- java.lang.Object
-
- java.util.AbstractMap<K,V>
-
- java.util.HashMap<K,V>
-
- Type Parameters:
K
- the type of keys maintained by this mapV
- the type of mapped values
- All Implemented Interfaces:
Serializable
,Cloneable
,Map<K,V>
- Direct Known Subclasses:
LinkedHashMap
,PrinterStateReasons
public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable
Hash table based implementation of theMap
interface. This implementation provides all of the optional map operations, and permitsnull
values and thenull
key. (TheHashMap
class is roughly equivalent toHashtable
, 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
andput
), assuming the hash function disperses the elements properly among the buckets. Iteration over collection views requires time proportional to the "capacity" of theHashMap
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, includingget
andput
). 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 using many keys with the samehashCode()
is a sure way to slow down performance of any hash table. To ameliorate impact, when keys areComparable
, this class may use comparison order among keys to help break ties.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 aConcurrentModificationException
. 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
- See Also:
Object.hashCode()
,Collection
,Map
,TreeMap
,Hashtable
, Serialized Form
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-
Nested Class Summary
-
Nested classes/interfaces declared in class java.util.AbstractMap
AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>
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-
Constructor Summary
Constructors Constructor Description HashMap()
Constructs an emptyHashMap
with the default initial capacity (16) and the default load factor (0.75).HashMap(int initialCapacity)
Constructs an emptyHashMap
with the specified initial capacity and the default load factor (0.75).HashMap(int initialCapacity, float loadFactor)
Constructs an emptyHashMap
with the specified initial capacity and load factor.HashMap(Map<? extends K,? extends V> m)
Constructs a newHashMap
with the same mappings as the specifiedMap
.
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Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description void
clear()
Removes all of the mappings from this map.Object
clone()
Returns a shallow copy of thisHashMap
instance: the keys and values themselves are not cloned.V
compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
Attempts to compute a mapping for the specified key and its current mapped value (ornull
if there is no current mapping).V
computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)
If the specified key is not already associated with a value (or is mapped tonull
), attempts to compute its value using the given mapping function and enters it into this map unlessnull
.V
computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.boolean
containsKey(Object key)
Returnstrue
if this map contains a mapping for the specified key.boolean
containsValue(Object value)
Returnstrue
if this map maps one or more keys to the specified value.Set<Map.Entry<K,V>>
entrySet()
Returns aSet
view of the mappings contained in this map.V
get(Object key)
Returns the value to which the specified key is mapped, ornull
if this map contains no mapping for the key.boolean
isEmpty()
Returnstrue
if this map contains no key-value mappings.Set<K>
keySet()
Returns aSet
view of the keys contained in this map.V
merge(K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction)
If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.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.int
size()
Returns the number of key-value mappings in this map.Collection<V>
values()
Returns aCollection
view of the values contained in this map.-
Methods declared in class java.util.AbstractMap
equals, hashCode, toString
-
Methods declared in interface java.util.Map
equals, forEach, getOrDefault, hashCode, putIfAbsent, remove, replace, replace, replaceAll
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-
-
-
Constructor Detail
-
HashMap
public HashMap(int initialCapacity, float loadFactor)
Constructs an emptyHashMap
with the specified initial capacity and load factor.- Parameters:
initialCapacity
- the initial capacityloadFactor
- the load factor- Throws:
IllegalArgumentException
- if the initial capacity is negative or the load factor is nonpositive
-
HashMap
public HashMap(int initialCapacity)
Constructs an emptyHashMap
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 emptyHashMap
with the default initial capacity (16) and the default load factor (0.75).
-
HashMap
public HashMap(Map<? extends K,? extends V> m)
Constructs a newHashMap
with the same mappings as the specifiedMap
. TheHashMap
is created with default load factor (0.75) and an initial capacity sufficient to hold the mappings in the specifiedMap
.- Parameters:
m
- the map whose mappings are to be placed in this map- Throws:
NullPointerException
- if the specified map is null
-
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Method Detail
-
size
public int size()
Returns the number of key-value mappings in this map.
-
isEmpty
public boolean isEmpty()
Returnstrue
if this map contains no key-value mappings.
-
get
public V get(Object key)
Returns the value to which the specified key is mapped, ornull
if this map contains no mapping for the key.More formally, if this map contains a mapping from a key
k
to a valuev
such that(key==null ? k==null : key.equals(k))
, then this method returnsv
; otherwise it returnsnull
. (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 tonull
. ThecontainsKey
operation may be used to distinguish these two cases.- Specified by:
get
in interfaceMap<K,V>
- Overrides:
get
in classAbstractMap<K,V>
- Parameters:
key
- the key whose associated value is to be returned- Returns:
- the value to which the specified key is mapped, or
null
if this map contains no mapping for the key - See Also:
put(Object, Object)
-
containsKey
public boolean containsKey(Object key)
Returnstrue
if this map contains a mapping for the specified key.- Specified by:
containsKey
in interfaceMap<K,V>
- Overrides:
containsKey
in classAbstractMap<K,V>
- 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.
