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 *  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 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 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 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 Collections.synchronizedMap(Map<Object,Object>). This class may throw exceptions when accessed by concurrent threads without synchronization. 
Author:
Stephen Colebourne, Rafael U.C. Afonso
See Also:
LazyMap
Since:
Commons Collections 3.2
Version:
$Revision: 1.7 $ $Date: 2008-04-10 14:33:15 +0200 (Thu, 10 Apr 2008) $/**
 * Decorates another <code>Map</code> returning a default value if the map
 * does not contain the requested key.
 * <p>
 * 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.
 * <p>
 * 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).
 * <p>
 * For instance:
 * <pre>
 * Map map = new DefaultedMap("NULL");
 * Object obj = map.get("Surname");
 * // obj == "NULL"
 * </pre>
 * After the above code is executed the map is still empty.
 * <p>
 * <strong>Note that DefaultedMap 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.
 *
 * @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 /** Serialization version */
    private static final long serialVersionUID = 19698628745827L;

    
The transformer to use if the map does not contain a key /** 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.
Params:
map –  the map to decorate, must not be null
defaultValue –  the default value to return when the key is not found
Throws:
IllegalArgumentException – if map is null/**
     * Factory method to create a defaulting map.
     * <p>
     * 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.
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 defaulting map.
     * <p>
     * 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.
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 defaulting map.
     * <p>
     * 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.
Params:
defaultValue –  the default value to return when the key is not found/**
     * Constructs a new empty <code>DefaultedMap</code> that decorates
     * a <code>HashMap</code>.
     * <p>
     * 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).
Params:
map –  the map to decorate, must not be null
value –  the value to use
Throws:
IllegalArgumentException – if map or transformer is null/**
     * 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.
Params:
out –  the output stream
Throws:
IOException – /**
     * 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.
Params:
in –  the input stream
Throws:
IOException – 
ClassNotFoundException – /**
     * 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.
}