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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;

import java.util.Iterator;

/**
 * Defines an iterator that operates over a {@code Map}.
 * 
 * This iterator is a special version designed for maps. It can be more
 * efficient to use this rather than an entry set iterator where the option
 * is available, and it is certainly more convenient.
 * 
 * 
 * A map that provides this interface may not hold the data internally using
 * Map Entry objects, thus this interface can avoid lots of object creation.
 * 
 * 
 * In use, this iterator iterates through the keys in the map. After each call
 * to {@code next()}, the {@code getValue()} method provides direct
 * access to the value. The value can also be set using {@code setValue()}.
 * 
 * {@code
 * MapIterator it = map.mapIterator();
 * while (it.hasNext()) {
 *   String key = it.next();
 *   Integer value = it.getValue();
 *   it.setValue(value + 1);
 * }
 * }
 *
 * @param  the type of the keys in the map
 * @param  the type of the values in the map
 * @since 3.0
 */
public interface MapIterator extends Iterator {

    /**
     * Gets the current key, which is the key returned by the last call
     * to {@code next()}.
     *
     * @return the current key
     * @throws IllegalStateException if {@code next()} has not yet been called
     */
    K getKey();

    /**
     * Gets the current value, which is the value associated with the last key
     * returned by {@code next()}.
     *
     * @return the current value
     * @throws IllegalStateException if {@code next()} has not yet been called
     */
    V getValue();

    /**
     * Checks to see if there are more entries still to be iterated.
     *
     * @return {@code true} if the iterator has more elements
     */
    @Override
    boolean hasNext();

    /**
     * Gets the next key from the {@code Map}.
     *
     * @return the next key in the iteration
     * @throws java.util.NoSuchElementException if the iteration is finished
     */
    @Override
    K next();

    /**
     * Removes the last returned key from the underlying {@code Map} (optional operation).
     * 
     * This method can be called once per call to {@code next()}.
     * 
     *
     * @throws UnsupportedOperationException if remove is not supported by the map
     * @throws IllegalStateException if {@code next()} has not yet been called
     * @throws IllegalStateException if {@code remove()} has already been called
     *  since the last call to {@code next()}
     */
    @Override
    void remove();

    /**
     * Sets the value associated with the current key (optional operation).
     *
     * @param value  the new value
     * @return the previous value
     * @throws UnsupportedOperationException if setValue is not supported by the map
     * @throws IllegalStateException if {@code next()} has not yet been called
     * @throws IllegalStateException if {@code remove()} has been called since the
     *  last call to {@code next()}
     */
    V setValue(V value);

}