org.apache.commons.collections
Class DoubleOrderedMap

java.lang.Object
  |
  +--java.util.AbstractMap
        |
        +--org.apache.commons.collections.DoubleOrderedMap

All Implemented Interfaces:

java.util.Map

public final class DoubleOrderedMap

extends java.util.AbstractMap

Red-Black tree-based implementation of Map. This class guarantees that the map will be in both ascending key order and ascending value order, sorted according to the natural order for the key's and value's classes.

This Map is intended for applications that need to be able to look up a key-value pairing by either key or value, and need to do so with equal efficiency.

While that goal could be accomplished by taking a pair of TreeMaps and redirecting requests to the appropriate TreeMap (e.g., containsKey would be directed to the TreeMap that maps values to keys, containsValue would be directed to the TreeMap that maps keys to values), there are problems with that implementation, particularly when trying to keep the two TreeMaps synchronized with each other. And if the data contained in the TreeMaps is large, the cost of redundant storage becomes significant.

This solution keeps the data properly synchronized and minimizes the data storage. The red-black algorithm is based on TreeMap's, but has been modified to simultaneously map a tree node by key and by value. This doubles the cost of put operations (but so does using two TreeMaps), and nearly doubles the cost of remove operations (there is a savings in that the lookup of the node to be removed only has to be performed once). And since only one node contains the key and value, storage is significantly less than that required by two TreeMaps.

There are some limitations placed on data kept in this Map. The biggest one is this:

When performing a put operation, neither the key nor the value may already exist in the Map. In the java.util Map implementations (HashMap, TreeMap), you can perform a put with an already mapped key, and neither cares about duplicate values at all ... but this implementation's put method with throw an IllegalArgumentException if either the key or the value is already in the Map.

Obviously, that same restriction (and consequence of failing to heed that restriction) applies to the putAll method.

The Map.Entry instances returned by the appropriate methods will not allow setValue() and will throw an UnsupportedOperationException on attempts to call that method.

New methods are added to take advantage of the fact that values are kept sorted independently of their keys:

Object getKeyForValue(Object value) is the opposite of get; it takes a value and returns its key, if any.

Object removeValue(Object value) finds and removes the specified value and returns the now un-used key.

Set entrySetByValue() returns the Map.Entry's in a Set whose iterator will iterate over the Map.Entry's in ascending order by their corresponding values.

Set keySetByValue() returns the keys in a Set whose iterator will iterate over the keys in ascending order by their corresponding values.

Collection valuesByValue() returns the values in a Collection whose iterator will iterate over the values in ascending order.

Since:

2.0

Author:

Marc Johnson (marcj at users dot sourceforge dot net)

Constructor Summary

DoubleOrderedMap()
Construct a new DoubleOrderedMap

DoubleOrderedMap(java.util.Map map)
Constructs a new DoubleOrderedMap from an existing Map, with keys and values sorted

Method Summary

void	clear() Removes all mappings from this map
boolean	containsKey(java.lang.Object key) Returns true if this map contains a mapping for the specified key.
boolean	containsValue(java.lang.Object value) Returns true if this map maps one or more keys to the specified value.
java.util.Set	entrySet() Returns a set view of the mappings contained in this map.
java.util.Set	entrySetByValue() Returns a set view of the mappings contained in this map.
java.lang.Object	get(java.lang.Object key) Returns the value to which this map maps the specified key.
java.lang.Object	getKeyForValue(java.lang.Object value) Returns the key to which this map maps the specified value.
java.util.Set	keySet() Returns a set view of the keys contained in this map.
java.util.Set	keySetByValue() Returns a set view of the keys contained in this map.
java.lang.Object	put(java.lang.Object key, java.lang.Object value) Associates the specified value with the specified key in this map.
java.lang.Object	remove(java.lang.Object key) Removes the mapping for this key from this map if present
java.lang.Object	removeValue(java.lang.Object value) Removes the mapping for this value from this map if present
int	size() Returns the number of key-value mappings in this map.
java.util.Collection	values() Returns a collection view of the values contained in this map.
java.util.Collection	valuesByValue() Returns a collection view of the values contained in this map.

Methods inherited from class java.util.AbstractMap

equals, hashCode, isEmpty, putAll, toString

Methods inherited from class java.lang.Object

getClass, notify, notifyAll, wait, wait, wait

Constructor Detail

DoubleOrderedMap

public DoubleOrderedMap()

Construct a new DoubleOrderedMap

DoubleOrderedMap

public DoubleOrderedMap(java.util.Map map)
                 throws java.lang.ClassCastException,
                        java.lang.NullPointerException,
                        java.lang.IllegalArgumentException

Constructs a new DoubleOrderedMap from an existing Map, with keys and values sorted

Parameters:

map - the map whose mappings are to be placed in this map.

Throws:

java.lang.ClassCastException - if the keys in the map are not Comparable, or are not mutually comparable; also if the values in the map are not Comparable, or are not mutually Comparable

java.lang.NullPointerException - if any key or value in the map is null

java.lang.IllegalArgumentException - if there are duplicate keys or duplicate values in the map

Method Detail

getKeyForValue

public java.lang.Object getKeyForValue(java.lang.Object value)
                                throws java.lang.ClassCastException,
                                       java.lang.NullPointerException

Returns the key to which this map maps the specified value. Returns null if the map contains no mapping for this value.

