src/main/java/com/google/devtools/build/lib/collect/EquivalenceRelation.java

// Copyright 2014 Google Inc. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
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//
//    http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
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package com.google.devtools.build.lib.collect;

/**
 * A comparison function, which imposes an equivalence relation on some
 * collection of objects.
 *
 * <p>The ordering imposed by an EquivalenceRelation <tt>e</tt> on a set of
 * elements <tt>S</tt> is said to be <i>consistent with equals</i> if and only
 * if <tt>(compare((Object)e1, (Object)e2)==0)</tt> has the same boolean value
 * as <tt>e1.equals((Object)e2)</tt> for every <tt>e1</tt> and <tt>e2</tt> in
 * <tt>S</tt>.<p>
 *
 * <p>Unlike {@link java.util.Comparator}, whose implementations are often
 * consistent with equals, the applications for which EquivalenceRelation
 * instances are used means that its implementations rarely are.  They may are
 * usually more or less discriminative than the default equivalence relation
 * for the type.
 *
 * <p>For example, consider possible equivalence relations for {@link
 * java.lang.Integer}: the default equivalence defined by Integer.equals() is
 * based on the integer value is represents, but two alternative equivalences
 * would be {@link EquivalenceRelation#IDENTITY} (object identity&mdash;a more
 * discriminative relation) or <i>parity</i> (under which all even numbers, odd
 * numbers are considered equivalent to each other&mdash;a less discriminative
 * relation).
 */
public interface EquivalenceRelation<T> {
  // This should be a superinterface of Comparator.

  /**
   * Compares its two arguments for equivalence.  Returns zero if they are
   * considered equivalent, or non-zero otherwise.<p>
   *
   * The implementor must ensure that the relation is
   *
   * reflexive (<tt>compare(x,x)==0</tt> for all x),
   *
   * symmetric (<tt>compare(x,y)==compare(y,x)<tt> for all x, y),
   *
   * and transitive <tt>(compare(x, y)==0 &amp;&amp; compare(y,
   * z)==0</tt> implies <tt>compare(x, z)==0</tt>.<p>
   *
   * @param o1 the first object to be compared.
   * @param o2 the second object to be compared.
   * @return zero if the two objects are equivalent; some other integer value
   *   otherwise.
   * @throws ClassCastException if the arguments' types prevent them from
   *   being compared by this EquivalenceRelation.
   */
  int compare(T o1, T o2);

  /**
   * The object-identity equivalence relation.  This is the strictest possible
   * equivalence relation for objects, and considers two values equal iff they
   * are references to the same object instance.
   */
  public static final EquivalenceRelation<?> IDENTITY =
      new EquivalenceRelation<Object>() {
        @Override
        public int compare(Object o1, Object o2) {
          return o1 == o2 ? 0 : -1;
        }
      };

  /**
   * The default equivalence relation for type T, using T.equals().  This
   * relation considers two values equivalent if either they are both null, or
   * o1.equals(o2).
   */
  public static final EquivalenceRelation<?> DEFAULT =
      new EquivalenceRelation<Object>() {
        @Override
        public int compare(Object o1, Object o2) {
          return (o1 == null ? o2 == null : o1.equals(o2))
              ? 0
              : -1;
        }
      };
}