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.map; import java.io.IOException; import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.io.Serializable; import java.util.HashMap; import java.util.Map; import org.apache.commons.collections4.Factory; import org.apache.commons.collections4.Transformer; import org.apache.commons.collections4.functors.ConstantTransformer; import org.apache.commons.collections4.functors.FactoryTransformer; /** * Decorates another <code>Map</code> returning a default value if the map * does not contain the requested key. * <p> * When the {@link #get(Object)} method is called with a key that does not * exist in the map, this map will return the default value specified in * the constructor/factory. Only the get method is altered, so the * {@link Map#containsKey(Object)} can be used to determine if a key really * is in the map or not. * <p> * The defaulted value is not added to the map. * Compare this behaviour with {@link LazyMap}, which does add the value * to the map (via a Transformer). * <p> * For instance: * <pre> * Map map = new DefaultedMap("NULL"); * Object obj = map.get("Surname"); * // obj == "NULL" * </pre> * After the above code is executed the map is still empty. * <p> * <strong>Note that DefaultedMap is not synchronized and is not thread-safe.</strong> * If you wish to use this map from multiple threads concurrently, you must use * appropriate synchronization. The simplest approach is to wrap this map * using {@link java.util.Collections#synchronizedMap(Map)}. This class may throw * exceptions when accessed by concurrent threads without synchronization. * * @since 3.2 * @version $Id: DefaultedMap.java 1479407 2013-05-05 22:07:53Z tn $ * * @see LazyMap */ public class DefaultedMap<K, V> extends AbstractMapDecorator<K, V> implements Serializable { /** Serialization version */ private static final long serialVersionUID = 19698628745827L; /** The transformer to use if the map does not contain a key */ private final Transformer<? super K, ? extends V> value; //----------------------------------------------------------------------- /** * Factory method to create a defaulting map. * <p> * The value specified is returned when a missing key is found. * * @param <K> the key type * @param <V> the value type * @param map the map to decorate, must not be null * @param defaultValue the default value to return when the key is not found * @return a new defaulting map * @throws IllegalArgumentException if map is null * @since 4.0 */ public static <K, V> DefaultedMap<K, V> defaultedMap(final Map<K, V> map, final V defaultValue) { return new DefaultedMap<K, V>(map, ConstantTransformer.constantTransformer(defaultValue)); } /** * Factory method to create a defaulting map. * <p> * The factory specified is called when a missing key is found. * The result will be returned as the result of the map get(key) method. * * @param <K> the key type * @param <V> the value type * @param map the map to decorate, must not be null * @param factory the factory to use to create entries, must not be null * @return a new defaulting map * @throws IllegalArgumentException if map or factory is null * @since 4.0 */ public static <K, V> DefaultedMap<K, V> defaultedMap(final Map<K, V> map, final Factory<? extends V> factory) { if (factory == null) { throw new IllegalArgumentException("Factory must not be null"); } return new DefaultedMap<K, V>(map, FactoryTransformer.factoryTransformer(factory)); } /** * Factory method to create a defaulting map. * <p> * The transformer specified is called when a missing key is found. * The key is passed to the transformer as the input, and the result * will be returned as the result of the map get(key) method. * * @param <K> the key type * @param <V> the value type * @param map the map to decorate, must not be null * @param transformer the transformer to use as a factory to create entries, must not be null * @return a new defaulting map * @throws IllegalArgumentException if map or factory is null * @since 4.0 */ public static <K, V> Map<K, V> defaultedMap(final Map<K, V> map, final Transformer<? super K, ? extends V> transformer) { if (transformer == null) { throw new IllegalArgumentException("Transformer must not be null"); } return new DefaultedMap<K, V>(map, transformer); } //----------------------------------------------------------------------- /** * Constructs a new empty <code>DefaultedMap</code> that decorates * a <code>HashMap</code>. * <p> * The object passed in will be returned by the map whenever an * unknown key is requested. * * @param defaultValue the default value to return when the key is not found */ public DefaultedMap(final V defaultValue) { this(ConstantTransformer.constantTransformer(defaultValue)); } /** * Constructs a new empty <code>DefaultedMap</code> that decorates a <code>HashMap</code>. * * @param defaultValueTransformer transformer to use to generate missing values. */ public DefaultedMap(final Transformer<? super K, ? extends V> defaultValueTransformer) { this(new HashMap<K, V>(), defaultValueTransformer); } /** * Constructor that wraps (not copies). * * @param map the map to decorate, must not be null * @param defaultValueTransformer the value transformer to use * @throws IllegalArgumentException if map or transformer is null */ protected DefaultedMap(final Map<K, V> map, final Transformer<? super K, ? extends V> defaultValueTransformer) { super(map); if (defaultValueTransformer == null) { throw new IllegalArgumentException("transformer must not be null"); } this.value = defaultValueTransformer; } //----------------------------------------------------------------------- /** * Write the map out using a custom routine. * * @param out the output stream * @throws IOException */ private void writeObject(final ObjectOutputStream out) throws IOException { out.defaultWriteObject(); out.writeObject(map); } /** * Read the map in using a custom routine. * * @param in the input stream * @throws IOException * @throws ClassNotFoundException */ @SuppressWarnings("unchecked") private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException { in.defaultReadObject(); map = (Map<K, V>) in.readObject(); } //----------------------------------------------------------------------- @Override @SuppressWarnings("unchecked") public V get(final Object key) { // create value for key if key is not currently in the map if (map.containsKey(key) == false) { return value.transform((K) key); } return map.get(key); } // no need to wrap keySet, entrySet or values as they are views of // existing map entries - you can't do a map-style get on them. }