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Guava testlib is a set of java classes used for more convenient unit testing - particularly to assist the tests for Guava itself.
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/*
 * Copyright (C) 2007 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.testing;

import static com.google.common.base.Preconditions.checkNotNull;
import static junit.framework.Assert.assertEquals;
import static junit.framework.Assert.assertTrue;

import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Equivalence;
import com.google.common.collect.Iterables;
import com.google.common.collect.Lists;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.util.ArrayList;
import java.util.List;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * Tester for equals() and hashCode() methods of a class.
 *
 * The simplest use case is:
 *
 * 
 * new EqualsTester().addEqualityGroup(foo).testEquals();
 * 
 *
 * This tests {@code foo.equals(foo)}, {@code foo.equals(null)}, and a few other operations.
 *
 * 
For more extensive testing, add multiple equality groups. Each group should contain objects
 * that are equal to each other but unequal to the objects in any other group. For example:
 *
 * 
 * new EqualsTester()
 *     .addEqualityGroup(new User("page"), new User("page"))
 *     .addEqualityGroup(new User("sergey"))
 *     .testEquals();
 * 
 *
 * This tests:
 *
 * 

 *   comparing each object against itself returns true
 *   
comparing each object against null returns false
 *   
comparing each object against an instance of an incompatible class returns false
 *   
comparing each pair of objects within the same equality group returns true
 *   
comparing each pair of objects from different equality groups returns false
 *   
the hash codes of any two equal objects are equal
 * 
 *
 * When a test fails, the error message labels the objects involved in the failed comparison as
 * follows:
 *
 * 

 *   "{@code [group }i{@code , item }j{@code ]}" refers to the
 *       j^th item in the i^th equality group, where both equality
 *       groups and the items within equality groups are numbered starting from 1. When either a
 *       constructor argument or an equal object is provided, that becomes group 1.
 * 
 *
 * @author Jim McMaster
 * @author Jige Yu
 * @since 10.0
 */
@GwtCompatible
@ElementTypesAreNonnullByDefault
public final class EqualsTester {
  private static final int REPETITIONS = 3;

  private final List> equalityGroups = Lists.newArrayList();
  private final RelationshipTester.ItemReporter itemReporter;

  /** Constructs an empty EqualsTester instance */
  public EqualsTester() {
    this(new RelationshipTester.ItemReporter());
  }

  EqualsTester(RelationshipTester.ItemReporter itemReporter) {
    this.itemReporter = checkNotNull(itemReporter);
  }

  /**
   * Adds {@code equalityGroup} with objects that are supposed to be equal to each other and not
   * equal to any other equality groups added to this tester.
   *
   * The {@code @Nullable} annotations on the {@code equalityGroup} parameter imply that the
   * objects, and the array itself, can be null. That is for programmer convenience, when the
   * objects come from factory methods that are themselves {@code @Nullable}. In reality neither the
   * array nor its contents can be null, but it is not useful to force the use of {@code
   * requireNonNull} or the like just to assert that.
   *
   * 
{@code EqualsTester} will always check that every object it is given returns false from
   * {@code equals(null)}, so it is neither useful nor allowed to include a null value in any
   * equality group.
   */
  @CanIgnoreReturnValue
  public EqualsTester addEqualityGroup(@Nullable Object @Nullable ... equalityGroup) {
    checkNotNull(equalityGroup);
    List