org.apache.commons.collections.map.DefaultedMap 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.HashMap;
import java.util.Map;
import org.apache.commons.collections.Factory;
import org.apache.commons.collections.Transformer;
import org.apache.commons.collections.functors.ConstantTransformer;
import org.apache.commons.collections.functors.FactoryTransformer;
/**
* Decorates another Map returning a default value if the map
* does not contain the requested key.
*
* 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.
*
* 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).
*
* For instance:
*
* Map map = new DefaultedMap("NULL");
* Object obj = map.get("Surname");
* // obj == "NULL"
*
* After the above code is executed the map is still empty.
*
* Note that DefaultedMap 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.
*
* @since Commons Collections 3.2
* @version $Revision: 1.7 $ $Date: 2008-04-10 14:33:15 +0200 (Thu, 10 Apr 2008) $
*
* @author Stephen Colebourne
* @author Rafael U.C. Afonso
* @see LazyMap
*/
public class DefaultedMap
extends AbstractMapDecorator
implements Map, Serializable {
/** Serialization version */
private static final long serialVersionUID = 19698628745827L;
/** The transformer to use if the map does not contain a key */
protected final Object value;
//-----------------------------------------------------------------------
/**
* Factory method to create a defaulting map.
*
* The value specified is returned when a missing key is found.
*
* @param map the map to decorate, must not be null
* @param defaultValue the default value to return when the key is not found
* @throws IllegalArgumentException if map is null
*/
public static Map decorate(Map map, Object defaultValue) {
if (defaultValue instanceof Transformer) {
defaultValue = ConstantTransformer.getInstance(defaultValue);
}
return new DefaultedMap(map, defaultValue);
}
/**
* Factory method to create a defaulting map.
*
* 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 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) {
if (factory == null) {
throw new IllegalArgumentException("Factory must not be null");
}
return new DefaultedMap(map, FactoryTransformer.getInstance(factory));
}
/**
* Factory method to create a defaulting map.
*
* 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 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) {
if (factory == null) {
throw new IllegalArgumentException("Transformer must not be null");
}
return new DefaultedMap(map, factory);
}
//-----------------------------------------------------------------------
/**
* Constructs a new empty DefaultedMap that decorates
* a HashMap.
*
* 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(Object defaultValue) {
super(new HashMap());
if (defaultValue instanceof Transformer) {
defaultValue = ConstantTransformer.getInstance(defaultValue);
}
this.value = defaultValue;
}
/**
* Constructor that wraps (not copies).
*
* @param map the map to decorate, must not be null
* @param value the value to use
* @throws IllegalArgumentException if map or transformer is null
*/
protected DefaultedMap(Map map, Object value) {
super(map);
this.value = value;
}
//-----------------------------------------------------------------------
/**
* Write the map out using a custom routine.
*
* @param out the output stream
* @throws IOException
*/
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
*/
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) {
if (value instanceof Transformer) {
return ((Transformer) value).transform(key);
}
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.
}