DefaultedMap.java
/*
* 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.collections4.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 java.util.Objects;
import org.apache.commons.collections4.Factory;
import org.apache.commons.collections4.Transformer;
import org.apache.commons.collections4.functors.ConstantTransformer;
import org.apache.commons.collections4.functors.FactoryTransformer;
/**
* Decorates another {@code Map} 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>
* <p>
* The defaulted value is not added to the map.
* Compare this behavior with {@link LazyMap}, which does add the value
* to the map (via a Transformer).
* </p>
* <p>
* For instance:
* </p>
* <pre>
* Map map = new DefaultedMap("NULL");
* Object obj = map.get("Surname");
* // obj == "NULL"
* </pre>
* <p>
* After the above code is executed the map is still empty.
* </p>
* <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.
* </p>
*
* @param <K> the type of the keys in this map
* @param <V> the type of the values in this map
*
* @since 3.2
* @see LazyMap
*/
public class DefaultedMap<K, V> extends AbstractMapDecorator<K, V> implements Serializable {
/** Serialization version */
private static final long serialVersionUID = 19698628745827L;
/**
* 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 <K> the key type
* @param <V> the value type
* @param map the map to decorate, must not be null
* @param factory the factory to use to create entries, must not be null
* @return a new defaulting map
* @throws NullPointerException if map or factory is null
* @since 4.0
*/
public static <K, V> DefaultedMap<K, V> defaultedMap(final Map<K, V> map, final Factory<? extends V> factory) {
return new DefaultedMap<>(map, FactoryTransformer.factoryTransformer(
Objects.requireNonNull(factory, "Factory must not be 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 <K> the key type
* @param <V> the value type
* @param map the map to decorate, must not be null
* @param transformer the transformer to use as a factory to create entries, must not be null
* @return a new defaulting map
* @throws NullPointerException if map or transformer is null
* @since 4.0
*/
public static <K, V> Map<K, V> defaultedMap(final Map<K, V> map,
final Transformer<? super K, ? extends V> transformer) {
return new DefaultedMap<>(map, Objects.requireNonNull(transformer, "Transformer must not be null"));
}
/**
* Factory method to create a defaulting map.
* <p>
* The value specified is returned when a missing key is found.
*
* @param <K> the key type
* @param <V> the value type
* @param map the map to decorate, must not be null
* @param defaultValue the default value to return when the key is not found
* @return a new defaulting map
* @throws NullPointerException if map is null
* @since 4.0
*/
public static <K, V> DefaultedMap<K, V> defaultedMap(final Map<K, V> map, final V defaultValue) {
return new DefaultedMap<>(map, ConstantTransformer.constantTransformer(defaultValue));
}
/** The transformer to use if the map does not contain a key */
private final Transformer<? super K, ? extends V> value;
/**
* Constructor that wraps (not copies).
*
* @param map the map to decorate, must not be null
* @param defaultValueTransformer the value transformer to use
* @throws NullPointerException if map or transformer is null
*/
protected DefaultedMap(final Map<K, V> map, final Transformer<? super K, ? extends V> defaultValueTransformer) {
super(map);
this.value = Objects.requireNonNull(defaultValueTransformer, "defaultValueTransformer");
}
/**
* Constructs a new empty {@code DefaultedMap} that decorates a {@code HashMap}.
*
* @param defaultValueTransformer transformer to use to generate missing values.
*/
public DefaultedMap(final Transformer<? super K, ? extends V> defaultValueTransformer) {
this(new HashMap<>(), defaultValueTransformer);
}
/**
* Constructs a new empty {@code DefaultedMap} that decorates
* a {@code HashMap}.
* <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(final V defaultValue) {
this(ConstantTransformer.constantTransformer(defaultValue));
}
@Override
@SuppressWarnings("unchecked")
public V get(final Object key) {
final V v;
return (v = map.get(key)) != null || map.containsKey(key)
? v
: value.transform((K) key);
}
/**
* Read the map in using a custom routine.
*
* @param in the input stream
* @throws IOException if an error occurs while reading from the stream
* @throws ClassNotFoundException if an object read from the stream can not be loaded
*/
@SuppressWarnings("unchecked")
private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
in.defaultReadObject();
map = (Map<K, V>) in.readObject();
}
/**
* Write the map out using a custom routine.
*
* @param out the output stream
* @throws IOException if an error occurs while writing to the stream
*/
private void writeObject(final ObjectOutputStream out) throws IOException {
out.defaultWriteObject();
out.writeObject(map);
}
// 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.
}