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Java Platform 1.2 Beta 4 |
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SUMMARY: INNER | FIELD | CONSTR | METHOD | DETAIL: FIELD | CONSTR | METHOD |
java.lang.Object | +--java.util.AbstractMap | +--java.util.HashMap
This implementation provides constant-time performance for the basic operations (get and put), assuming the the hash function disperses the elements properly among the buckets. Iteration over Collection views requires time proportional to the "capacity" of the HashMap (the number of buckets) plus its size (the number of key-value mappings). Thus, it's very important not to set the intial capacity too high (or the load factor too low) if iteration performance is important.
An instance of HashMap has two parameters that affect its efficiency: its capacity and its load factor. The load factor should be between 0.0 (exclusive) and 1.0 (inclusive). When the number of mappings in the HashMap exceeds the product of the load factor and the current capacity, the capacity is increased by calling the rehash method. Larger load factors use memory more efficiently, at the expense of larger expected time per lookup.
If many mappings are to be stored in a HashMap, 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 HashMap concurrently, and at least one of the threads modifies the HashMap 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 is already contained in the Table is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the HashMap. If no such object exists, the HashMap should be "wrapped" using the Collections.synchronizedMap method. This is best done at creation time, to prevent accidental unsynchronized access to the HashMap:
Map m = Collections.synchronizedMap(new HashMap(...));
The Iterators returned by the iterator methods of the Collections returned by all of HashMap's "collection view methods" are fail-fast: if the HashMap 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.
Object.hashCode()
,
Collection
,
Map
,
TreeMap
,
Hashtable
, Serialized FormConstructor Summary | |
HashMap()
Constructs a new, empty HashMap with a default capacity and load factor, which is 0.75. |
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HashMap(int initialCapacity,
float loadFactor)
Constructs a new, empty HashMap with the specified initial capacity and the specified load factor. |
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HashMap(int initialCapacity)
Constructs a new, empty HashMap with the specified initial capacity and default load factor, which is 0.75. |
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HashMap(Map t)
Constructs a new HashMap with the same mappings as the given Map. |
Method Summary | |
void | clear()
Removes all mappings from this HashMap. |
Object | clone()
Returns a shallow copy of this HashMap. |
boolean | containsKey(Object key)
Returns true if this HashMap contains a mapping for the specified key. |
boolean | containsValue(Object value)
Returns true if this HashMap maps one or more keys to the specified value. |
Set | entrySet()
Returns a Collection view of the mappings contained in this HashMap. |
Object | get(Object key)
Returns the value to which this HashMap maps the specified 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 HashMap. |
Object | put(Object key,
Object value)
Associates the specified value with the specified key in this HashMap. |
void | putAll(Map t)
Copies all of the mappings from the specified Map to this HashMap These mappings will replace any mappings that this HashMap had for any of the keys currently in the specified Map. |
Object | remove(Object key)
Removes the mapping for this key from this HashMap 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 HashMap. |
Methods inherited from class java.util.AbstractMap | |
equals , hashCode , toString |
Methods inherited from class java.lang.Object | |
finalize , getClass , notify , notifyAll , wait , wait , wait |
Constructor Detail |
public HashMap(int initialCapacity, float loadFactor)
initialCapacity
- the initial capacity of the HashMap.
loadFactor
- the load factor of the HashMappublic HashMap(int initialCapacity)
initialCapacity
- the initial capacity of the HashMap.public HashMap()
public HashMap(Map t)
Method Detail |
public int size()
public boolean isEmpty()
public boolean containsValue(Object value)
value
- value whose presence in this Map is to be tested.public boolean containsKey(Object key)
key
- key whose presence in this Map is to be tested.public Object get(Object key)
key
- key whose associated value is to be returned.public Object put(Object key, Object value)
key
- key with which the specified value is to be associated.
value
- value to be associated with the specified key.public Object remove(Object key)
key
- key whose mapping is to be removed from the Map.public void putAll(Map t)
t
- Mappings to be stored in this Map.public void clear()
public Object clone()
public Set keySet()
public Collection values()
public Set entrySet()
Map.Entry
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Java Platform 1.2 Beta 4 |
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