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Java Source Code / Java Documentation » 6.0 JDK Modules » j2me » java.util 
Source Cross Reference  Class Diagram Java Document (Java Doc) 


java.lang.Object
   java.util.AbstractMap
      java.util.HashMap

All known Subclasses:   java.util.LinkedHashMap,
HashMap
public class HashMap extends AbstractMap implements Map,Cloneable,Serializable(Code)
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 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 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 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.

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.
author:
   Doug Lea
author:
   Josh Bloch
author:
   Arthur van Hoff
version:
   1.38, 02/02/00
See Also:   Object.hashCode
See Also:   Collection
See Also:   Map
See Also:   TreeMap
See Also:   Hashtable
since:
   1.2


Inner Class :static class Entry implements Map.Entry

Field Summary
final static  intDEFAULT_INITIAL_CAPACITY
     The default initial capacity - MUST be a power of two.
final static  floatDEFAULT_LOAD_FACTOR
     The load factor used when none specified in constructor.
final static  intMAXIMUM_CAPACITY
     The maximum capacity, used if a higher value is implicitly specified by either of the constructors with arguments.
final static  ObjectNULL_KEY
     Value representing null keys inside tables.
final  floatloadFactor
     The load factor for the hash table.
transient volatile  intmodCount
     The number of times this HashMap has been structurally modified Structural modifications are those that change the number of mappings in the HashMap or otherwise modify its internal structure (e.g., rehash).
transient  intsize
     The number of key-value mappings contained in this identity hash map.
transient  Entry[]table
     The table, resized as necessary.
 intthreshold
     The next size value at which to resize (capacity * load factor).

Constructor Summary
public  HashMap(int initialCapacity, float loadFactor)
     Constructs an empty HashMap with the specified initial capacity and load factor.
public  HashMap(int initialCapacity)
     Constructs an empty HashMap with the specified initial capacity and the default load factor (0.75).
public  HashMap()
     Constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75).
public  HashMap(Map m)
     Constructs a new HashMap with the same mappings as the specified Map.

Method Summary
 voidaddEntry(int hash, Object key, Object value, int bucketIndex)
     Add a new entry with the specified key, value and hash code to the specified bucket.
 intcapacity()
    
public  voidclear()
     Removes all mappings from this map.
public  Objectclone()
     Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned.
public  booleancontainsKey(Object key)
     Returns true if this map contains a mapping for the specified key.
public  booleancontainsValue(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.
 voidcreateEntry(int hash, Object key, Object value, int bucketIndex)
     Like addEntry except that this version is used when creating entries as part of Map construction or "pseudo-construction" (cloning, deserialization).
public  SetentrySet()
     Returns a collection view of the mappings contained in this map.
static  booleaneq(Object x, Object y)
     Check for equality of non-null reference x and possibly-null y.
public  Objectget(Object key)
     Returns the value to which the specified key is mapped in this identity hash map, or 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 is also possible that the map explicitly maps the key to null.
 EntrygetEntry(Object key)
     Returns the entry associated with the specified key in the HashMap.
static  inthash(Object x)
     Returns a hash value for the specified object.
static  intindexFor(int h, int length)
     Returns index for hash code h.
 voidinit()
     Initialization hook for subclasses.
public  booleanisEmpty()
     Returns true if this map contains no key-value mappings.
public  SetkeySet()
     Returns a set view of the keys contained in this map.
 floatloadFactor()
    
static  ObjectmaskNull(Object key)
     Returns internal representation for key.
 IteratornewEntryIterator()
    
 IteratornewKeyIterator()
    
 IteratornewValueIterator()
    
public  Objectput(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.
Parameters:
  value - value to be associated with the specified key.
public  voidputAll(Map m)
     Copies all of the mappings from the specified map to this map These mappings will replace any mappings that this map had for any of the keys currently in the specified map.
 voidputAllForCreate(Map m)
    
public  Objectremove(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.
 EntryremoveEntryForKey(Object key)
     Removes and returns the entry associated with the specified key in the HashMap.
 EntryremoveMapping(Object o)
     Special version of remove for EntrySet.
 voidresize(int newCapacity)
     Rehashes the contents of this map into a new array with a larger capacity.
public  intsize()
     Returns the number of key-value mappings in this map.
 voidtransfer(Entry[] newTable)
     Transfer all entries from current table to newTable.
static  ObjectunmaskNull(Object key)
     Returns key represented by specified internal representation.
public  Collectionvalues()
     Returns a collection view of the values contained in this map.

