LazyMap.java
/*
* 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.Map;
import java.util.Objects;
import org.apache.commons.collections4.Factory;
import org.apache.commons.collections4.Transformer;
import org.apache.commons.collections4.functors.FactoryTransformer;
/**
* Decorates another {@code Map} to create objects in the map on demand.
* <p>
* When the {@link #get(Object)} method is called with a key that does not
* exist in the map, the factory is used to create the object. The created
* object will be added to the map using the requested key.
* </p>
* <p>
* For instance:
* </p>
* <pre>
* Factory<Date> factory = new Factory<Date>() {
* public Date create() {
* return new Date();
* }
* }
* Map<String, Date> lazy = LazyMap.lazyMap(new HashMap<String, Date>(), factory);
* Date date = lazy.get("NOW");
* </pre>
*
* <p>
* After the above code is executed, {@code date} will refer to
* a new {@code Date} instance. Furthermore, that {@code Date}
* instance is mapped to the "NOW" key in the map.
* </p>
* <p>
* <strong>Note that LazyMap 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.
* </p>
* <p>
* This class is Serializable from Commons Collections 3.1.
* </p>
*
* @param <K> the type of the keys in this map
* @param <V> the type of the values in this map
* @since 3.0
*/
public class LazyMap<K, V> extends AbstractMapDecorator<K, V> implements Serializable {
/** Serialization version */
private static final long serialVersionUID = 7990956402564206740L;
/**
* Factory method to create a lazily instantiated map.
*
* @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, must not be null
* @return a new lazy map
* @throws NullPointerException if map or factory is null
* @since 4.0
*/
public static <K, V> LazyMap<K, V> lazyMap(final Map<K, V> map, final Factory<? extends V> factory) {
return new LazyMap<>(map, factory);
}
/**
* Factory method to create a lazily instantiated map.
*
* @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, must not be null
* @return a new lazy map
* @throws NullPointerException if map or factory is null
* @since 4.0
*/
public static <V, K> LazyMap<K, V> lazyMap(final Map<K, V> map, final Transformer<? super K, ? extends V> factory) {
return new LazyMap<>(map, factory);
}
/** The factory to use to construct elements */
protected final Transformer<? super K, ? extends V> factory;
/**
* Constructor that wraps (not copies).
*
* @param map the map to decorate, must not be null
* @param factory the factory to use, must not be null
* @throws NullPointerException if map or factory is null
*/
protected LazyMap(final Map<K, V> map, final Factory<? extends V> factory) {
super(map);
this.factory = FactoryTransformer.factoryTransformer(Objects.requireNonNull(factory, "factory"));
}
/**
* Constructor that wraps (not copies).
*
* @param map the map to decorate, must not be null
* @param factory the factory to use, must not be null
* @throws NullPointerException if map or factory is null
*/
protected LazyMap(final Map<K, V> map, final Transformer<? super K, ? extends V> factory) {
super(map);
this.factory = Objects.requireNonNull(factory, "factory");
}
@Override
public V get(final Object key) {
// create value for key if key is not currently in the map
if (!map.containsKey(key)) {
@SuppressWarnings("unchecked")
final K castKey = (K) key;
final V value = factory.transform(castKey);
map.put(castKey, value);
return value;
}
return map.get(key);
}
/**
* Read the map in using a custom routine.
*
* @param in the input stream
* @throws IOException if an error occurs while reading from the stream
* @throws ClassNotFoundException if an object read from the stream can not be loaded
* @since 3.1
*/
@SuppressWarnings("unchecked")
private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
in.defaultReadObject();
map = (Map<K, V>) in.readObject();
}
/**
* Write the map out using a custom routine.
*
* @param out the output stream
* @throws IOException if an error occurs while writing to the stream
* @since 3.1
*/
private void writeObject(final ObjectOutputStream out) throws IOException {
out.defaultWriteObject();
out.writeObject(map);
}
// 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.
}