java::util::Map Class Reference

Inheritance diagram for java::util::Map:

Inheritance graph
java::util::AbstractMapjava::util::SortedMapjava::lang::Interfacejava::lang::Objectjava::util::HashMapjava::util::TreeMap
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Collaboration diagram for java::util::Map:

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java::lang::Interfacejava::lang::Object
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List of all members.


Detailed Description

An object that maps keys to values.

A map cannot contain duplicate keys; each key can map to at most one value.

This interface takes the place of the Dictionary class, which was a totally abstract class rather than an interface.

The Map interface provides three collection views, which allow a map's contents to be viewed as a set of keys, collection of values, or set of key-value mappings. The order of a map is defined as the order in which the iterators on the map's collection views return their elements. Some map implementations, like the TreeMap class, make specific guarantees as to their order; others, like the HashMap class, do not.

Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map. A special case of this prohibition is that it is not permissible for a map to contain itself as a key. While it is permissible for a map to contain itself as a value, extreme caution is advised: the equals and hashCode methods are no longer well defined on a such a map.

All general-purpose map implementation classes should provide two "standard" constructors: a void (no arguments) constructor which creates an empty map, and a constructor with a single argument of type Map, which creates a new map with the same key-value mappings as its argument. In effect, the latter constructor allows the user to copy any map, producing an equivalent map of the desired class. There is no way to enforce this recommendation (as interfaces cannot contain constructors) but all of the general-purpose map implementations in the SDK comply.

The "destructive" methods contained in this interface, that is, the methods that modify the map on which they operate, are specified to throw UnsupportedOperationException if this map does not support the operation. If this is the case, these methods may, but are not required to, throw an UnsupportedOperationException if the invocation would have no effect on the map. For example, invoking the putAll(Map) method on an unmodifiable map may, but is not required to, throw the exception if the map whose mappings are to be "superimposed" is empty.

Some map implementations have restrictions on the keys and values they may contain. For example, some implementations prohibit null keys and values, and some have restrictions on the types of their keys. Attempting to insert an ineligible key or value throws an unchecked exception, typically NullPointerException or ClassCastException. Attempting to query the presence of an ineligible key or value may throw an exception, or it may simply return false; some implementations will exhibit the former behavior and some will exhibit the latter. More generally, attempting an operation on an ineligible key or value whose completion would not result in the insertion of an ineligible element into the map may throw an exception or it may succeed, at the option of the implementation. Such exceptions are marked as "optional" in the specification for this interface.

This interface is a member of the Java Collections Framework.

Author:
Josh Bloch
Version:
1.39, 01/23/03
See also:
HashMap

TreeMap

Hashtable

SortedMap

Collection

Set

Since:
1.2

Public Member Functions

virtual jint size () const =0
 Returns the number of key-value mappings in this map.
virtual jboolean isEmpty () const =0
 Returns true if this map contains no key-value mappings.
virtual jboolean containsKey (const ObjectRef &key) const =0
 Returns true if this map contains a mapping for the specified key.
virtual jboolean containsValue (const ObjectRef &value) const =0
 Returns true if this map maps one or more keys to the specified value.
virtual ObjectRef get (const ObjectRef &key) const =0
 Returns the value to which this map maps the specified key.
virtual ObjectRef put (const ObjectRef &key, const ObjectRef &value)=0
 Associates the specified value with the specified key in this map (optional operation).
virtual ObjectRef remove (const ObjectRef &key)=0
 Removes the mapping for this key from this map if it is present (optional operation).
virtual void putAll (const Map &t)=0
 Copies all of the mappings from the specified map to this map (optional operation).
virtual void clear ()=0
 Removes all mappings from this map (optional operation).
virtual Ref< SetkeySet () const =0
 Returns a set view of the keys contained in this map.
virtual Ref< Collectionvalues () const =0
 Returns a collection view of the values contained in this map.
virtual Ref< SetentrySet () const =0
 Returns a set view of the mappings contained in this map.

