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HashMap

public class HashMap
extends AbstractMap<K, V> implements Map<K, V>, Cloneable, Serializable

java.lang.Object
   ↳ java.util.AbstractMap<K, V>
     ↳ java.util.HashMap<K, V>
Known Direct Subclasses


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 hash table is rehashed (that is, internal data structures are rebuilt) so that the hash table has approximately twice the number of buckets.

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 a hash 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 method, 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.

See also:

Summary

Public constructors

HashMap(int initialCapacity, float loadFactor)

Constructs an empty HashMap with the specified initial capacity and load factor.

HashMap(int initialCapacity)

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

HashMap()

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

HashMap(Map<? extends K, ? extends V> m)

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

Public methods

void clear()

Removes all of the 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.

boolean containsValue(Object value)

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

Set<Entry<K, V>> entrySet()

Returns a Set view of the mappings contained in this map.

void forEach(BiConsumer<? super K, ? super V> action)

Performs the given action for each entry in this map until all entries have been processed or the action throws an exception.

V get(Object key)

Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

boolean isEmpty()

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

Set<K> keySet()

Returns a Set view of the keys contained in this map.

V put(K key, V value)

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

void putAll(Map<? extends K, ? extends V> m)

Copies all of the mappings from the specified map to this map.

V remove(Object key)

Removes the mapping for the specified key from this map if present.

boolean replace(K key, V oldValue, V newValue)

Replaces the entry for the specified key only if currently mapped to the specified value.

void replaceAll(BiFunction<? super K, ? super V, ? extends V> function)

Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception.

int size()

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

Collection<V> values()

Returns a Collection view of the values contained in this map.

Inherited methods

From class java.util.AbstractMap
From class java.lang.Object
From interface java.util.Map

Public constructors

HashMap

Added in API level 1
HashMap (int initialCapacity, 
                float loadFactor)

Constructs an empty HashMap with the specified initial capacity and load factor.

Parameters
initialCapacity int: the initial capacity
loadFactor float: the load factor
Throws
IllegalArgumentException if the initial capacity is negative or the load factor is nonpositive

HashMap

Added in API level 1
HashMap (int initialCapacity)

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

Parameters
initialCapacity int: the initial capacity.
Throws
IllegalArgumentException if the initial capacity is negative.

HashMap

Added in API level 1
HashMap ()

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

HashMap

Added in API level 1
HashMap (Map<? extends K, ? extends V> 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 Map: the map whose mappings are to be placed in this map
Throws
NullPointerException if the specified map is null

Public methods

clear

Added in API level 1
void clear ()

Removes all of the mappings from this map. The map will be empty after this call returns.

clone

Added in API level 1
Object clone ()

Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned.

Returns
Object a shallow copy of this map

containsKey

Added in API level 1
boolean containsKey (Object key)

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

Parameters
key Object: The key whose presence in this map is to be tested
Returns
boolean true if this map contains a mapping for the specified key.

containsValue

Added in API level 1
boolean containsValue (Object value)

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

Parameters
value Object: value whose presence in this map is to be tested
Returns
boolean true if this map maps one or more keys to the specified value

entrySet

Added in API level 1
Set<Entry<K, V>> entrySet ()

Returns a Set view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation, or through the setValue operation on a map entry returned by the iterator) the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations.

Returns
Set<Entry<K, V>> a set view of the mappings contained in this map

forEach

Added in API level 24
void forEach (BiConsumer<? super K, ? super V> action)

Performs the given action for each entry in this map until all entries have been processed or the action throws an exception. Unless otherwise specified by the implementing class, actions are performed in the order of entry set iteration (if an iteration order is specified.) Exceptions thrown by the action are relayed to the caller.

Parameters
action BiConsumer: The action to be performed for each entry

get

Added in API level 1
V get (Object key)

Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

More formally, if this map contains a mapping from a key k to a value v such that (key==null ? k==null : key.equals(k)), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)

A return value of null does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to null. The containsKey operation may be used to distinguish these two cases.

Parameters
key Object: the key whose associated value is to be returned
Returns
V the value to which the specified key is mapped, or null if this map contains no mapping for the key

See also:

isEmpty

Added in API level 1
boolean isEmpty ()

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

Returns
boolean true if this map contains no key-value mappings

keySet

Added in API level 1
Set<K> 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. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

Returns
Set<K> a set view of the keys contained in this map

put

Added in API level 1
V put (K key, 
                V value)

Associates the specified value with the specified key in this map. If the map previously contained a mapping for the key, the old value is replaced.

Parameters
key K: key with which the specified value is to be associated
value V: value to be associated with the specified key
Returns
V the previous value associated with key, or null if there was no mapping for key. (A null return can also indicate that the map previously associated null with key.)

putAll

Added in API level 1
void putAll (Map<? extends K, ? extends V> 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.

Parameters
m Map: mappings to be stored in this map
Throws
NullPointerException if the specified map is null

remove

Added in API level 1
V remove (Object key)

Removes the mapping for the specified key from this map if present.

Parameters
key Object: key whose mapping is to be removed from the map
Returns
V the previous value associated with key, or null if there was no mapping for key. (A null return can also indicate that the map previously associated null with key.)

replace

Added in API level 24
boolean replace (K key, 
                V oldValue, 
                V newValue)

Replaces the entry for the specified key only if currently mapped to the specified value.

Parameters
key K: key with which the specified value is associated
oldValue V: value expected to be associated with the specified key
newValue V: value to be associated with the specified key
Returns
boolean true if the value was replaced

replaceAll

Added in API level 24
void replaceAll (BiFunction<? super K, ? super V, ? extends V> function)

Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception. Exceptions thrown by the function are relayed to the caller.

Parameters
function BiFunction: the function to apply to each entry

size

Added in API level 1
int size ()

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

Returns
int the number of key-value mappings in this map

values

Added in API level 1
Collection<V> 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. If the map is modified while an iteration over the collection is in progress (except through the iterator's own remove operation), the results of the iteration are undefined. 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.

Returns
Collection<V> a collection view of the values contained in this map
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