java.util
Class HashMap

java.lang.Object
  extended byjava.util.AbstractMap
      extended byjava.util.HashMap
All Implemented Interfaces:
Cloneable, Map, Serializable
Direct Known Subclasses:
LinkedHashMap

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 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.

Since:
1.2
Author:
Doug Lea, Josh Bloch, Arthur van Hoff
See Also:
Object.hashCode(), Collection, Map, TreeMap, Hashtable, Serialized Form

Nested Class Summary
(package private) static class HashMap.Entry
           
private  class HashMap.EntryIterator
           
private  class HashMap.EntrySet
           
private  class HashMap.HashIterator
           
private  class HashMap.KeyIterator
           
private  class HashMap.KeySet
           
private  class HashMap.ValueIterator
           
private  class HashMap.Values
           
 
Nested classes inherited from class java.util.AbstractMap
AbstractMap.SimpleEntry
 
Field Summary
(package private) static int DEFAULT_INITIAL_CAPACITY
          The default initial capacity - MUST be a power of two.
(package private) static float DEFAULT_LOAD_FACTOR
          The load factor used when none specified in constructor.
private  Set entrySet
           
(package private)  float loadFactor
          The load factor for the hash table.
(package private) static int MAXIMUM_CAPACITY
          The maximum capacity, used if a higher value is implicitly specified by either of the constructors with arguments.
(package private)  int modCount
          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).
(package private) static Object NULL_KEY
          Value representing null keys inside tables.
private static long serialVersionUID
           
(package private)  int size
          The number of key-value mappings contained in this identity hash map.
(package private)  HashMap.Entry[] table
          The table, resized as necessary.
(package private)  int threshold
          The next size value at which to resize (capacity * load factor).
 
Fields inherited from class java.util.AbstractMap
keySet, values
 
Constructor Summary
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 m)
          Constructs a new HashMap with the same mappings as the specified Map.
 
Method Summary
(package private)  void addEntry(int hash, Object key, Object value, int bucketIndex)
          Add a new entry with the specified key, value and hash code to the specified bucket.
(package private)  int capacity()
           
 void clear()
          Removes all 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.
private  boolean containsNullValue()
          Special-case code for containsValue with null argument
 boolean containsValue(Object value)
          Returns true if this map maps one or more keys to the specified value.
(package private)  void createEntry(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).
 Set entrySet()
          Returns a collection view of the mappings contained in this map.
(package private) static boolean eq(Object x, Object y)
          Check for equality of non-null reference x and possibly-null y.
 Object get(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.
(package private)  HashMap.Entry getEntry(Object key)
          Returns the entry associated with the specified key in the HashMap.
(package private) static int hash(Object x)
          Returns a hash value for the specified object.
(package private) static int indexFor(int h, int length)
          Returns index for hash code h.
(package private)  void init()
          Initialization hook for subclasses.
 boolean isEmpty()
          Returns true if this map contains no key-value mappings.
 Set keySet()
          Returns a set view of the keys contained in this map.
(package private)  float loadFactor()
           
(package private) static Object maskNull(Object key)
          Returns internal representation for key.
(package private)  Iterator newEntryIterator()
           
(package private)  Iterator newKeyIterator()
           
(package private)  Iterator newValueIterator()
           
 Object put(Object key, Object value)
          Associates the specified value with the specified key in this map.
 void putAll(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.
(package private)  void putAllForCreate(Map m)
           
private  void putForCreate(Object key, Object value)
          This method is used instead of put by constructors and pseudoconstructors (clone, readObject).
private  void readObject(ObjectInputStream s)
          Reconstitute the HashMap instance from a stream (i.e., deserialize it).
 Object remove(Object key)
          Removes the mapping for this key from this map if present.
(package private)  HashMap.Entry removeEntryForKey(Object key)
          Removes and returns the entry associated with the specified key in the HashMap.
(package private)  HashMap.Entry removeMapping(Object o)
          Special version of remove for EntrySet.
(package private)  void resize(int newCapacity)
          Rehashes the contents of this map into a new array with a larger capacity.
 int size()
          Returns the number of key-value mappings in this map.
(package private)  void transfer(HashMap.Entry[] newTable)
          Transfer all entries from current table to newTable.
(package private) static Object unmaskNull(Object key)
          Returns key represented by specified internal representation.
 Collection values()
          Returns a collection view of the values contained in this map.
private  void writeObject(ObjectOutputStream s)
          Save the state of the HashMap instance to a stream (i.e., serialize it).
 
Methods inherited from class java.util.AbstractMap
equals, hashCode, toString
 
Methods inherited from class java.lang.Object
finalize, getClass, notify, notifyAll, wait, wait, wait
 
Methods inherited from interface java.util.Map
equals, hashCode
 

Field Detail

DEFAULT_INITIAL_CAPACITY

static final int DEFAULT_INITIAL_CAPACITY
The default initial capacity - MUST be a power of two.

