org.apache.commons.collections.map.LazyMap Maven / Gradle / Ivy
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*
* This software consists of voluntary contributions made by many
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* 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.
*
* @since Commons Collections 3.0
* @version $Revision: 1.3 $ $Date: 2004/01/14 21:43:11 $
*
* @author Stephen Colebourne
* @author Paul Jack
*/
public class LazyMap
extends AbstractMapDecorator implements Map {
/** 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 = TransformerUtils.asTransformer(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;
}
//-----------------------------------------------------------------------
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.
}