Field Detail
DEFAULT_INITIAL_CAPACITY
final static int DEFAULT_INITIAL_CAPACITY(Code)
The default initial capacity - MUST be a power of two.



DEFAULT_LOAD_FACTOR
final static float DEFAULT_LOAD_FACTOR(Code)
The load factor used when none specified in constructor.



MAXIMUM_CAPACITY
final static int MAXIMUM_CAPACITY(Code)
The maximum capacity, used if a higher value is implicitly specified by either of the constructors with arguments. MUST be a power of two <= 1<<30.



NULL_KEY
final static Object NULL_KEY(Code)
Value representing null keys inside tables.



loadFactor
final float loadFactor(Code)
The load factor for the hash table.



modCount
transient volatile int modCount(Code)
The number of times this HashMap has been structurally modified Structural modifications are those that change the number of mappings in the HashMap or otherwise modify its internal structure (e.g., rehash). This field is used to make iterators on Collection-views of the HashMap fail-fast. (See ConcurrentModificationException).



size
transient int size(Code)
The number of key-value mappings contained in this identity hash map.



table
transient Entry[] table(Code)
The table, resized as necessary. Length MUST Always be a power of two.



threshold
int threshold(Code)
The next size value at which to resize (capacity * load factor).




Constructor Detail
HashMap
public HashMap(int initialCapacity, float loadFactor)(Code)
Constructs an empty HashMap with the specified initial capacity and load factor.
Parameters:
  initialCapacity - The initial capacity.
Parameters:
  loadFactor - The load factor.
throws:
  IllegalArgumentException - if the initial capacity is negativeor the load factor is nonpositive.



HashMap
public HashMap(int initialCapacity)(Code)
Constructs an empty HashMap with the specified initial capacity and the default load factor (0.75).
Parameters:
  initialCapacity - the initial capacity.
throws:
  IllegalArgumentException - if the initial capacity is negative.



HashMap
public HashMap()(Code)
Constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75).



HashMap
public HashMap(Map m)(Code)
Constructs a new HashMap with the same mappings as the specified Map. The HashMap is created with default load factor (0.75) and an initial capacity sufficient to hold the mappings in the specified Map.
Parameters:
  m - the map whose mappings are to be placed in this map.
throws:
  NullPointerException - if the specified map is null.




Method Detail
addEntry
void addEntry(int hash, Object key, Object value, int bucketIndex)(Code)
Add a new entry with the specified key, value and hash code to the specified bucket. It is the responsibility of this method to resize the table if appropriate. Subclass overrides this to alter the behavior of put method.



capacity
int capacity()(Code)



clear
public void clear()(Code)
Removes all mappings from this map.



clone
public Object clone()(Code)
Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned. a shallow copy of this map.



containsKey
public boolean containsKey(Object key)(Code)
Returns true if this map contains a mapping for the specified key.
Parameters:
  key - The key whose presence in this map is to be tested true if this map contains a mapping for the specifiedkey.



containsValue
public boolean containsValue(Object value)(Code)
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. true if this map maps one or more keys to thespecified value.



createEntry
void createEntry(int hash, Object key, Object value, int bucketIndex)(Code)
Like addEntry except that this version is used when creating entries as part of Map construction or "pseudo-construction" (cloning, deserialization). This version needn't worry about resizing the table. Subclass overrides this to alter the behavior of HashMap(Map), clone, and readObject.



entrySet
public Set entrySet()(Code)
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. a collection view of the mappings contained in this map.
See Also:   Map.Entry



eq
static boolean eq(Object x, Object y)(Code)
Check for equality of non-null reference x and possibly-null y.



get
public Object get(Object key)(Code)
Returns the value to which the specified key is mapped in this identity hash map, or 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 is also possible that the map explicitly maps the key to null. The containsKey method may be used to distinguish these two cases.
Parameters:
  key - the key whose associated value is to be returned. the value to which this map maps the specified key, ornull if the map contains no mapping for this key.
See Also:   HashMap.put(Object,Object)



getEntry
Entry getEntry(Object key)(Code)
Returns the entry associated with the specified key in the HashMap. Returns null if the HashMap contains no mapping for this key.



hash
static int hash(Object x)(Code)
Returns a hash value for the specified object. In addition to the object's own hashCode, this method applies a "supplemental hash function," which defends against poor quality hash functions. This is critical because HashMap uses power-of two length hash tables.

