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Class com.sun.java.util.collections.HashMap

java.lang.Object
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   +----com.sun.java.util.collections.AbstractMap
           |
           +----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. (HashMap 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 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 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 hashtable 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 HashMap operations, including get and put). The expected number of entries in the HashMap 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, 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.

See Also:
hashCode, Collection, Map, TreeMap, Hashtable

Constructor Index

 o HashMap()
Constructs a new, empty HashMap with a default capacity and load factor, which is 0.75.
 o HashMap(int)
Constructs a new, empty HashMap with the specified initial capacity and default load factor, which is 0.75.
 o HashMap(int, float)
Constructs a new, empty HashMap with the specified initial capacity and the specified load factor.
 o HashMap(Map)
Constructs a new HashMap with the same mappings as the given Map.

Method Index

 o capacity()
 o clear()
Removes all mappings from this HashMap.
 o clone()
Returns a shallow copy of this HashMap.
 o containsKey(Object)
Returns true if this HashMap contains a mapping for the specified key.
 o containsValue(Object)
Returns true if this HashMap maps one or more keys to the specified value.
 o entrySet()
Returns a Collection view of the mappings contained in this HashMap.
 o get(Object)
Returns the value to which this HashMap maps the specified key.
 o isEmpty()
Returns true if this Map contains no key-value mappings.
 o keySet()
Returns a Set view of the keys contained in this HashMap.
 o loadFactor()
 o put(Object, Object)
Associates the specified value with the specified key in this HashMap.
 o putAll(Map)
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.
 o remove(Object)
Removes the mapping for this key from this HashMap if present.
 o size()
Returns the number of key-value mappings in this Map.
 o values()
Returns a Collection view of the values contained in this HashMap.

Constructors

 o HashMap
 public HashMap(int initialCapacity,
                float loadFactor)
Constructs a new, empty HashMap 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
Throws: IllegalArgumentException
if the initial capacity is less than zero, or if the load factor is nonpositive.
 o HashMap
 public HashMap(int initialCapacity)
Constructs a new, empty HashMap with the specified initial capacity and default load factor, which is 0.75.

Parameters:
initialCapacity - the initial capacity of the HashMap.
Throws: IllegalArgumentException
if the initial capacity is less than zero.
 o HashMap
 public HashMap()
Constructs a new, empty HashMap with a default capacity and load factor, which is 0.75.

 o HashMap
 public HashMap(Map t)
Constructs a new HashMap with the same mappings as the given Map. The HashMap 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.

Methods

 o 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
 o 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
 o containsValue
 public boolean containsValue(Object value)
Returns true if this HashMap 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 HashMap maps one or more keys to the specified value.
Overrides:
containsValue in class AbstractMap
 o containsKey
 public boolean containsKey(Object key)
Returns true if this HashMap contains a mapping for the specified key.

Parameters:
key - key whose presence in this Map is to be tested.
Returns:
true if this HashMap contains a mapping for the specified key.
Overrides:
containsKey in class AbstractMap
 o get
 public Object get(Object key)
Returns the value to which this HashMap maps the specified key. Returns null if the HashMap contains no mapping for this key. A return value of null does not necessarily indicate that the HashMap contains no mapping for the key; it's also possible that the HashMap 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 HashMap maps the specified key.
Overrides:
get in class AbstractMap
 o put
 public Object put(Object key,
                   Object value)
Associates the specified value with the specified key in this HashMap. If the HashMap 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
 o remove
 public Object remove(Object key)
Removes the mapping for this key from this HashMap 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 HashMap previously associated null with the specified key.
Overrides:
remove in class AbstractMap
 o putAll
 public 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.

Parameters:
t - Mappings to be stored in this Map.
Overrides:
putAll in class AbstractMap
 o clear
 public void clear()
Removes all mappings from this HashMap.

Overrides:
clear in class AbstractMap
 o clone
 public Object clone()
Returns a shallow copy of this HashMap. The keys and values themselves are not cloned.

Returns:
a shallow copy of this HashMap.
Overrides:
clone in class Object
 o keySet
 public Set keySet()
Returns a Set view of the keys contained in this HashMap. The Set is backed by the HashMap, so changes to the HashMap are reflected in the Set, and vice-versa. The Set supports element removal, which removes the corresponding mapping from the HashMap, 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 HashMap.
Overrides:
keySet in class AbstractMap
 o values
 public Collection values()
Returns a Collection view of the values contained in this HashMap. The Collection is backed by the HashMap, so changes to the HashMap are reflected in the Collection, and vice-versa. The Collection supports element removal, which removes the corresponding mapping from the HashMap, 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 HashMap.
Overrides:
values in class AbstractMap
 o entrySet
 public Set entrySet()
Returns a Collection view of the mappings contained in this HashMap. Each element in the returned collection is a Map.Entry. The Collection is backed by the HashMap, so changes to the HashMap are reflected in the Collection, and vice-versa. The Collection supports element removal, which removes the corresponding mapping from the HashMap, 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 HashMap.
Overrides:
entrySet in class AbstractMap
See Also:
Entry
 o capacity
 int capacity()
 o loadFactor
 float loadFactor()

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