java::util::HashMap Class Reference

Inheritance diagram for java::util::HashMap:

Inheritance graph
java::util::AbstractMapjava::util::Mapjava::lang::Interfacejava::lang::Object
[legend]
Collaboration diagram for java::util::HashMap:

Collaboration graph
java::util::AbstractMapjava::util::Mapjava::lang::Interfacejava::lang::Comparablejava::lang::Objectjava::lang::ObjectRef
[legend]

List of all members.


Detailed Description

Hash table based implementation of the Map interface.

This implementation provides all of the optional map operations, and permits null values and the null key. (The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.) This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.

This implementation provides constant-time performance for the basic operations (get and put), assuming the hash function disperses the elements properly among the buckets. Iteration over collection views requires time proportional to the "capacity" of the HashMap instance (the number of buckets) plus its size (the number of key-value mappings). Thus, it's very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.

An instance of HashMap has two parameters that affect its performance: initial capacity and load factor. The capacity is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created. The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the capacity is roughly doubled by calling the rehash method.

As a general rule, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the lookup cost (reflected in most of the operations of the HashMap class, including get and put). The expected number of entries in the map and its load factor should be taken into account when setting its initial capacity, so as to minimize the number of rehash operations. If the initial capacity is greater than the maximum number of entries divided by the load factor, no rehash operations will ever occur.

If many mappings are to be stored in a HashMap instance, creating it with a sufficiently large capacity will allow the mappings to be stored more efficiently than letting it perform automatic rehashing as needed to grow the table.

Note that this implementation is not synchronized. If multiple threads access this map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more mappings; merely changing the value associated with a key that an instance already contains is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the map. If no such object exists, the map should be "wrapped" using the Collections.synchronizedMap method. This is best done at creation time, to prevent accidental unsynchronized access to the map:

 Map m = Collections.synchronizedMap(new HashMap(...));
 

The iterators returned by all of this class's "collection view methods" are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

Author:
Doug Lea

Josh Bloch

Arthur van Hoff

Version:
1.51, 01/24/02
See also:
Object::hashCode()

Collection

Map

TreeMap

Hashtable

Since:
1.2

Public Member Functions

 HashMap (jint initialCapacity=DEFAULT_INITIAL_CAPACITY, jfloat loadFactor=DEFAULT_LOAD_FACTOR)
 HashMap (const Map &m)
 Constructs an empty HashMap with the specified initial capacity and the default load factor (0.75).
virtual jint size () const
 Returns the number of elements in this collection.
virtual jboolean isEmpty () const
 Returns true if this collection contains no elements.
virtual jboolean containsKey (const ObjectRef &key) const
 Returns true if this map contains a mapping for the specified key.
virtual ObjectRef get (const ObjectRef &key) const
 Returns the value to which this map maps the specified key.
virtual ObjectRef put (const ObjectRef &key, const ObjectRef &value)
 Associates the specified value with the specified key in this map (optional operation).
virtual ObjectRef remove (const ObjectRef &key)
 Removes the mapping for this key from this map if it is present (optional operation).
virtual void putAll (const Map &t)
 Copies all of the mappings from the specified map to this map (optional operation).
virtual void clear ()
 Removes all mappings from this map (optional operation).
virtual Ref< SetentrySet () const
 Returns a set view of the mappings contained in this map.

Static Public Attributes

static const jint DEFAULT_INITIAL_CAPACITY
 The default initial capacity - MUST be a power of two.
static const jint MAXIMUM_CAPACITY
 The maximum capacity, used if a higher value is implicitly specified by either of the constructors with arguments.
static const jfloat DEFAULT_LOAD_FACTOR
 The load fast used when none specified in constructor.

Constructor & Destructor Documentation

java::util::HashMap::HashMap ( jint  initialCapacity = DEFAULT_INITIAL_CAPACITY,
jfloat  loadFactor = DEFAULT_LOAD_FACTOR 
)

java::util::HashMap::HashMap ( const Map m  ) 

Constructs an empty HashMap with the specified initial capacity and the default load factor (0.75).

Parameters:
initialCapacity the initial capacity.
Exceptions:
IllegalArgumentException if the initial capacity is negative.


Member Function Documentation

virtual jint java::util::HashMap::size (  )  const [virtual]

Returns the number of elements in this collection.

If this collection contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.

Returns:
the number of elements in this collection

Reimplemented from java::util::AbstractMap.

virtual jboolean java::util::HashMap::isEmpty (  )  const [virtual]

Returns true if this collection contains no elements.

This implementation returns size() == 0.

Returns:
true if this collection contains no elements.

Reimplemented from java::util::AbstractMap.

virtual jboolean java::util::HashMap::containsKey ( const ObjectRef key  )  const [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).

Reimplemented from java::util::AbstractMap.

virtual ObjectRef java::util::HashMap::get ( const ObjectRef key  )  const [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)

Reimplemented from java::util::AbstractMap.

virtual ObjectRef java::util::HashMap::put ( const ObjectRef key,
const ObjectRef value 
) [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.

Reimplemented from java::util::AbstractMap.

virtual ObjectRef java::util::HashMap::remove ( const ObjectRef key  )  [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.

Reimplemented from java::util::AbstractMap.

virtual void java::util::HashMap::putAll ( const Map t  )  [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.

Reimplemented from java::util::AbstractMap.

virtual void java::util::HashMap::clear (  )  [virtual]

Removes all mappings from this map (optional operation).

Exceptions:
UnsupportedOperationException clear is not supported by this map.

Reimplemented from java::util::AbstractMap.

virtual Ref<Set> java::util::HashMap::entrySet (  )  const [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.

Implements java::util::Map.


Member Data Documentation

const jint java::util::HashMap::DEFAULT_INITIAL_CAPACITY [static]

The default initial capacity - MUST be a power of two.

const jint java::util::HashMap::MAXIMUM_CAPACITY [static]

The maximum capacity, used if a higher value is implicitly specified by either of the constructors with arguments.

MUST be a power of two <= 1<<30.

const jfloat java::util::HashMap::DEFAULT_LOAD_FACTOR [static]

The load fast used when none specified in constructor.


The documentation for this class was generated from the following file:
Generated on Fri May 16 11:56:48 2008 for CrossPlatformJavaLikeC++API by  doxygen 1.5.3