java.util
Class HashMap

java.lang.Object
  java.util.AbstractMap
      java.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()