org.apache.commons.collections.map.LazyMap Maven / Gradle / Ivy
/*
* 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.collections.map;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.Map;
import org.apache.commons.collections.Factory;
import org.apache.commons.collections.Transformer;
import org.apache.commons.collections.functors.FactoryTransformer;
/**
* Decorates another Map to create objects in the map on demand.
*
* 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.
*
* For instance:
*
* Factory factory = new Factory() {
* public Object create() {
* return new Date();
* }
* }
* Map lazy = Lazy.map(new HashMap(), factory);
* Object obj = lazy.get("NOW");
*
*
* After the above code is executed, obj will contain
* a new Date instance. Furthermore, that Date
* instance is mapped to the "NOW" key in the map.
*
* Note that LazyMap is not synchronized and is not thread-safe.
* 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.
*
* This class is Serializable from Commons Collections 3.1.
*
* @since Commons Collections 3.0
* @version $Revision: 646777 $ $Date: 2008-04-10 14:33:15 +0200 (Thu, 10 Apr 2008) $
*
* @author Stephen Colebourne
* @author Paul Jack
*/
public class LazyMap
extends AbstractMapDecorator
implements Map, Serializable {
/** Serialization version */
private static final long serialVersionUID = 7990956402564206740L;
/** The factory to use to construct elements */
protected final Transformer factory;
/**
* Factory method to create a lazily instantiated map.
*
* @param map the map to decorate, must not be null
* @param factory the factory to use, must not be null
* @throws IllegalArgumentException if map or factory is null
*/
public static Map decorate(Map map, Factory factory) {
return new LazyMap(map, factory);
}
/**
* Factory method to create a lazily instantiated map.
*
* @param map the map to decorate, must not be null
* @param factory the factory to use, must not be null
* @throws IllegalArgumentException if map or factory is null
*/
public static Map decorate(Map map, Transformer factory) {
return new LazyMap(map, 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 IllegalArgumentException if map or factory is null
*/
protected LazyMap(Map map, Factory factory) {
super(map);
if (factory == null) {
throw new IllegalArgumentException("Factory must not be null");
}
this.factory = FactoryTransformer.getInstance(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 IllegalArgumentException if map or factory is null
*/
protected LazyMap(Map map, Transformer factory) {
super(map);
if (factory == null) {
throw new IllegalArgumentException("Factory must not be null");
}
this.factory = factory;
}
//-----------------------------------------------------------------------
/**
* Write the map out using a custom routine.
*
* @param out the output stream
* @throws IOException
* @since Commons Collections 3.1
*/
private void writeObject(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
* @since Commons Collections 3.1
*/
private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
in.defaultReadObject();
map = (Map) in.readObject();
}
//-----------------------------------------------------------------------
public Object get(Object key) {
// create value for key if key is not currently in the map
if (map.containsKey(key) == false) {
Object value = factory.transform(key);
map.put(key, value);
return value;
}
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.
}