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The Apache Commons Collections package contains types that extend and augment the Java Collections Framework.

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/*
 * 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.collections4;

/**
 * An equation function, which determines equality between objects of type T.
 * 
 * It is the functional sibling of {@link java.util.Comparator}; {@link Equator} is to
 * {@link Object} as {@link java.util.Comparator} is to {@link Comparable}.
 * 
 *
 * @param  the types of object this {@link Equator} can evaluate.
 * @since 4.0
 */
public interface Equator {
    /**
     * 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 Object#equals(Object) equals(Object)} method to an
     * Equator (and so must also delegate their {@link Object#hashCode() hashCode()} method), or for implementations
     * of {@link org.apache.commons.collections4.map.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);
}