K - the type of keys maintained by this mapV - the type of mapped valuespublic class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, 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 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.
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
Object.hashCode(),
Collection,
Map,
Hashtable,
Serialized FormAbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>| 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<? extends K,? extends V> m)
Constructs a new HashMap with the same mappings as the specified Map.
|
| Modifier and Type | Method and Description |
|---|---|
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. |
equals, hashCode, toStringpublic HashMap()
public HashMap(int initialCapacity)
initialCapacity - the initial capacity.IllegalArgumentException - if the initial capacity is negative.public HashMap(int initialCapacity,
float loadFactor)
initialCapacity - the initial capacityloadFactor - the load factorIllegalArgumentException - if the initial capacity is negative or the load factor is nonpositivepublic HashMap(Map<? extends K,? extends V> m)
m - the map whose mappings are to be placed in this mapNullPointerException - if the specified map is nullpublic void clear()
public Object clone()
clone in class AbstractMap<K,V>Cloneablepublic boolean containsKey(Object key)
containsKey in interface Map<K,V>containsKey in class AbstractMap<K,V>key - The key whose presence in this map is to be testedpublic boolean containsValue(Object value)
containsValue in interface Map<K,V>containsValue in class AbstractMap<K,V>value - value whose presence in this map is to be testedpublic Set<Map.Entry<K,V>> entrySet()
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.@Nullable public V get(Object key)
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.
get in interface Map<K,V>get in class AbstractMap<K,V>key - the key whose associated value is to be returnednull if this map contains no
mapping for the keyput(Object, Object)public boolean isEmpty()
public Set<K> keySet()
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.@Nullable public V put(K key, V value)
put in interface Map<K,V>put in class AbstractMap<K,V>key - key with which the specified value is to be associatedvalue - value to be associated with the specified keypublic void putAll(Map<? extends K,? extends V> m)
putAll in interface Map<K,V>putAll in class AbstractMap<K,V>m - mappings to be stored in this mapNullPointerException - if the specified map is null@Nullable public V remove(Object key)
remove in interface Map<K,V>remove in class AbstractMap<K,V>key - key whose mapping is to be removed from the mappublic int size()
public Collection<V> values()
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.