-
put
public 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.- Specified by:
put
in interfaceMap<K,V>
- Overrides:
put
in classAbstractMap<K,V>
- Parameters:
key
- key with which the specified value is to be associatedvalue
- value to be associated with the specified key- Returns:
- the previous value associated with
key
, ornull
if there was no mapping forkey
. (Anull
return can also indicate that the map previously associatednull
withkey
.)
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putAll
public 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.- Specified by:
putAll
in interfaceMap<K,V>
- Overrides:
putAll
in classAbstractMap<K,V>
- Parameters:
m
- mappings to be stored in this map- Throws:
NullPointerException
- if the specified map is null
-
remove
public V remove(Object key)
Removes the mapping for the specified key from this map if present.- Specified by:
remove
in interfaceMap<K,V>
- Overrides:
remove
in classAbstractMap<K,V>
- Parameters:
key
- key whose mapping is to be removed from the map- Returns:
- the previous value associated with
key
, ornull
if there was no mapping forkey
. (Anull
return can also indicate that the map previously associatednull
withkey
.)
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clear
public void clear()
Removes all of the mappings from this map. The map will be empty after this call returns.
-
containsValue
public boolean containsValue(Object value)
Returnstrue
if this map maps one or more keys to the specified value.- Specified by:
containsValue
in interfaceMap<K,V>
- Overrides:
containsValue
in classAbstractMap<K,V>
- 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
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keySet
public Set<K> keySet()
Returns aSet
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 ownremove
operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via theIterator.remove
,Set.remove
,removeAll
,retainAll
, andclear
operations. It does not support theadd
oraddAll
operations.
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values
public Collection<V> values()
Returns aCollection
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 ownremove
operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via theIterator.remove
,Collection.remove
,removeAll
,retainAll
andclear
operations. It does not support theadd
oraddAll
operations.
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entrySet
public Set<Map.Entry<K,V>> entrySet()
Returns aSet
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 ownremove
operation, or through thesetValue
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 theIterator.remove
,Set.remove
,removeAll
,retainAll
andclear
operations. It does not support theadd
oraddAll
operations.
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computeIfAbsent
public V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)
If the specified key is not already associated with a value (or is mapped tonull
), attempts to compute its value using the given mapping function and enters it into this map unlessnull
.If the mapping function returns
null
, no mapping is recorded. If the mapping function itself throws an (unchecked) exception, the exception is rethrown, and no mapping is recorded. The most common usage is to construct a new object serving as an initial mapped value or memoized result, as in:map.computeIfAbsent(key, k -> new Value(f(k)));
Or to implement a multi-value map,
Map<K,Collection<V>>
, supporting multiple values per key:map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);
The mapping function should not modify this map during computation.
This method will, on a best-effort basis, throw a
ConcurrentModificationException
if it is detected that the mapping function modifies this map during computation.- Specified by:
computeIfAbsent
in interfaceMap<K,V>
- Parameters:
key
- key with which the specified value is to be associatedmappingFunction
- the mapping function to compute a value- Returns:
- the current (existing or computed) value associated with the specified key, or null if the computed value is null
- Throws:
ConcurrentModificationException
- if it is detected that the mapping function modified this map
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computeIfPresent
public V computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.If the remapping function returns
null
, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.The remapping function should not modify this map during computation.
This method will, on a best-effort basis, throw a
ConcurrentModificationException
if it is detected that the remapping function modifies this map during computation.- Specified by:
computeIfPresent
in interfaceMap<K,V>
- Parameters:
key
- key with which the specified value is to be associatedremappingFunction
- the remapping function to compute a value- Returns:
- the new value associated with the specified key, or null if none
- Throws:
ConcurrentModificationException
- if it is detected that the remapping function modified this map
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compute
public V compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
Attempts to compute a mapping for the specified key and its current mapped value (ornull
if there is no current mapping). For example, to either create or append aString
msg to a value mapping:map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))
merge()
is often simpler to use for such purposes.)If the remapping function returns
null
, the mapping is removed (or remains absent if initially absent). If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.The remapping function should not modify this map during computation.
This method will, on a best-effort basis, throw a
ConcurrentModificationException
if it is detected that the remapping function modifies this map during computation.- Specified by:
compute
in interfaceMap<K,V>
- Parameters:
key
- key with which the specified value is to be associatedremappingFunction
- the remapping function to compute a value- Returns:
- the new value associated with the specified key, or null if none
- Throws:
ConcurrentModificationException
- if it is detected that the remapping function modified this map
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merge
public V merge(K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction)
If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value. Otherwise, replaces the associated value with the results of the given remapping function, or removes if the result isnull
. This method may be of use when combining multiple mapped values for a key. For example, to either create or append aString msg
to a value mapping:map.merge(key, msg, String::concat)
If the remapping function returns
null
, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.The remapping function should not modify this map during computation.
This method will, on a best-effort basis, throw a
ConcurrentModificationException
if it is detected that the remapping function modifies this map during computation.- Specified by:
merge
in interfaceMap<K,V>
- Parameters:
key
- key with which the resulting value is to be associatedvalue
- the non-null value to be merged with the existing value associated with the key or, if no existing value or a null value is associated with the key, to be associated with the keyremappingFunction
- the remapping function to recompute a value if present- Returns:
- the new value associated with the specified key, or null if no value is associated with the key
- Throws:
ConcurrentModificationException
- if it is detected that the remapping function modified this map
-
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