Static Public Member Functions

static Ref< MapemptyMap ()
 The empty map (immutable).
static Ref< MapsingletonMap (const ObjectRef &key, const ObjectRef &value)
 Returns an immutable map, mapping only the specified key to the specified value.
static Ref< MapsynchronizedMap (const Ref< Map > &map, const Ref< Sync > &sync=0)
 Returns a synchronized (thread-safe) map backed by the specified map.
static Ref< MapimmutableMap (const Ref< Map > &map)
 Returns an unmodifiable view of the specified map.

Classes

class  Entry
 A map entry (key-value pair). More...

Member Function Documentation

static Ref<Map> java::util::Map::emptyMap (  )  [static]

The empty map (immutable).

static Ref<Map> java::util::Map::singletonMap ( const ObjectRef key,
const ObjectRef value 
) [static]

Returns an immutable map, mapping only the specified key to the specified value.

Parameters:
key the sole key to be stored in the returned map.
value the value to which the returned map maps key.
Returns:
an immutable map containing only the specified key-value mapping.
Since:
1.3

static Ref<Map> java::util::Map::synchronizedMap ( const Ref< Map > &  map,
const Ref< Sync > &  sync = 0 
) [static]

Returns a synchronized (thread-safe) map backed by the specified map.

In order to guarantee serial access, it is critical that all access to the backing map is accomplished through the returned map.

It is imperative that the user manually synchronize on the returned map when iterating over any of its collection views:

  Map m = Collections.synchronizedMap(new HashMap());
      ...
  Set s = m.keySet();  // Needn't be in synchronized block
      ...
  synchronized(m) {  // Synchronizing on m, not s!
      Iterator i = s.iterator(); // Must be in synchronized block
      while (i.hasNext())
          foo(i.next());
  }
 
Failure to follow this advice may result in non-deterministic behavior.

The returned map will be serializable if the specified map is serializable.

Parameters:
m the map to be "wrapped" in a synchronized map.
Returns:
a synchronized view of the specified map.

static Ref<Map> java::util::Map::immutableMap ( const Ref< Map > &  map  )  [static]

Returns an unmodifiable view of the specified map.

This method allows modules to provide users with "read-only" access to internal maps. Query operations on the returned map "read through" to the specified map, and attempts to modify the returned map, whether direct or via its collection views, result in an UnsupportedOperationException.

Parameters:
m the map for which an unmodifiable view is to be returned.
Returns:
an unmodifiable view of the specified map.

virtual jint java::util::Map::size (  )  const [pure virtual]

Returns the number of key-value mappings in this map.

If the map contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.

Returns:
the number of key-value mappings in this map.

Implemented in java::util::AbstractMap, java::util::HashMap, and java::util::TreeMap.

virtual jboolean java::util::Map::isEmpty (  )  const [pure virtual]

Returns true if this map contains no key-value mappings.

Returns:
true if this map contains no key-value mappings.

Implemented in java::util::AbstractMap, java::util::HashMap, and java::util::TreeMap.

virtual jboolean java::util::Map::containsKey ( const ObjectRef key  )  const [pure virtual]

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

More formally, returns true if and only if this map contains at a mapping for a key k such that (key==null ? k==null : key.equals(k)). (There can be at most one such mapping.)

Parameters:
key key whose presence in this map is to be tested.
Returns:
true if this map contains a mapping for the specified key.
Exceptions:
ClassCastException if the key is of an inappropriate type for this map (optional).
NullPointerException if the key is null and this map does not not permit null keys (optional).

Implemented in java::util::AbstractMap, java::util::HashMap, and java::util::TreeMap.

virtual jboolean java::util::Map::containsValue ( const ObjectRef value  )  const [pure virtual]

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

More formally, returns true if and only if this map contains at least one mapping to a value v such that (value==null ? v==null : value.equals(v)). This operation will probably require time linear in the map size for most implementations of the Map interface.

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.
Exceptions:
ClassCastException if the value is of an inappropriate type for this map (optional).
NullPointerException if the value is null and this map does not not permit null values (optional).

Implemented in java::util::AbstractMap, and java::util::TreeMap.

virtual ObjectRef java::util::Map::get ( const ObjectRef key  )  const [pure virtual]

Returns the value to which this map maps the specified key.

