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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.mutable;

/**
 * Provides mutable access to a value.
 * 
 * Mutable is used as a generic interface to the implementations in this package.
 * 
 * A typical use case would be to enable a primitive or string to be passed to a method and allow that method to
 * effectively change the value of the primitive/string. Another use case is to store a frequently changing primitive in
 * a collection (for example a total in a map) without needing to create new Integer/Long wrapper objects.
 *
 * @param  the type to set and get
 * @since 2.1
 */
public interface Mutable {

    /**
     * Gets the value of this mutable.
     *
     * @return the stored value
     */
    T getValue();

    /**
     * Sets the value of this mutable.
     *
     * @param value
     *            the value to store
     * @throws NullPointerException
     *             if the object is null and null is invalid
     * @throws ClassCastException
     *             if the type is invalid
     */
    void setValue(T value);

}