return true; |
| } |
| if (obj instanceof EquivalentToPredicate) { |
| EquivalentToPredicate<?> that = (EquivalentToPredicate<?>) obj; |
| return equivalence.equals(that.equivalence) |
| && Objects.equal(target, that.target); |
| } |
| return false; |
| } |
| |
| @Override public int hashCode() { |
| return Objects.hashCode(equivalence, target); |
| } |
| |
| @Override public String toString() { |
| return equivalence + ".equivalentTo(" + target + ")"; |
| } |
| |
| private static final long serialVersionUID = 0; |
| } |
| |
| /** |
| * Returns an equivalence that delegates to {@link Object#equals} and {@link Object#hashCode}. |
| * {@link Equivalence#equivalent} returns {@code true} if both values are null, or if neither |
| * value is null and {@link Object#equals} returns {@code true}. {@link Equivalence#hash} returns |
| * {@code 0} if passed a null value. |
| * |
| * @since 13.0 |
| * @since 8.0 (in Equivalences with null-friendly behavior) |
| * @since 4.0 (in Equivalences) |
| */ |
| public static Equivalence<Object> equals() { |
| return Equals.INSTANCE; |
| } |
| |
| /** |
| * Returns an equivalence that uses {@code ==} to compare values and {@link |
| * System#identityHashCode(Object)} to compute the hash code. {@link Equivalence#equivalent} |
| * returns {@code true} if {@code a == b}, including in the case that a and b are both null. |
| * |
| * @since 13.0 |
| * @since 4.0 (in Equivalences) |
| */ |
| public static Equivalence<Object> identity() { |
| return Identity.INSTANCE; |
| } |
| |
| static final class Equals extends Equivalence<Object> |
| implements Serializable { |
| |
| static final Equals INSTANCE = new Equals(); |
| |
| @Override protected boolean doEquivalent(Object a, Object b) { |
| return a.equals(b); |
| } |
| @Override public int doHash(Object o) { |
| return o.hashCode(); |
| } |
| |
| private Object readResolve() { |
| return INSTANCE; |
| } |
| private static final long serialVersionUID = 1; |
| } |
| |
| static final class Identity extends Equivalence<Object> |
| implements Serializable { |
| |
| static final Identity INSTANCE = new Identity(); |
| |
| @Override protected boolean doEquivalent(Object a, Object b) { |
| return false; |
| } |
| |
| @Override protected int doHash(Object o) { |
| return System.identityHashCode(o); |
| } |
| |
| private Object readResolve() { |
| return INSTANCE; |
| } |
| private static final long serialVersionUID = 1; |
| } |
| } |