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The Commons Collections SingletonMap.java source code
/*
* 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.Serializable;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import org.apache.commons.collections.BoundedMap;
import org.apache.commons.collections.KeyValue;
import org.apache.commons.collections.MapIterator;
import org.apache.commons.collections.OrderedMap;
import org.apache.commons.collections.OrderedMapIterator;
import org.apache.commons.collections.ResettableIterator;
import org.apache.commons.collections.iterators.SingletonIterator;
import org.apache.commons.collections.keyvalue.TiedMapEntry;
/**
* A <code>Map implementation that holds a single item and is fixed size.
* <p>
* The single key/value pair is specified at creation.
* The map is fixed size so any action that would change the size is disallowed.
* However, the <code>put or setValue methods can change
* the value associated with the key.
* <p>
* If trying to remove or clear the map, an UnsupportedOperationException is thrown.
* If trying to put a new mapping into the map, an IllegalArgumentException is thrown.
* The put method will only suceed if the key specified is the same as the
* singleton key.
* <p>
* The key and value can be obtained by:
* <ul>
* <li>normal Map methods and views
* <li>the MapIterator, see {@link #mapIterator()}
* <li>the KeyValue interface (just cast - no object creation)
* </ul>
*
* @since Commons Collections 3.1
* @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
*
* @author Stephen Colebourne
*/
public class SingletonMap
implements OrderedMap, BoundedMap, KeyValue, Serializable, Cloneable {
/** Serialization version */
private static final long serialVersionUID = -8931271118676803261L;
/** Singleton key */
private final Object key;
/** Singleton value */
private Object value;
/**
* Constructor that creates a map of <code>null to null.
*/
public SingletonMap() {
super();
this.key = null;
}
/**
* Constructor specifying the key and value.
*
* @param key the key to use
* @param value the value to use
*/
public SingletonMap(Object key, Object value) {
super();
this.key = key;
this.value = value;
}
/**
* Constructor specifying the key and value as a <code>KeyValue.
*
* @param keyValue the key value pair to use
*/
public SingletonMap(KeyValue keyValue) {
super();
this.key = keyValue.getKey();
this.value = keyValue.getValue();
}
/**
* Constructor specifying the key and value as a <code>MapEntry.
*
* @param mapEntry the mapEntry to use
*/
public SingletonMap(Map.Entry mapEntry) {
super();
this.key = mapEntry.getKey();
this.value = mapEntry.getValue();
}
/**
* Constructor copying elements from another map.
*
* @param map the map to copy, must be size 1
* @throws NullPointerException if the map is null
* @throws IllegalArgumentException if the size is not 1
*/
public SingletonMap(Map map) {
super();
if (map.size() != 1) {
throw new IllegalArgumentException("The map size must be 1");
}
Map.Entry entry = (Map.Entry) map.entrySet().iterator().next();
this.key = entry.getKey();
this.value = entry.getValue();
}
// KeyValue
//-----------------------------------------------------------------------
/**
* Gets the key.
*
* @return the key
*/
public Object getKey() {
return key;
}
/**
* Gets the value.
*
* @return the value
*/
public Object getValue() {
return value;
}
/**
* Sets the value.
*
* @param value the new value to set
* @return the old value
*/
public Object setValue(Object value) {
Object old = this.value;
this.value = value;
return old;
}
// BoundedMap
//-----------------------------------------------------------------------
/**
* Is the map currently full, always true.
*
* @return true always
*/
public boolean isFull() {
return true;
}
/**
* Gets the maximum size of the map, always 1.
*
* @return 1 always
*/
public int maxSize() {
return 1;
}
// Map
//-----------------------------------------------------------------------
/**
* Gets the value mapped to the key specified.
*
* @param key the key
* @return the mapped value, null if no match
*/
public Object get(Object key) {
if (isEqualKey(key)) {
return value;
}
return null;
}
/**
* Gets the size of the map, always 1.
*
* @return the size of 1
*/
public int size() {
return 1;
}
/**
* Checks whether the map is currently empty, which it never is.
*
* @return false always
*/
public boolean isEmpty() {
return false;
}
//-----------------------------------------------------------------------
/**
* Checks whether the map contains the specified key.
