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// 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.
// 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.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—a more
* discriminative relation) or <i>parity</i> (under which all even numbers, odd
* numbers are considered equivalent to each other—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 && 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;
}
};
}
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