The shift distances in this function were chosen as the result of an automated search over the entire four-dimensional search space.




indexFor
static int indexFor(int h, int length)(Code)
Returns index for hash code h.



init
void init()(Code)
Initialization hook for subclasses. This method is called in all constructors and pseudo-constructors (clone, readObject) after HashMap has been initialized but before any entries have been inserted. (In the absence of this method, readObject would require explicit knowledge of subclasses.)



isEmpty
public boolean isEmpty()(Code)
Returns true if this map contains no key-value mappings. true if this map contains no key-value mappings.



keySet
public Set keySet()(Code)
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. a set view of the keys contained in this map.



loadFactor
float loadFactor()(Code)



maskNull
static Object maskNull(Object key)(Code)
Returns internal representation for key. Use NULL_KEY if key is null.



newEntryIterator
Iterator newEntryIterator()(Code)



newKeyIterator
Iterator newKeyIterator()(Code)



newValueIterator
Iterator newValueIterator()(Code)



put
public Object put(Object key, Object value)(Code)
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.
Parameters:
  value - value to be associated with the specified key. previous value associated with specified key, or nullif there was no mapping for key. A null return canalso indicate that the HashMap previously associatednull with the specified key.



putAll
public void putAll(Map m)(Code)
Copies all of the mappings from the specified map to this map These mappings will replace any mappings that this map had for any of the keys currently in the specified map.
Parameters:
  m - mappings to be stored in this map.
throws:
  NullPointerException - if the specified map is null.



putAllForCreate
void putAllForCreate(Map m)(Code)



remove
public Object remove(Object key)(Code)
Removes the mapping for this key from this map if present.
Parameters:
  key - key whose mapping is to be removed from the map. previous value associated with specified key, or nullif there was no mapping for key. A null return canalso indicate that the map previously associated nullwith the specified key.



removeEntryForKey
Entry removeEntryForKey(Object key)(Code)
Removes and returns the entry associated with the specified key in the HashMap. Returns null if the HashMap contains no mapping for this key.



removeMapping
Entry removeMapping(Object o)(Code)
Special version of remove for EntrySet.



resize
void resize(int newCapacity)(Code)
Rehashes the contents of this map into a new array with a larger capacity. This method is called automatically when the number of keys in this map reaches its threshold. If current capacity is MAXIMUM_CAPACITY, this method does not resize the map, but but sets threshold to Integer.MAX_VALUE. This has the effect of preventing future calls.
Parameters:
  newCapacity - the new capacity, MUST be a power of two;must be greater than current capacity unless currentcapacity is MAXIMUM_CAPACITY (in which case valueis irrelevant).



size
public int size()(Code)
Returns the number of key-value mappings in this map. the number of key-value mappings in this map.



transfer
void transfer(Entry[] newTable)(Code)
Transfer all entries from current table to newTable.



unmaskNull
static Object unmaskNull(Object key)(Code)
Returns key represented by specified internal representation.



values
public Collection values()(Code)
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. a collection view of the values contained in this map.



Fields inherited from java.util.AbstractMap
transient volatile Set keySet(Code)(Java Doc)
transient volatile Collection values(Code)(Java Doc)

Methods inherited from java.util.AbstractMap
public void clear()(Code)(Java Doc)
protected Object clone() throws CloneNotSupportedException(Code)(Java Doc)
public boolean containsKey(Object key)(Code)(Java Doc)
public boolean containsValue(Object value)(Code)(Java Doc)
abstract public Set entrySet()(Code)(Java Doc)
public boolean equals(Object o)(Code)(Java Doc)
public Object get(Object key)(Code)(Java Doc)
public int hashCode()(Code)(Java Doc)
public boolean isEmpty()(Code)(Java Doc)
public Set keySet()(Code)(Java Doc)
public Object put(Object key, Object value)(Code)(Java Doc)
public void putAll(Map t)(Code)(Java Doc)
public Object remove(Object key)(Code)(Java Doc)
public int size()(Code)(Java Doc)
public String toString()(Code)(Java Doc)
public Collection values()(Code)(Java Doc)

Methods inherited from java.lang.Object
public boolean equals(Object obj)(Code)(Java Doc)
final native public Class getClass()(Code)(Java Doc)
native public int hashCode()(Code)(Java Doc)
final native public void notify()(Code)(Java Doc)
final native public void notifyAll()(Code)(Java Doc)
public String toString()(Code)(Java Doc)
final native public void wait(long timeout) throws InterruptedException(Code)(Java Doc)
final public void wait(long timeout, int nanos) throws InterruptedException(Code)(Java Doc)
final public void wait() throws InterruptedException(Code)(Java Doc)

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