/*
 *  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 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 Collections.synchronizedMap(Map<Object,Object>). This class may throw exceptions when accessed by concurrent threads without synchronization. 

This class is Serializable from Commons Collections 3.1.
Author:
Stephen Colebourne, Paul Jack
Since:
Commons Collections 3.0
Version:
$Revision: 646777 $ $Date: 2008-04-10 14:33:15 +0200 (Thu, 10 Apr 2008) $/**
 * Decorates another <code>Map</code> 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>
 * For instance:
 * <pre>
 * Factory factory = new Factory() {
 *     public Object create() {
 *         return new Date();
 *     }
 * }
 * Map lazy = Lazy.map(new HashMap(), factory);
 * Object obj = lazy.get("NOW");
 * </pre>
 *
 * After the above code is executed, <code>obj</code> will contain
 * a new <code>Date</code> instance. Furthermore, that <code>Date</code>
 * instance is mapped to the "NOW" key in the map.
 * <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>
 * 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 /** Serialization version */
    private static final long serialVersionUID = 7990956402564206740L;

    
The factory to use to construct elements /** The factory to use to construct elements */
    protected final Transformer factory;

    
Factory method to create a lazily instantiated map.
Params:
map –  the map to decorate, must not be null
factory –  the factory to use, must not be null
Throws:
IllegalArgumentException – if map or factory is null/**
     * 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.
Params:
map –  the map to decorate, must not be null
factory –  the factory to use, must not be null
Throws:
IllegalArgumentException – if map or factory is null/**
     * 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).
Params:
map –  the map to decorate, must not be null
factory –  the factory to use, must not be null
Throws:
IllegalArgumentException – if map or factory is null/**
     * 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).
Params:
map –  the map to decorate, must not be null
factory –  the factory to use, must not be null
Throws:
IllegalArgumentException – if map or factory is null/**
     * 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.
Params:
out –  the output stream
Throws:
IOException – 
Since:
Commons Collections 3.1/**
     * 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.
Params:
in –  the input stream
Throws:
IOException – 
ClassNotFoundException – 
Since:
Commons Collections 3.1/**
     * 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.
}