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Class com.sun.java.util.collections.HashMap
java.lang.Object
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+----com.sun.java.util.collections.AbstractMap
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+----com.sun.java.util.collections.HashMap
- public class HashMap
- extends AbstractMap
- implements Map, 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 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
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, te 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.
- See Also:
- hashCode, Collection, Map, TreeMap, Hashtable
-
HashMap()
- Constructs a new, empty map with a default capacity and load
factor, which is 0.75.
-
HashMap(int)
- Constructs a new, empty map with the specified initial capacity
and default load factor, which is 0.75.
-
HashMap(int, float)
- Constructs a new, empty map with the specified initial
capacity and the specified load factor.
-
HashMap(Map)
- Constructs a new map with the same mappings as the given map.
-
capacity()
-
-
clear()
- Removes all mappings from this map.
-
clone()
- Returns a shallow copy of this HashMap instance: the keys and
values themselves are not cloned.
-
containsKey(Object)
- Returns true if this map contains a mapping for the specified
key.
-
containsValue(Object)
- Returns true if this map maps one or more keys to the
specified value.
-
entrySet()
- Returns a collection view of the mappings contained in this map.
-
get(Object)
- Returns the value to which this map maps the specified key.
-
isEmpty()
- Returns true if this map contains no key-value mappings.
-
keySet()
- Returns a set view of the keys contained in this map.
-
loadFactor()
-
-
put(Object, Object)
- Associates the specified value with the specified key in this map.
-
putAll(Map)
- Copies all of the mappings from the specified map to this one.
-
remove(Object)
- Removes the mapping for this key from this map if present.
-
size()
- Returns the number of key-value mappings in this map.
-
values()
- Returns a collection view of the values contained in this map.
HashMap
public HashMap(int initialCapacity,
float loadFactor)
- Constructs a new, empty map with the specified initial
capacity and the specified load factor.
- Parameters:
- initialCapacity - the initial capacity of the HashMap.
- loadFactor - the load factor of the HashMap
HashMap
public HashMap(int initialCapacity)
- Constructs a new, empty map with the specified initial capacity
and default load factor, which is 0.75.
- Parameters:
- initialCapacity - the initial capacity of the HashMap.
HashMap
public HashMap()
- Constructs a new, empty map with a default capacity and load
factor, which is 0.75.
HashMap
public HashMap(Map t)
- Constructs a new map with the same mappings as the given map. The
map is created with a capacity of twice the number of mappings in
the given map or 11 (whichever is greater), and a default load factor,
which is 0.75.
size
public int size()
- Returns the number of key-value mappings in this map.
- Returns:
- the number of key-value mappings in this map.
- Overrides:
- size in class AbstractMap
isEmpty
public boolean isEmpty()
- Returns true if this map contains no key-value mappings.
- Returns:
- true if this map contains no key-value mappings.
- Overrides:
- isEmpty in class AbstractMap
containsValue
public boolean containsValue(Object value)
- Returns true if this map maps one or more keys to the
specified value.
- 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.
- Overrides:
- containsValue in class AbstractMap
containsKey
public boolean containsKey(Object key)
- Returns true if this map contains a mapping for the specified
key.
- Parameters:
- key - key whose presence in this Map is to be tested.
- Returns:
- true if this map contains a mapping for the specified
key.
- Overrides:
- containsKey in class AbstractMap
get
public Object get(Object key)
- Returns the value to which this map maps the specified key. Returns
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's also possible that the map
explicitly maps the key to null. The containsKey
operation may be used to distinguish these two cases.
- Parameters:
- key - key whose associated value is to be returned.
- Returns:
- the value to which this map maps the specified key.
- Overrides:
- get in class AbstractMap
put
public Object put(Object key,
Object 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.
- 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.
- Overrides:
- put in class AbstractMap
remove
public Object remove(Object key)
- Removes the mapping for this key from this map if present.
- 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.
- Overrides:
- remove in class AbstractMap
putAll
public void putAll(Map t)
- Copies all of the mappings from the specified map to this one.
These mappings replace any mappings that this map had for any of the
keys currently in the specified Map.
- Parameters:
- t - Mappings to be stored in this map.
- Overrides:
- putAll in class AbstractMap
clear
public void clear()
- Removes all mappings from this map.
- Overrides:
- clear in class AbstractMap
clone
public Object clone()
- Returns a shallow copy of this HashMap instance: the keys and
values themselves are not cloned.
- Returns:
- a shallow copy of this map.
- Overrides:
- clone in class Object
keySet
public Set 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.
- Returns:
- a set view of the keys contained in this map.
- Overrides:
- keySet in class AbstractMap
values
public Collection 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.
- Returns:
- a collection view of the values contained in this map.
- Overrides:
- values in class AbstractMap
entrySet
public Set 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.
- Returns:
- a collection view of the mappings contained in this map.
- Overrides:
- entrySet in class AbstractMap
- See Also:
- Entry
capacity
int capacity()
loadFactor
float loadFactor()
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