public class HashMap extends AbstractMap implements Map, Cloneable, Serializable
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.
Object.hashCode(),
Collection,
Map,
TreeMap,
Hashtable,
Serialized Form| Constructor and Description |
|---|
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 m)
Constructs a new HashMap with the same mappings as the
specified Map.
|
| Modifier and Type | Method and Description |
|---|---|
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 |
entrySet()
Returns a collection view of the mappings contained in this map.
|
Object |
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 |
keySet()
Returns a set view of the keys contained in this map.
|
Object |
put(Object key,
Object value)
Associates the specified value with the specified key in this map.
|
void |
putAll(Map 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.
|
Object |
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 |
values()
Returns a collection view of the values contained in this map.
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equals, hashCode, toStringpublic HashMap(int initialCapacity,
float loadFactor)
initialCapacity - The initial capacity.loadFactor - The load factor.IllegalArgumentException - if the initial capacity is negative
or the load factor is nonpositive.public HashMap(int initialCapacity)
initialCapacity - the initial capacity.IllegalArgumentException - if the initial capacity is negative.public HashMap()
public HashMap(Map m)
m - the map whose mappings are to be placed in this map.NullPointerException - if the specified map is null.public int size()
size in interface Mapsize in class AbstractMappublic boolean isEmpty()
isEmpty in interface MapisEmpty in class AbstractMappublic Object get(Object key)
get in interface Mapget in class AbstractMapkey - the key whose associated value is to be returned.put(Object, Object)public boolean containsKey(Object key)
containsKey in interface MapcontainsKey in class AbstractMapkey - The key whose presence in this map is to be testedpublic Object put(Object key, Object value)
put in interface Mapput in class AbstractMapkey - key with which the specified value is to be associated.value - value to be associated with the specified key.public void putAll(Map m)
putAll in interface MapputAll in class AbstractMapm - mappings to be stored in this map.NullPointerException - if the specified map is null.public Object remove(Object key)
remove in interface Mapremove in class AbstractMapkey - key whose mapping is to be removed from the map.public void clear()
clear in interface Mapclear in class AbstractMappublic boolean containsValue(Object value)
containsValue in interface MapcontainsValue in class AbstractMapvalue - value whose presence in this map is to be tested.public Object clone()
clone in class AbstractMapCloneablepublic Set keySet()
keySet in interface MapkeySet in class AbstractMappublic Collection values()
values in interface Mapvalues in class AbstractMappublic Set entrySet()
entrySet in interface MapentrySet in class AbstractMapMap.EntryCopyright © 2012. All Rights Reserved.