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Class java.util.HashMap

java.lang.Object
    |
    +----java.util.AbstractMap
            |
            +----java.util.HashMap

public class HashMap

extends AbstractMap

implements Map, Cloneable, Serializable

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 requires time proportional to the number of buckets in the hash table plus the size of the Map (the number of key-value mappings).

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 and 1.0. When the number of entries 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 entries are to be made into a HashMap, creating it with a sufficiently large capacity will allow the entries to be inserted 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.synchronizedSet 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.

Since:

JDK1.2

See Also:

hashCode, Collection, Map, TreeMap, ArrayMap, Hashtable

HashMap()

Constructs a new, empty HashMap with a default capacity and load factor.

HashMap(int)

Constructs a new, empty HashMap with the specified initial capacity and default load factor.

HashMap(int, float)

Constructs a new, empty HashMap with the specified initial capacity and the specified load factor.

HashMap(Map)

Constructs a new HashMap with the same mappings as the given Map.

clear()

Removes all mappings from this HashMap.

clone()

Returns a shallow copy of this HashMap.

containsKey(Object)

Returns true if this HashMap contains a mapping for the specified key.

containsValue(Object)

Returns true if this HashMap maps one or more keys to the specified value.

entries()

Returns a Collection view of the mappings contained in this HashMap.

get(Object)

Returns the value to which this HashMap 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 HashMap.

put(Object, Object)

Associates the specified value with the specified key in this HashMap.

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.

remove(Object)

Removes the mapping for this key from this HashMap if present.

size()

Returns the number of key-value mappings in this Map.

values()

Returns a Collection view of the values contained in this 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 - a number between 0.0 and 1.0.

Throws: IllegalArgumentException

if the initial capacity is less than or equal to zero, or if the load factor is less than or equal to zero.

public HashMap(int initialCapacity)

Constructs a new, empty HashMap with the specified initial capacity and default load factor.

Parameters:

initialCapacity - the initial capacity of the HashMap.

public HashMap()

Constructs a new, empty HashMap with a default capacity and load factor.

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 entries in the given Map, and a default load factor.

public int size()

Returns the number of key-value mappings in this Map.

Overrides:

size in class AbstractMap

public boolean isEmpty()

Returns true if this Map contains no key-value mappings.

Overrides:

isEmpty in class AbstractMap

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.

Overrides:

containsValue in class AbstractMap

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.

Throws: NullPointerException

the specified key is null.

Overrides:

containsKey in class AbstractMap

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.

Throws: NullPointerException

the specified key is null.

Overrides:

get in class AbstractMap

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 forthis 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 Map previously associated null with the specified key, if the implementation supports null values.

Throws: NullPointerException

the specified key is null.

Overrides:

put in class AbstractMap

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

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.

Throws: NullPointerException

a key in the mapping specified by t is null.

Overrides:

putAll in class AbstractMap

public void clear()

Removes all mappings from this HashMap.

Overrides:

clear in class AbstractMap

public Object clone()

Returns a shallow copy of this HashMap. The keys and values themselves are not cloned.

Overrides:

clone in class Object

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.

Overrides:

keySet in class AbstractMap

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.

Overrides:

values in class AbstractMap

public Collection entries()

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.

Overrides:

entries in class AbstractMap

See Also:

Map.Entry

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