Parameters:

value - value whose associated key is to be returned.

Returns:

the key to which this map maps the specified value, or null if the map contains no mapping for this value.

Throws:

java.lang.ClassCastException - if the value is of an inappropriate type for this map.

java.lang.NullPointerException - if the value is null

removeValue

public java.lang.Object removeValue(java.lang.Object value)

Removes the mapping for this value from this map if present

Parameters:

value - value whose mapping is to be removed from the map.

Returns:

previous key associated with specified value, or null if there was no mapping for value.

entrySetByValue

public java.util.Set entrySetByValue()

Returns a set view of the mappings contained in this map. Each element in the returned set is a Map.Entry. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress, the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations.

The difference between this method and entrySet is that entrySet's iterator() method returns an iterator that iterates over the mappings in ascending order by key. This method's iterator method iterates over the mappings in ascending order by value.

Returns:

a set view of the mappings contained in this map.

keySetByValue

public java.util.Set keySetByValue()

Returns a set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress, the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

The difference between this method and keySet is that keySet's iterator() method returns an iterator that iterates over the keys in ascending order by key. This method's iterator method iterates over the keys in ascending order by value.

Returns:

a set view of the keys contained in this map.

valuesByValue

public java.util.Collection valuesByValue()

Returns a collection view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. If the map is modified while an iteration over the collection is in progress, the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Collection.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations.

The difference between this method and values is that values's iterator() method returns an iterator that iterates over the values in ascending order by key. This method's iterator method iterates over the values in ascending order by key.

Returns:

a collection view of the values contained in this map.

size

public int size()

Returns the number of key-value mappings in this map. If the map contains more than Integer.MAXVALUE elements, returns Integer.MAXVALUE.

Specified by:

size in interface java.util.Map

Overrides:

size in class java.util.AbstractMap

Returns:

the number of key-value mappings in this map.

containsKey

public boolean containsKey(java.lang.Object key)
                    throws java.lang.ClassCastException,
                           java.lang.NullPointerException

Returns true if this map contains a mapping for the specified key.

Specified by:

containsKey in interface java.util.Map

Overrides:

containsKey in class java.util.AbstractMap

Parameters:

key - key whose presence in this map is to be tested.

Returns:

true if this map contains a mapping for the specified key.

Throws:

java.lang.ClassCastException - if the key is of an inappropriate type for this map.

java.lang.NullPointerException - if the key is null

containsValue

public boolean containsValue(java.lang.Object value)

Returns true if this map maps one or more keys to the specified value.

Specified by:

containsValue in interface java.util.Map

Overrides:

containsValue in class java.util.AbstractMap

Parameters:

value - value whose presence in this map is to be tested.

Returns:

true if this map maps one or more keys to the specified value.

get

public java.lang.Object get(java.lang.Object key)
                     throws java.lang.ClassCastException,
                            java.lang.NullPointerException

Returns the value to which this map maps the specified key. Returns null if the map contains no mapping for this key.

Specified by:

get in interface java.util.Map

Overrides:

get in class java.util.AbstractMap

Parameters:

key - key whose associated value is to be returned.

Returns:

the value to which this map maps the specified key, or null if the map contains no mapping for this key.

Throws:

java.lang.ClassCastException - if the key is of an inappropriate type for this map.

java.lang.NullPointerException - if the key is null

put

public java.lang.Object put(java.lang.Object key,
                            java.lang.Object value)
                     throws java.lang.ClassCastException,
                            java.lang.NullPointerException,
                            java.lang.IllegalArgumentException

Associates the specified value with the specified key in this map.

Specified by:

put in interface java.util.Map

Overrides:

put in class java.util.AbstractMap

Parameters:

key - key with which the specified value is to be associated.

value - value to be associated with the specified key.

Returns:

null

Throws:

java.lang.ClassCastException - if the class of the specified key or value prevents it from being stored in this map.

java.lang.NullPointerException - if the specified key or value is null

java.lang.IllegalArgumentException - if the key duplicates an existing key, or if the value duplicates an existing value

remove

public java.lang.Object remove(java.lang.Object key)

Removes the mapping for this key from this map if present

Specified by:

remove in interface java.util.Map

Overrides:

remove in class java.util.AbstractMap

Parameters:

key - key whose mapping is to be removed from the map.

Returns:

previous value associated with specified key, or null if there was no mapping for key.

clear

public void clear()

Removes all mappings from this map

Specified by:

clear in interface java.util.Map

Overrides:

clear in class java.util.AbstractMap

keySet

public java.util.Set keySet()

Returns a set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress, the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

Specified by:

keySet in interface java.util.Map

Overrides:

keySet in class java.util.AbstractMap

Returns:

a set view of the keys contained in this map.

values

public java.util.Collection values()

Returns a collection view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. If the map is modified while an iteration over the collection is in progress, the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Collection.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations.

Specified by:

values in interface java.util.Map

Overrides:

values in class java.util.AbstractMap

Returns:

a collection view of the values contained in this map.

entrySet

public java.util.Set entrySet()

Returns a set view of the mappings contained in this map. Each element in the returned set is a Map.Entry. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress, the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations.

Specified by:

entrySet in interface java.util.Map

Specified by:

entrySet in class java.util.AbstractMap

Returns:

a set view of the mappings contained in this map.