*
* @param key the key to search for
* @return true if the map contains the key
*/
public boolean containsKey(Object key) {
return (isEqualKey(key));
}
/**
* Checks whether the map contains the specified value.
*
* @param value the value to search for
* @return true if the map contains the key
*/
public boolean containsValue(Object value) {
return (isEqualValue(value));
}
//-----------------------------------------------------------------------
/**
* Puts a key-value mapping into this map where the key must match the existing key.
* <p>
* An IllegalArgumentException is thrown if the key does not match as the map
* is fixed size.
*
* @param key the key to set, must be the key of the map
* @param value the value to set
* @return the value previously mapped to this key, null if none
* @throws IllegalArgumentException if the key does not match
*/
public Object put(Object key, Object value) {
if (isEqualKey(key)) {
return setValue(value);
}
throw new IllegalArgumentException("Cannot put new key/value pair - Map is fixed size singleton");
}
/**
* Puts the values from the specified map into this map.
* <p>
* The map must be of size 0 or size 1.
* If it is size 1, the key must match the key of this map otherwise an
* IllegalArgumentException is thrown.
*
* @param map the map to add, must be size 0 or 1, and the key must match
* @throws NullPointerException if the map is null
* @throws IllegalArgumentException if the key does not match
*/
public void putAll(Map map) {
switch (map.size()) {
case 0:
return;
case 1:
Map.Entry entry = (Map.Entry) map.entrySet().iterator().next();
put(entry.getKey(), entry.getValue());
return;
default:
throw new IllegalArgumentException("The map size must be 0 or 1");
}
}
/**
* Unsupported operation.
*
* @param key the mapping to remove
* @return the value mapped to the removed key, null if key not in map
* @throws UnsupportedOperationException always
*/
public Object remove(Object key) {
throw new UnsupportedOperationException();
}
/**
* Unsupported operation.
*/
public void clear() {
throw new UnsupportedOperationException();
}
//-----------------------------------------------------------------------
/**
* Gets the entrySet view of the map.
* Changes made via <code>setValue affect this map.
* To simply iterate through the entries, use {@link #mapIterator()}.
*
* @return the entrySet view
*/
public Set entrySet() {
Map.Entry entry = new TiedMapEntry(this, getKey());
return Collections.singleton(entry);
}
/**
* Gets the unmodifiable keySet view of the map.
* Changes made to the view affect this map.
* To simply iterate through the keys, use {@link #mapIterator()}.
*
* @return the keySet view
*/
public Set keySet() {
return Collections.singleton(key);
}
/**
* Gets the unmodifiable values view of the map.
* Changes made to the view affect this map.
* To simply iterate through the values, use {@link #mapIterator()}.
*
* @return the values view
*/
public Collection values() {
return new SingletonValues(this);
}
/**
* Gets an iterator over the map.
* Changes made to the iterator using <code>setValue affect this map.
* The <code>remove method is unsupported.
* <p>
* A MapIterator returns the keys in the map. It also provides convenient
* methods to get the key and value, and set the value.
* It avoids the need to create an entrySet/keySet/values object.
* It also avoids creating the Map Entry object.
*
* @return the map iterator
*/
public MapIterator mapIterator() {
return new SingletonMapIterator(this);
}
// OrderedMap
//-----------------------------------------------------------------------
/**
* Obtains an <code>OrderedMapIterator over the map.
* <p>
* A ordered map iterator is an efficient way of iterating over maps
* in both directions.
*
* @return an ordered map iterator
*/
public OrderedMapIterator orderedMapIterator() {
return new SingletonMapIterator(this);
}
/**
* Gets the first (and only) key in the map.
*
* @return the key
*/
public Object firstKey() {
return getKey();
}
/**
* Gets the last (and only) key in the map.
*
* @return the key
*/
public Object lastKey() {
return getKey();
}
/**
* Gets the next key after the key specified, always null.
*
* @param key the next key
* @return null always
*/
public Object nextKey(Object key) {
return null;
}
/**
* Gets the previous key before the key specified, always null.
