Java tutorial
/* * 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.ArrayList; import java.util.Collection; import java.util.HashSet; import java.util.List; import java.util.Set; import org.apache.commons.collections4.bag.HashBag; import org.apache.commons.collections4.multimap.ArrayListValuedHashMap; import org.apache.commons.collections4.multimap.HashSetValuedHashMap; import org.apache.commons.collections4.multimap.TransformedMultiValuedMap; import org.apache.commons.collections4.multimap.UnmodifiableMultiValuedMap; /** * Provides utility methods and decorators for {@link MultiValuedMap} instances. * <p> * It contains various type safe and null safe methods. Additionally, it provides * the following decorators: * <ul> * <li>{@link #unmodifiableMultiValuedMap(MultiValuedMap)}</li> * <li>{@link #transformedMultiValuedMap(MultiValuedMap, Transformer, Transformer)}</li> * </ul> * * @since 4.1 * @version $Id: MultiMapUtils.java 1715302 2015-11-19 23:08:01Z tn $ */ public class MultiMapUtils { /** * <code>MultiMapUtils</code> should not normally be instantiated. */ private MultiMapUtils() { } /** * An empty {@link UnmodifiableMultiValuedMap}. */ @SuppressWarnings({ "rawtypes", "unchecked" }) public static final MultiValuedMap EMPTY_MULTI_VALUED_MAP = UnmodifiableMultiValuedMap .unmodifiableMultiValuedMap(new ArrayListValuedHashMap(0, 0)); /** * Returns immutable EMPTY_MULTI_VALUED_MAP with generic type safety. * * @param <K> the type of key in the map * @param <V> the type of value in the map * @return immutable and empty <code>MultiValuedMap</code> */ @SuppressWarnings("unchecked") public static <K, V> MultiValuedMap<K, V> emptyMultiValuedMap() { return EMPTY_MULTI_VALUED_MAP; } // Null safe methods /** * Returns an immutable empty <code>MultiValuedMap</code> if the argument is * <code>null</code>, or the argument itself otherwise. * * @param <K> the type of key in the map * @param <V> the type of value in the map * @param map the map, may be null * @return an empty {@link MultiValuedMap} if the argument is null */ @SuppressWarnings("unchecked") public static <K, V> MultiValuedMap<K, V> emptyIfNull(final MultiValuedMap<K, V> map) { return map == null ? EMPTY_MULTI_VALUED_MAP : map; } /** * Null-safe check if the specified <code>MultiValuedMap</code> is empty. * <p> * If the provided map is null, returns true. * * @param map the map to check, may be null * @return true if the map is empty or null */ public static boolean isEmpty(final MultiValuedMap<?, ?> map) { return map == null || map.isEmpty(); } // Null safe getters // ------------------------------------------------------------------------- /** * Gets a Collection from <code>MultiValuedMap</code> in a null-safe manner. * * @param <K> the key type * @param <V> the value type * @param map the {@link MultiValuedMap} to use * @param key the key to look up * @return the Collection in the {@link MultiValuedMap}, or null if input map is null */ public static <K, V> Collection<V> getCollection(final MultiValuedMap<K, V> map, final K key) { if (map != null) { return map.get(key); } return null; } // TODO: review the getValuesAsXXX methods - depending on the actual MultiValuedMap type, changes // to the returned collection might update the backing map. This should be clarified and/or prevented. /** * Gets a List from <code>MultiValuedMap</code> in a null-safe manner. * * @param <K> the key type * @param <V> the value type * @param map the {@link MultiValuedMap} to use * @param key the key to look up * @return the Collection in the {@link MultiValuedMap} as List, or null if input map is null */ public static <K, V> List<V> getValuesAsList(final MultiValuedMap<K, V> map, final K key) { if (map != null) { Collection<V> col = map.get(key); if (col instanceof List) { return (List<V>) col; } return new ArrayList<V>(col); } return null; } /** * Gets a Set from <code>MultiValuedMap</code> in a null-safe manner. * * @param <K> the key type * @param <V> the value type * @param map the {@link MultiValuedMap} to use * @param key the key to look up * @return the Collection in the {@link MultiValuedMap} as Set, or null if input map is null */ public static <K, V> Set<V> getValuesAsSet(final MultiValuedMap<K, V> map, final K key) { if (map != null) { Collection<V> col = map.get(key); if (col instanceof Set) { return (Set<V>) col; } return new HashSet<V>(col); } return null; } /** * Gets a Bag from <code>MultiValuedMap</code> in a null-safe manner. * * @param <K> the key type * @param <V> the value type * @param map the {@link MultiValuedMap} to use * @param key the key to look up * @return the Collection in the {@link MultiValuedMap} as Bag, or null if input map is null */ public static <K, V> Bag<V> getValuesAsBag(final MultiValuedMap<K, V> map, final K key) { if (map != null) { Collection<V> col = map.get(key); if (col instanceof Bag) { return (Bag<V>) col; } return new HashBag<V>(col); } return null; } // Factory Methods // ----------------------------------------------------------------------- /** * Creates a {@link ListValuedMap} with an {@link java.util.ArrayList ArrayList} as * collection class to store the values mapped to a key. * * @param <K> the key type * @param <V> the value type * @return a new <code>ListValuedMap</code> */ public static <K, V> ListValuedMap<K, V> newListValuedHashMap() { return new ArrayListValuedHashMap<K, V>(); } /** * Creates a {@link SetValuedMap} with an {@link java.util.HashSet HashSet} as * collection class to store the values mapped to a key. * * @param <K> the key type * @param <V> the value type * @return a new {@link SetValuedMap} */ public static <K, V> SetValuedMap<K, V> newSetValuedHashMap() { return new HashSetValuedHashMap<K, V>(); } // MultiValuedMap Decorators // ----------------------------------------------------------------------- /** * Returns an <code>UnmodifiableMultiValuedMap</code> backed by the given * map. * * @param <K> the key type * @param <V> the value type * @param map the {@link MultiValuedMap} to decorate, must not be null * @return an unmodifiable {@link MultiValuedMap} backed by the provided map * @throws NullPointerException if map is null */ public static <K, V> MultiValuedMap<K, V> unmodifiableMultiValuedMap( final MultiValuedMap<? extends K, ? extends V> map) { return UnmodifiableMultiValuedMap.<K, V>unmodifiableMultiValuedMap(map); } /** * Returns a <code>TransformedMultiValuedMap</code> backed by the given map. * <p> * This method returns a new <code>MultiValuedMap</code> (decorating the * specified map) that will transform any new entries added to it. Existing * entries in the specified map will not be transformed. If you want that * behaviour, see {@link TransformedMultiValuedMap#transformedMap}. * <p> * Each object is passed through the transformers as it is added to the Map. * It is important not to use the original map after invoking this method, * as it is a back door for adding untransformed objects. * <p> * If there are any elements already in the map being decorated, they are * NOT transformed. * * @param <K> the key type * @param <V> the value type * @param map the {@link MultiValuedMap} to transform, must not be null, typically empty * @param keyTransformer the transformer for the map keys, null means no transformation * @param valueTransformer the transformer for the map values, null means no transformation * @return a transformed <code>MultiValuedMap</code> backed by the given map * @throws NullPointerException if map is null */ public static <K, V> MultiValuedMap<K, V> transformedMultiValuedMap(final MultiValuedMap<K, V> map, final Transformer<? super K, ? extends K> keyTransformer, final Transformer<? super V, ? extends V> valueTransformer) { return TransformedMultiValuedMap.transformingMap(map, keyTransformer, valueTransformer); } }