Merging from -r468106:814127 of collections_jdk5_branch - namely where this code was generified; mostly in r738956.

Also see the following revisions:

    ------------------------------------------------------------------------
    r643590 | skestle | 2008-04-01 14:36:59 -0700 (Tue, 01 Apr 2008) | 1 line
    
    Added Equator interface for COLLECTIONS-242.
    ------------------------------------------------------------------------


git-svn-id: https://svn.apache.org/repos/asf/commons/proper/collections/trunk@815047 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/commons/collections/functors/Equator.java

@@ -0,0 +1,48 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more contributor license
+ * agreements. See the NOTICE file distributed with this work for additional information regarding
+ * copyright ownership. The ASF licenses this file to You 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 org.apache.commons.collections.functors;
+
+import org.apache.commons.collections.map.HashedMap;
+
+/**
+ * <p>
+ * An equation function, which determines equality between objects of type T.
+ * </p>
+ * <p>
+ * It is the functional sibling of {@link java.util.Comparator}; {@link Equator} is to
+ * {@link Object} as {@link java.util.Comparator} is to {@link java.lang.Comparable}.
+ * </p>
+ * 
+ * @param <T> the types of object this {@link Equator} can evaluate.
+ * @author Stephen Kestle
+ * @since 4.0
+ */
+public interface Equator<T> {
+    /**
+     * Evaluates the two arguments for their equality.
+     * 
+     * @param o1 the first object to be equated.
+     * @param o2 the second object to be equated.
+     * @return whether the two objects are equal.
+     */
+    boolean equate(T o1, T o2);
+
+    /**
+     * Calculates the hash for the object, based on the method of equality used in the equate
+     * method. This is used for classes that delegate their {@link #equals(Object)} method to an
+     * Equator (and so must also delegate their {@link #hashCode()} method), or for implementations
+     * of {@link HashedMap} that use an Equator for the key objects.
+     * 
+     * @param o the object to calculate the hash for.
+     * @return the hash of the object.
+     */
+    int hash(T o);
+}