/*
 * 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
 *
 *      https://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}.
 * <p>
 * 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.
 * </p>
 * <p>
 * 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.
 * </p>
 * <p>
 * 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()}.
 * </p>
 *
 * <pre>{@code
 * MapIterator<String, Integer> it = map.mapIterator();
 * while (it.hasNext()) {
 *     String key = it.next();
 *     Integer value = it.getValue();
 *     it.setValue(value + 1);
 * }
 * }</pre>
 *
 * @param <K> The type of the keys in the map.
 * @param <V> The type of the values in the map.
 * @since 3.0
 */
public interface MapIterator<K, V> extends Iterator<K> {

    /**
     * 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 <em>key</em> 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).
     * <p>
     * This method can be called once per call to {@code next()}.
     * </p>
     *
     * @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);
}