*
* @param key the next key
* @return null always
*/
public Object previousKey(Object key) {
return null;
}
//-----------------------------------------------------------------------
/**
* Compares the specified key to the stored key.
*
* @param key the key to compare
* @return true if equal
*/
protected boolean isEqualKey(Object key) {
return (key == null ? getKey() == null : key.equals(getKey()));
}
/**
* Compares the specified value to the stored value.
*
* @param value the value to compare
* @return true if equal
*/
protected boolean isEqualValue(Object value) {
return (value == null ? getValue() == null : value.equals(getValue()));
}
//-----------------------------------------------------------------------
/**
* SingletonMapIterator.
*/
static class SingletonMapIterator implements OrderedMapIterator, ResettableIterator {
private final SingletonMap parent;
private boolean hasNext = true;
private boolean canGetSet = false;
SingletonMapIterator(SingletonMap parent) {
super();
this.parent = parent;
}
public boolean hasNext() {
return hasNext;
}
public Object next() {
if (hasNext == false) {
throw new NoSuchElementException(AbstractHashedMap.NO_NEXT_ENTRY);
}
hasNext = false;
canGetSet = true;
return parent.getKey();
}
public boolean hasPrevious() {
return (hasNext == false);
}
public Object previous() {
if (hasNext == true) {
throw new NoSuchElementException(AbstractHashedMap.NO_PREVIOUS_ENTRY);
}
hasNext = true;
return parent.getKey();
}
public void remove() {
throw new UnsupportedOperationException();
}
public Object getKey() {
if (canGetSet == false) {
throw new IllegalStateException(AbstractHashedMap.GETKEY_INVALID);
}
return parent.getKey();
}
public Object getValue() {
if (canGetSet == false) {
throw new IllegalStateException(AbstractHashedMap.GETVALUE_INVALID);
}
return parent.getValue();
}
public Object setValue(Object value) {
if (canGetSet == false) {
throw new IllegalStateException(AbstractHashedMap.SETVALUE_INVALID);
}
return parent.setValue(value);
}
public void reset() {
hasNext = true;
}
public String toString() {
if (hasNext) {
return "Iterator[]";
} else {
return "Iterator[" + getKey() + "=" + getValue() + "]";
}
}
}
/**
* Values implementation for the SingletonMap.
* This class is needed as values is a view that must update as the map updates.
*/
static class SingletonValues extends AbstractSet implements Serializable {
private static final long serialVersionUID = -3689524741863047872L;
private final SingletonMap parent;
SingletonValues(SingletonMap parent) {
super();
this.parent = parent;
}
public int size() {
return 1;
}
public boolean isEmpty() {
return false;
}
public boolean contains(Object object) {
return parent.containsValue(object);
}
public void clear() {
throw new UnsupportedOperationException();
}
public Iterator iterator() {
return new SingletonIterator(parent.getValue(), false);
}
}
//-----------------------------------------------------------------------
/**
* Clones the map without cloning the key or value.
*
* @return a shallow clone
*/
public Object clone() {
try {
SingletonMap cloned = (SingletonMap) super.clone();
return cloned;
} catch (CloneNotSupportedException ex) {
throw new InternalError();
}
}
/**
* Compares this map with another.
*
* @param obj the object to compare to
* @return true if equal
*/
public boolean equals(Object obj) {
if (obj == this) {
return true;
}
if (obj instanceof Map == false) {
return false;
}
Map other = (Map) obj;
if (other.size() != 1) {
return false;
}
Map.Entry entry = (Map.Entry) other.entrySet().iterator().next();
return isEqualKey(entry.getKey()) && isEqualValue(entry.getValue());
}
/**
* Gets the standard Map hashCode.
*
* @return the hash code defined in the Map interface
*/
public int hashCode() {
return (getKey() == null ? 0 : getKey().hashCode()) ^
(getValue() == null ? 0 : getValue().hashCode());
}
/**
* Gets the map as a String.
*
* @return a string version of the map
*/
public String toString() {
return new StringBuffer(128)
.append('{')
.append((getKey() == this ? "(this Map)" : getKey()))
.append('=')
.append((getValue() == this ? "(this Map)" : getValue()))
.append('}')
.toString();
}
}
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