See Also:
Constant Field Values

MAXIMUM_CAPACITY

static final int MAXIMUM_CAPACITY
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.

See Also:
Constant Field Values

DEFAULT_LOAD_FACTOR

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

See Also:
Constant Field Values

table

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


size

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


threshold

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


loadFactor

final float loadFactor
The load factor for the hash table.


modCount

transient volatile int modCount
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).


NULL_KEY

static final Object NULL_KEY
Value representing null keys inside tables.


entrySet

private transient Set entrySet

serialVersionUID

private static final long serialVersionUID
See Also:
Constant Field Values
Constructor Detail

HashMap

public HashMap(int initialCapacity,
               float loadFactor)
Constructs an empty HashMap with the specified initial capacity and load factor.

Parameters:
initialCapacity - The initial capacity.
loadFactor - The load factor.
Throws:
IllegalArgumentException - if the initial capacity is negative or the load factor is nonpositive.

HashMap

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


HashMap

public HashMap(Map m)
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

init

void init()
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.)


maskNull

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


unmaskNull

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


hash

static int hash(Object x)
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.


eq

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


indexFor

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


size

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

Specified by:
size in interface Map
Overrides:
size in class AbstractMap
Returns:
the number of key-value mappings in this map.

isEmpty

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

Specified by:
isEmpty in interface Map
Overrides:
isEmpty in class AbstractMap
Returns:
true if this map contains no key-value mappings.

get

public Object get(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. The containsKey method may be used to distinguish these two cases.

Specified by:
get in interface Map
Overrides:
get in class AbstractMap
Parameters:
key - the key whose associated value is to be returned.
Returns:
the value to which this map maps the specified key, or null if the map contains no mapping for this key.
See Also:
put(Object, Object)

containsKey

public boolean containsKey(Object key)
Returns true if this map contains a mapping for the specified key.

Specified by:
containsKey in interface Map
Overrides:
containsKey in class AbstractMap
Parameters:
key - The key whose presence in this map is to be tested
Returns:
true if this map contains a mapping for the specified key.

getEntry

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


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.

Specified by:
put in interface Map
Overrides:
put in class AbstractMap
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.

putForCreate

private void putForCreate(Object key,
                          Object value)
This method is used instead of put by constructors and pseudoconstructors (clone, readObject). It does not resize the table, check for comodification, etc. It calls createEntry rather than addEntry.


putAllForCreate

void putAllForCreate(Map m)

resize

void resize(int newCapacity)
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 current capacity is MAXIMUM_CAPACITY (in which case value is irrelevant).

transfer

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


putAll

public void putAll(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.

Specified by:
putAll in interface Map
Overrides:
putAll in class AbstractMap
Parameters:
m - mappings to be stored in this map.
Throws:
NullPointerException - if the specified map is null.

remove

public Object remove(Object key)
Removes the mapping for this key from this map if present.

Specified by:
remove in interface Map
Overrides:
remove in class AbstractMap
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.

removeEntryForKey

HashMap.Entry removeEntryForKey(Object key)
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

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


clear

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

Specified by:
clear in interface Map
Overrides:
clear in class AbstractMap

containsValue

public boolean containsValue(Object value)
Returns true if this map maps one or more keys to the specified value.

Specified by:
containsValue in interface Map
Overrides:
containsValue in class AbstractMap
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.

containsNullValue

private boolean containsNullValue()
Special-case code for containsValue with null argument


clone

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

Overrides:
clone in class AbstractMap
Returns:
a shallow copy of this map.

addEntry

void addEntry(int hash,
              Object key,
              Object value,
              int bucketIndex)
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.


createEntry

void createEntry(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). This version needn't worry about resizing the table. Subclass overrides this to alter the behavior of HashMap(Map), clone, and readObject.


newKeyIterator

Iterator newKeyIterator()

newValueIterator

Iterator newValueIterator()

newEntryIterator

Iterator newEntryIterator()

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.

Specified by:
keySet in interface Map
Overrides:
keySet in class AbstractMap
Returns:
a set view of the keys contained in this map.

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.

Specified by:
values in interface Map
Overrides:
values in class AbstractMap
Returns:
a collection view of the values contained in this map.

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.

Specified by:
entrySet in interface Map
Specified by:
entrySet in class AbstractMap
Returns:
a collection view of the mappings contained in this map.
See Also:
HashMap.Entry

writeObject

private void writeObject(ObjectOutputStream s)
                  throws IOException
Save the state of the HashMap instance to a stream (i.e., serialize it).

Throws:
IOException

readObject

private void readObject(ObjectInputStream s)
                 throws IOException,
                        ClassNotFoundException
Reconstitute the HashMap instance from a stream (i.e., deserialize it).

Throws:
IOException
ClassNotFoundException

capacity

int capacity()

loadFactor

float loadFactor()