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Apache Commons Lang, a package of Java utility classes for the classes that are in java.lang's hierarchy, or are considered to be so standard as to justify existence in java.lang.

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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.lang3.builder;

/**
 * {@code Diffable} classes can be compared with other objects
 * for differences. The {@link DiffResult} object retrieved can be queried
 * for a list of differences or printed using the {@link DiffResult#toString()}.
 *
 * The calculation of the differences is consistent with equals if
 * and only if {@code d1.equals(d2)} implies {@code d1.diff(d2) == ""}.
 * It is strongly recommended that implementations are consistent with equals
 * to avoid confusion. Note that {@code null} is not an instance of any class
 * and {@code d1.diff(null)} should throw a {@code NullPointerException}.
 *
 * 
 * {@code Diffable} classes lend themselves well to unit testing, in which a
 * easily readable description of the differences between an anticipated result and
 * an actual result can be retrieved. For example:
 * 
 *  * Assert.assertEquals(expected.diff(result), expected, result);
 * 
 *
 * @param  the type of objects that this object may be differentiated against
 * @since 3.3
 */
@FunctionalInterface
public interface Diffable {

    /**
     * Retrieves a list of the differences between
     * this object and the supplied object.
     *
     * @param obj the object to diff against, can be {@code null}
     * @return a list of differences
     * @throws NullPointerException if the specified object is {@code null}
     */
    DiffResult diff(T obj);
}