/* * 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 java.io.Serializable; import javax.annotation.Nullable; import com.google.common.annotations.Beta; import com.google.common.annotations.GwtCompatible; /** * 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 * (mostly source-compatible since 4.0) */ @GwtCompatible public abstract class Equivalence { /** * Constructor for use by subclasses. */ protected Equivalence() { } /** * Returns {@code true} if the given objects are considered equivalent. * *

* The {@code equivalent} method implements an equivalence relation on object * references: * *

*/ 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. * *

* 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}. * *

* The {@code hash} has the following properties: *

*/ 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}. * *

* 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. * *

* For example: * *

	 * {
	 * 	@code
	 * 	Equivalence SAME_AGE = Equivalence.equals().onResultOf(GET_PERSON_AGE);
	 * }
	 * 
* *

* {@code function} will never be invoked with a null value. * *

* 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 Equivalence onResultOf(Function function) { return new FunctionalEquivalence(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 Wrapper wrap(@Nullable S reference) { return new Wrapper(this, reference); } /** * Wraps an object so that {@link #equals(Object)} and {@link #hashCode()} * delegate to an {@link Equivalence}. * *

* For example, given an {@link Equivalence} for {@link String strings} named * {@code equiv} that tests equivalence using their lengths: * *

	 *    {@code
	 *   equiv.wrap("a").equals(equiv.wrap("b")) // true
	 *   equiv.wrap("a").equals(equiv.wrap("hello")) // false}
	 * 
* *

* Note in particular that an equivalence wrapper is never equal to the object * it wraps. * *

	 *    {@code
	 *   equiv.wrap(obj).equals(obj) // always false}
	 * 
* * @since 10.0 */ public static final class Wrapper implements Serializable { private final Equivalence equivalence; @Nullable private final T reference; private Wrapper(Equivalence 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 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 equivalence = (Equivalence) 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. * *

* 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 Equivalence> pairwise() { // Ideally, the returned equivalence would support Iterable. // However, // the need for this is so rare that it's not worth making callers deal with the // ugly wildcard. return new PairwiseEquivalence(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 equivalentTo(@Nullable T target) { return new EquivalentToPredicate(this, target); } private static final class EquivalentToPredicate implements Predicate, Serializable { private final Equivalence equivalence; @Nullable private final T target; EquivalentToPredicate(Equivalence 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 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 identity() { return Identity.INSTANCE; } static final class Equals extends Equivalence 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 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; } }