JavaTM 2 Platform Std. Ed. v1.5.0
java.util
Class HashMap<K,V>
java.lang.Object
java.util.AbstractMap<K,V>
java.util.HashMap<K,V>
- All Implemented Interfaces:
- Serializable, Cloneable, Map<K,V>
- Direct Known Subclasses:
- LinkedHashMap, PrinterStateReasons
public class HashMap<K,V> - extends AbstractMap<K,V>
- implements Map<K,V>, Cloneable, Serializable
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.
This class is a member of the
Java Collections Framework.
- Since:
- 1.2
- See Also:
Object.hashCode() ,
Collection ,
Map ,
TreeMap ,
Hashtable ,
Serialized Form
Constructor Summary |
HashMap()
Constructs an empty HashMap with the default initial capacity
(16) and the default load factor (0.75). |
HashMap(int initialCapacity)
Constructs an empty HashMap with the specified initial
capacity and the default load factor (0.75). |
HashMap(int initialCapacity,
float loadFactor)
Constructs an empty HashMap with the specified initial
capacity and load factor. |
HashMap(Map<? extends K,? extends V> m)
Constructs a new HashMap with the same mappings as the
specified Map. |
Method Summary |
void |
clear()
Removes all mappings from this map. |
Object |
clone()
Returns a shallow copy of this HashMap instance: the keys and
values themselves are not cloned. |
boolean |
containsKey(Object key)
Returns true if this map contains a mapping for the
specified key. |
boolean |
containsValue(Object value)
Returns true if this map maps one or more keys to the
specified value. |
Set<Map.Entry<K,V>> |
entrySet()
Returns a collection view of the mappings contained in this map. |
V |
get(Object key)
Returns the value to which the specified key is mapped in this identity
hash map, or null if the map contains no mapping for this key. |
boolean |
isEmpty()
Returns true if this map contains no key-value mappings. |
Set<K> |
keySet()
Returns a set view of the keys contained in this map. |
V |
put(K key,
V value)
Associates the specified value with the specified key in this map. |
void |
putAll(Map<? extends K,? extends V> m)
Copies all of the mappings from the specified map to this map
These mappings will replace any mappings that
this map had for any of the keys currently in the specified map. |
V |
remove(Object key)
Removes the mapping for this key from this map if present. |
int |
size()
Returns the number of key-value mappings in this map. |
Collection<V> |
values()
Returns a collection view of the values contained in this map. |
HashMap
public HashMap(int initialCapacity,
float loadFactor)
- Constructs an empty HashMap with the specified initial
capacity and load factor.
- Parameters:
initialCapacity - The initial capacity.loadFactor - The load factor.
- Throws:
IllegalArgumentException - if the initial capacity is negative
or the load factor is nonpositive.
HashMap
public HashMap(int initialCapacity)
- Constructs an empty HashMap with the specified initial
capacity and the default load factor (0.75).
- Parameters:
initialCapacity - the initial capacity.
- Throws:
IllegalArgumentException - if the initial capacity is negative.
HashMap
public HashMap()
- Constructs an empty HashMap with the default initial capacity
(16) and the default load factor (0.75).
HashMap
public HashMap(Map<? extends K,? extends V> m)
- Constructs a new HashMap with the same mappings as the
specified Map. The HashMap is created with
default load factor (0.75) and an initial capacity sufficient to
hold the mappings in the specified Map.
- Parameters:
m - the map whose mappings are to be placed in this map.
- Throws:
NullPointerException - if the specified map is null.
size
public int size()
- Returns the number of key-value mappings in this map.
- Specified by:
size in interface Map<K,V> - Overrides:
size in class AbstractMap<K,V>
- Returns:
- the number of key-value mappings in this map.
isEmpty
public boolean isEmpty()
- Returns true if this map contains no key-value mappings.
- Specified by:
isEmpty in interface Map<K,V> - Overrides:
isEmpty in class AbstractMap<K,V>
- Returns:
- true if this map contains no key-value mappings.
get
public V get(Object key)
- Returns the value to which the specified key is mapped in this identity
hash map, or 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 is also possible that
the map explicitly maps the key to null. The
containsKey method may be used to distinguish these two cases.
- Specified by:
get in interface Map<K,V> - Overrides:
get in class AbstractMap<K,V>
- Parameters:
key - the 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.
- See Also:
put(Object, Object)
containsKey
public boolean containsKey(Object key)
- Returns true if this map contains a mapping for the
specified key.
- Specified by:
containsKey in interface Map<K,V> - Overrides:
containsKey in class AbstractMap<K,V>
- Parameters:
key - The key whose presence in this map is to be tested
- Returns:
- true if this map contains a mapping for the specified
key.
put
public V put(K key,
V value)
- Associates the specified value with the specified key in this map.
If the map previously contained a mapping for this key, the old
value is replaced.
- Specified by:
put in interface Map<K,V> - Overrides:
put in class AbstractMap<K,V>
- 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 HashMap previously associated
null with the specified key.
putAll
public void putAll(Map<? extends K,? extends V> m)
- Copies all of the mappings from the specified map to this map
These mappings will replace any mappings that
this map had for any of the keys currently in the specified map.
- Specified by:
putAll in interface Map<K,V> - Overrides:
putAll in class AbstractMap<K,V>
- Parameters:
m - mappings to be stored in this map.
- Throws:
NullPointerException - if the specified map is null.
remove
public V remove(Object key)
- Removes the mapping for this key from this map if present.
- Specified by:
remove in interface Map<K,V> - Overrides:
remove in class AbstractMap<K,V>
- 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. A null return can
also indicate that the map previously associated null
with the specified key.
clear
public void clear()
- Removes all mappings from this map.
- Specified by:
clear in interface Map<K,V> - Overrides:
clear in class AbstractMap<K,V>
containsValue
public boolean containsValue(Object value)
- Returns true if this map maps one or more keys to the
specified value.
- Specified by:
containsValue in interface Map<K,V> - Overrides:
containsValue in class AbstractMap<K,V>
- 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.
clone
public Object clone()
- Returns a shallow copy of this HashMap instance: the keys and
values themselves are not cloned.
- Overrides:
clone in class AbstractMap<K,V>
- Returns:
- a shallow copy of this map.
- See Also:
Cloneable
keySet
public Set<K> 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. The set supports element removal, which removes the
corresponding mapping from this 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 Map<K,V> - Overrides:
keySet in class AbstractMap<K,V>
- Returns:
- a set view of the keys contained in this map.
values
public Collection<V> 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. The collection supports element
removal, which removes the corresponding mapping from this 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 Map<K,V> - Overrides:
values in class AbstractMap<K,V>
- Returns:
- a collection view of the values contained in this map.
entrySet
public Set<Map.Entry<K,V>> entrySet()
- Returns a collection view of the mappings contained in this map. Each
element in the returned collection is a Map.Entry. The
collection is backed by the map, so changes to the map are reflected in
the collection, and vice-versa. 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:
entrySet in interface Map<K,V> - Specified by:
entrySet in class AbstractMap<K,V>
- Returns:
- a collection view of the mappings contained in this map.
- See Also:
Map.Entry
Copyright 2003 Sun Microsystems, Inc. All rights reserved
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