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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.keyvalue;

import java.util.Map;

import org.apache.commons.collections4.KeyValue;

/**
 * A mutable KeyValue pair that does not implement
 * {@link java.util.Map.Entry Map.Entry}.
 * 
 * Note that a DefaultKeyValue instance may not contain
 * itself as a key or value.
 * 
 *
 * @param  the type of keys
 * @param  the type of values
 * @since 3.0
 */
public class DefaultKeyValue extends AbstractKeyValue {

    /**
     * Constructs a new pair with a null key and null value.
     */
    public DefaultKeyValue() {
        super(null, null);
    }

    /**
     * Constructs a new pair with the specified key and given value.
     *
     * @param key  the key for the entry, may be null
     * @param value  the value for the entry, may be null
     */
    public DefaultKeyValue(final K key, final V value) {
        super(key, value);
    }

    /**
     * Constructs a new pair from the specified KeyValue.
     *
     * @param pair  the pair to copy, must not be null
     * @throws NullPointerException if the entry is null
     */
    public DefaultKeyValue(final KeyValue pair) {
        super(pair.getKey(), pair.getValue());
    }

    /**
     * Constructs a new pair from the specified Map.Entry.
     *
     * @param entry  the entry to copy, must not be null
     * @throws NullPointerException if the entry is null
     */
    public DefaultKeyValue(final Map.Entry entry) {
        super(entry.getKey(), entry.getValue());
    }

    //-----------------------------------------------------------------------
    /**
     * Sets the key.
     *
     * @param key  the new key
     * @return the old key
     * @throws IllegalArgumentException if key is this object
     */
    @Override
    public K setKey(final K key) {
        if (key == this) {
            throw new IllegalArgumentException("DefaultKeyValue may not contain itself as a key.");
        }

        return super.setKey(key);
    }

    /**
     * Sets the value.
     *
     * @return the old value of the value
     * @param value the new value
     * @throws IllegalArgumentException if value is this object
     */
    @Override
    public V setValue(final V value) {
        if (value == this) {
            throw new IllegalArgumentException("DefaultKeyValue may not contain itself as a value.");
        }

        return super.setValue(value);
    }

    //-----------------------------------------------------------------------
    /**
     * Returns a new Map.Entry object with key and value from this pair.
     *
     * @return a MapEntry instance
     */
    public Map.Entry toMapEntry() {
        return new DefaultMapEntry<>(this);
    }

    //-----------------------------------------------------------------------
    /**
     * Compares this Map.Entry with another Map.Entry.
     * 
     * Returns true if the compared object is also a DefaultKeyValue,
     * and its key and value are equal to this object's key and value.
     *
     * @param obj  the object to compare to
     * @return true if equal key and value
     */
    @Override
    public boolean equals(final Object obj) {
        if (obj == this) {
            return true;
        }
        if (obj instanceof DefaultKeyValue == false) {
            return false;
        }

        final DefaultKeyValue other = (DefaultKeyValue) obj;
        return
            (getKey() == null ? other.getKey() == null : getKey().equals(other.getKey())) &&
            (getValue() == null ? other.getValue() == null : getValue().equals(other.getValue()));
    }

    /**
     * Gets a hashCode compatible with the equals method.
     * 
     * Implemented per API documentation of {@link java.util.Map.Entry#hashCode()},
     * however subclasses may override this.
     *
     * @return a suitable hash code
     */
    @Override
    public int hashCode() {
        return (getKey() == null ? 0 : getKey().hashCode()) ^
               (getValue() == null ? 0 : getValue().hashCode());
    }

}