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diff --git a/src/main/java/org/apache/commons/lang3/builder/Diffable.java b/src/main/java/org/apache/commons/lang3/builder/Diffable.java
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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;
+
+/**
+ * {@link 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()}.
+ *
+ * <p>The calculation of the differences is <i>consistent with equals</i> 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 {@link NullPointerException}.</p>
+ *
+ * <p>
+ * {@link 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:
+ * </p>
+ * <pre>
+ * Assert.assertEquals(expected.diff(result), expected, result);
+ * </pre>
+ *
+ * @param <T> the type of objects that this object may be differentiated against
+ * @since 3.3
+ */
+@FunctionalInterface
+public interface Diffable<T> {
+
+    /**
+     * 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<T> diff(T obj);
+}