All Downloads are FREE. Search and download functionalities are using the official Maven repository.

com.google.common.base.Equivalence Maven / Gradle / Ivy

Go to download

This artifact provides a single jar that contains all classes required to use remote EJB and JMS, including all dependencies. It is intended for use by those not using maven, maven users should just import the EJB and JMS BOM's instead (shaded JAR's cause lots of problems with maven, as it is very easy to inadvertently end up with different versions on classes on the class path).

There is a newer version: 34.0.0.Final
Show newest version
/*
 * 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.GwtCompatible;
import com.google.errorprone.annotations.ForOverride;
import java.io.Serializable;
import java.util.function.BiPredicate;
import javax.annotation.CheckForNull;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * A strategy for determining whether two instances are considered equivalent, and for computing
 * hash codes in a manner consistent with that equivalence. Two examples of equivalences are the
 * {@linkplain #identity() identity equivalence} and the {@linkplain #equals "equals" equivalence}.
 *
 * @author Bob Lee
 * @author Ben Yu
 * @author Gregory Kick
 * @since 10.0 (mostly
 *     source-compatible since 4.0)
 */
@GwtCompatible
@ElementTypesAreNonnullByDefault
/*
 * The type parameter is  rather than  so that we can use T in the
 * doEquivalent and doHash methods to indicate that the parameter cannot be null.
 */
public abstract class Equivalence implements BiPredicate<@Nullable T, @Nullable T> {
  /** Constructor for use by subclasses. */
  protected Equivalence() {}

  /**
   * Returns {@code true} if the given objects are considered equivalent.
   *
   * 
This method describes an equivalence relation on object references, meaning that for
   * all references {@code x}, {@code y}, and {@code z} (any of which may be null):
   *
   * 
    
   *   
  • {@code equivalent(x, x)} is true (reflexive property)
   *   
  • {@code equivalent(x, y)} and {@code equivalent(y, x)} each return the same result
   *       (symmetric property)
   *   
  • If {@code equivalent(x, y)} and {@code equivalent(y, z)} are both true, then {@code
   *       equivalent(x, z)} is also true (transitive property)
   * 

   *
   * 
Note that all calls to {@code equivalent(x, y)} are expected to return the same result as
   * long as neither {@code x} nor {@code y} is modified.
   */
  public final boolean equivalent(@CheckForNull T a, @CheckForNull T b) {
    if (a == b) {
      return true;
    }
    if (a == null || b == null) {
      return false;
    }
    return doEquivalent(a, b);
  }

  /**
   * @deprecated Provided only to satisfy the {@link BiPredicate} interface; use {@link #equivalent}
   *     instead.
   * @since 21.0
   */
  @Deprecated
  @Override
  public final boolean test(@CheckForNull T t, @CheckForNull T u) {
    return equivalent(t, u);
  }

  /**
   * Implemented by the user to determine whether {@code a} and {@code b} are considered equivalent,
   * subject to the requirements specified in {@link #equivalent}.
   *
   * 
This method should not be called except by {@link #equivalent}. When {@link #equivalent}
   * calls this method, {@code a} and {@code b} are guaranteed to be distinct, non-null instances.
   *
   * @since 10.0 (previously, subclasses would override equivalent())
   */
  @ForOverride
  protected abstract boolean doEquivalent(T a, T b);

  /**
   * Returns a hash code for {@code t}.
   *
   * 
The {@code hash} has the following properties:
   *
   * 
    
   *   
  • It is consistent: 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.
   *   
  • It is distributable across equivalence: for any references {@code x} and {@code
   *       y}, if {@code equivalent(x, y)}, then {@code hash(x) == hash(y)}. It is not
   *       necessary that the hash be distributable across inequivalence. If {@code
   *       equivalence(x, y)} is false, {@code hash(x) == hash(y)} may still be true.
   *   
  • {@code hash(null)} is {@code 0}.
   * 

   */
  public final int hash(@CheckForNull T t) {
    if (t == null) {
      return 0;
    }
    return doHash(t);
  }

  /**
   * Implemented by the user to return a hash code for {@code t}, subject to the requirements
   * specified in {@link #hash}.
   *
   * 
This method should not be called except by {@link #hash}. When {@link #hash} calls this
   * method, {@code t} is guaranteed to be non-null.
   *
   * @since 10.0 (previously, subclasses would override hash())
   */
  @ForOverride
  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(@ParametricNullness S reference) {
    /*
     * I'm pretty sure that this warning "makes sense" but doesn't indicate a real problem.
     *
     * Why it "makes sense": If we pass a `@Nullable Foo`, then we should also pass an
     * `Equivalence`. And there's no such thing because Equivalence doesn't
     * permit nullable type arguments.
     *
     * Why there's no real problem: Every Equivalence can handle null.
     *
     * We could work around this by giving Wrapper 2 type parameters. In the terms of this method,
     * that would be both the T parameter (from the class) and the S parameter (from this method).
     * However, such a change would be source-incompatible. (Plus, there's no reason for the S
     * parameter from the user's perspective, so it would be a wart.)
     *
     * We could probably also work around this by making Wrapper non-final and putting the
     * implementation into a subclass with those 2 type parameters. But we like `final`, if only to
     * deter users from using mocking frameworks to construct instances. (And could also complicate
     * serialization, which is discussed more in the next paragraph.)
     *
     * We could probably also work around this by having Wrapper accept an instance of a new
     * WrapperGuts class, which would then be the class that would declare the 2 type parameters.
     * But that would break deserialization of previously serialized Wrapper instances. And while we
     * specifically say not to rely on serialization across Guava versions, users sometimes do. So
     * we'd rather not break them without a good enough reason.
     *
     * (We could work around the serialization problem by writing custom serialization code. But
     * even that helps only the case of serializing with an old version and deserializing with a
     * new, not vice versa -- unless we introduce WrapperGuts and the logic to handle it today, wait
     * until "everyone" has picked up a version of Guava with that code, and *then* change to use
     * WrapperGuts.)
     *
     * Anyway, a suppression isn't really a big deal. But I have tried to do some due diligence on
     * avoiding it :)
     */
    @SuppressWarnings("nullness")
    Wrapper w = new Wrapper<>(this, reference);
    return w;
  }

  /**
   * 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;
    @ParametricNullness private final T reference;

    private Wrapper(Equivalence equivalence, @ParametricNullness T reference) {
      this.equivalence = checkNotNull(equivalence);
      this.reference = reference;
    }

    /** Returns the (possibly null) reference wrapped by this instance. */
    @ParametricNullness
    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(@CheckForNull 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
   */
  public final Predicate<@Nullable T> equivalentTo(@CheckForNull T target) {
    return new EquivalentToPredicate(this, target);
  }

  private static final class EquivalentToPredicate
      implements Predicate<@Nullable T>, Serializable {

    private final Equivalence equivalence;
    @CheckForNull private final T target;

    EquivalentToPredicate(Equivalence equivalence, @CheckForNull T target) {
      this.equivalence = checkNotNull(equivalence);
      this.target = target;
    }

    @Override
    public boolean apply(@CheckForNull T input) {
      return equivalence.equivalent(input, target);
    }

    @Override
    public boolean equals(@CheckForNull 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
    protected 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;
  }
}
    

    

    

            

    

            



    
    






    
© 2015 - 2024 Weber Informatics LLC | Privacy Policy