Returns null if the map contains no mapping for this key. A return value of null does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to null. The containsKey operation may be used to distinguish these two cases.

More formally, if this map contains a mapping from a key k to a value v such that (key==null ? k==null : key.equals(k)), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)

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.
Exceptions:
ClassCastException if the key is of an inappropriate type for this map (optional).
NullPointerException key is null and this map does not not permit null keys (optional).
See also:
containsKey(Object)

Implemented in java::util::AbstractMap, java::util::HashMap, and java::util::TreeMap.

virtual ObjectRef java::util::Map::put ( const ObjectRef key,
const ObjectRef value 
) [pure virtual]

Associates the specified value with the specified key in this map (optional operation).

If the map previously contained a mapping for this key, the old value is replaced by the specified value. (A map m is said to contain a mapping for a key k if and only if m.containsKey(k) would return true.))

Parameters:
key key with which the specified value is to be associated.
value value to be associated with the specified key.
Returns:
previous value associated with specified key, or null if there was no mapping for key. A null return can also indicate that the map previously associated null with the specified key, if the implementation supports null values.
Exceptions:
UnsupportedOperationException if the put operation is not supported by this map.
ClassCastException if the class of the specified key or value prevents it from being stored in this map.
IllegalArgumentException if some aspect of this key or value prevents it from being stored in this map.
NullPointerException this map does not permit null keys or values, and the specified key or value is null.

Implemented in java::util::AbstractMap, java::util::HashMap, and java::util::TreeMap.

virtual ObjectRef java::util::Map::remove ( const ObjectRef key  )  [pure virtual]

Removes the mapping for this key from this map if it is present (optional operation).

More formally, if this map contains a mapping from key k to value v such that (key==null ? k==null : key.equals(k)), that mapping is removed. (The map can contain at most one such mapping.)

Returns the value to which the map previously associated the key, or null if the map contained no mapping for this key. (A null return can also indicate that the map previously associated null with the specified key if the implementation supports null values.) The map will not contain a mapping for the specified key once the call returns.

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.
Exceptions:
ClassCastException if the key is of an inappropriate type for this map (optional).
NullPointerException if the key is null and this map does not not permit null keys (optional).
UnsupportedOperationException if the remove method is not supported by this map.

Implemented in java::util::AbstractMap, java::util::HashMap, and java::util::TreeMap.

virtual void java::util::Map::putAll ( const Map t  )  [pure virtual]

Copies all of the mappings from the specified map to this map (optional operation).

The effect of this call is equivalent to that of calling put(k, v) on this map once for each mapping from key k to value v in the specified map. The behavior of this operation is unspecified if the specified map is modified while the operation is in progress.

Parameters:
t Mappings to be stored in this map.
Exceptions:
UnsupportedOperationException if the putAll method is not supported by this map.
ClassCastException if the class of a key or value in the specified map prevents it from being stored in this map.
IllegalArgumentException some aspect of a key or value in the specified map prevents it from being stored in this map.
NullPointerException the specified map is null, or if this map does not permit null keys or values, and the specified map contains null keys or values.

Implemented in java::util::AbstractMap, java::util::HashMap, and java::util::TreeMap.

virtual void java::util::Map::clear (  )  [pure virtual]

Removes all mappings from this map (optional operation).

Exceptions:
UnsupportedOperationException clear is not supported by this map.

Implemented in java::util::AbstractMap, java::util::HashMap, and java::util::TreeMap.

virtual Ref<Set> java::util::Map::keySet (  )  const [pure virtual]

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.

Returns:
a set view of the keys contained in this map.

Implemented in java::util::AbstractMap, and java::util::TreeMap.

virtual Ref<Collection> java::util::Map::values (  )  const [pure virtual]

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.

Returns:
a collection view of the values contained in this map.

Implemented in java::util::AbstractMap, and java::util::TreeMap.

virtual Ref<Set> java::util::Map::entrySet (  )  const [pure virtual]

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.

Returns:
a set view of the mappings contained in this map.

Implemented in java::util::HashMap, and java::util::TreeMap.


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