| JavaTM 2 Platform Std. Ed. v1.6.0
java.util
Class HashMap<K,V>
java.lang.Object
 java.util.AbstractMap<K,V>
java.util.HashMap<K,V>
- Type Parameters:
K - the type of keys maintained by this mapV - the type of mapped values
- 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 hash table is rehashed (that is, internal data
structures are rebuilt) so that the hash table has approximately twice the
number of buckets.
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 a hash 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 method, 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 of the 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 Set view of the mappings contained in this map. |
V |
get(Object key)
Returns the value to which the specified key is mapped,
or null if this map contains no mapping for the 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. |
V |
remove(Object key)
Removes the mapping for the specified 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 capacityloadFactor - 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,
or
null if this map contains no mapping for the key.
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.)
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.
- 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 the specified key is mapped, or
null if this map contains no mapping for the 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 the 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 associatedvalue - value to be associated with the specified key
- Returns:
- the previous value associated with key, or
null if there was no mapping for key.
(A null return can also indicate that the map
previously associated null with 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 the specified 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:
- the previous value associated with key, or
null if there was no mapping for key.
(A null return can also indicate that the map
previously associated null with key.)
clear
public void clear()
- Removes all of the mappings from this map.
The map will be empty after this call returns.
- 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. If the map is modified
while an iteration over the set is in progress (except through
the iterator's own remove operation), 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 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. If the map is
modified while an iteration over the collection is in progress
(except through the iterator's own remove operation),
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 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
Set view of the mappings 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 (except through
the iterator's own remove operation, or through the
setValue operation on a map entry returned by the
iterator) 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 Map<K,V>- Specified by:
entrySet in class AbstractMap<K,V>
- Returns:
- a set view of the mappings contained in this map
Copyright 2003 Sun Microsystems, Inc. All rights reserved
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