- 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
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 using
many keys with the same hashCode() is a sure way to slow
down performance of any hash table. To ameliorate impact, when keys
are Comparable, 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 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
- See Also:
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Nested Class Summary
Nested classes/interfaces declared in class java.util.AbstractMap
AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K, V> -
Constructor Summary
ConstructorsConstructorDescriptionHashMap()Constructs an emptyHashMapwith the default initial capacity (16) and the default load factor (0.75).HashMap(int initialCapacity) Constructs an emptyHashMapwith the specified initial capacity and the default load factor (0.75).HashMap(int initialCapacity, float loadFactor) Constructs an emptyHashMapwith the specified initial capacity and load factor.Constructs a newHashMapwith the same mappings as the specifiedMap. -
Method Summary
Modifier and TypeMethodDescriptionvoidclear()Removes all of the mappings from this map.clone()Returns a shallow copy of thisHashMapinstance: the keys and values themselves are not cloned.Attempts to compute a mapping for the specified key and its current mapped value (ornullif there is no current mapping).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.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.booleancontainsKey(Object key) Returnstrueif this map contains a mapping for the specified key.booleancontainsValue(Object value) Returnstrueif this map maps one or more keys to the specified value.entrySet()Returns aSetview of the mappings contained in this map.Returns the value to which the specified key is mapped, ornullif this map contains no mapping for the key.booleanisEmpty()Returnstrueif this map contains no key-value mappings.keySet()Returns aSetview of the keys contained in this map.If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.static <K,V> HashMap <K, V> newHashMap(int numMappings) Creates a new, empty HashMap suitable for the expected number of mappings.Associates the specified value with the specified key in this map.voidCopies all of the mappings from the specified map to this map.Removes the mapping for the specified key from this map if present.intsize()Returns the number of key-value mappings in this map.values()Returns aCollectionview of the values contained in this map.Methods declared in class java.util.AbstractMap
equals, hashCode, toStringMethods declared in interface java.util.Map
equals, forEach, getOrDefault, hashCode, putIfAbsent, remove, replace, replace, replaceAll
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Constructor Details
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HashMap
public HashMap(int initialCapacity, float loadFactor) Constructs an emptyHashMapwith the specified initial capacity and load factor.- API Note:
- To create a
HashMapwith an initial capacity that accommodates an expected number of mappings, usenewHashMap. - Parameters:
initialCapacity- the initial capacityloadFactor- the load factor- Throws:
IllegalArgumentException- if the initial capacity is negative or the load factor is nonpositive
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HashMap
public HashMap(int initialCapacity) Constructs an emptyHashMapwith the specified initial capacity and the default load factor (0.75).- API Note:
- To create a
HashMapwith an initial capacity that accommodates an expected number of mappings, usenewHashMap. - Parameters:
initialCapacity- the initial capacity.- Throws:
IllegalArgumentException- if the initial capacity is negative.
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HashMap
public HashMap()Constructs an emptyHashMapwith the default initial capacity (16) and the default load factor (0.75). -
HashMap
Constructs a newHashMapwith the same mappings as the specifiedMap. TheHashMapis 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 Details
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size
public int size()Returns the number of key-value mappings in this map. -
isEmpty
public boolean isEmpty()Returnstrueif this map contains no key-value mappings. -
get
Returns the value to which the specified key is mapped, ornullif this map contains no mapping for the key.More formally, if this map contains a mapping from a key
kto a valuevsuch 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
nulldoes not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key tonull. ThecontainsKeyoperation may be used to distinguish these two cases. -
containsKey
Returnstrueif this map contains a mapping for the specified key.- Specified by:
containsKeyin interfaceMap<K,V> - Overrides:
containsKeyin classAbstractMap<K,V> - Parameters:
key- The key whose presence in this map is to be tested- Returns:
trueif this map contains a mapping for the specified key.
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put
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:
putin interfaceMap<K,V> - Overrides:
putin 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, ornullif there was no mapping forkey. (Anullreturn can also indicate that the map previously associatednullwithkey.)
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putAll
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:
putAllin interfaceMap<K,V> - Overrides:
putAllin classAbstractMap<K,V> - Parameters:
m- mappings to be stored in this map- Throws:
NullPointerException- if the specified map is null
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remove
Removes the mapping for the specified key from this map if present.- Specified by:
removein interfaceMap<K,V> - Overrides:
removein classAbstractMap<K,V> - Parameters:
key- key whose mapping is to be removed from the map- Returns:
- the previous value associated with
key, ornullif there was no mapping forkey. (Anullreturn can also indicate that the map previously associatednullwithkey.)
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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
Returnstrueif this map maps one or more keys to the specified value.- Specified by:
containsValuein interfaceMap<K,V> - Overrides:
containsValuein classAbstractMap<K,V> - Parameters:
value- value whose presence in this map is to be tested- Returns:
trueif this map maps one or more keys to the specified value
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keySet
Returns aSetview 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 ownremoveoperation), 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, andclearoperations. It does not support theaddoraddAlloperations. -
values
Returns aCollectionview 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 ownremoveoperation), 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,retainAllandclearoperations. It does not support theaddoraddAlloperations. -
entrySet
Returns aSetview 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 ownremoveoperation, or through thesetValueoperation 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,retainAllandclearoperations. It does not support theaddoraddAlloperations. -
computeIfAbsent
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
ConcurrentModificationExceptionif it is detected that the mapping function modifies this map during computation.- Specified by:
computeIfAbsentin 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
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
ConcurrentModificationExceptionif it is detected that the remapping function modifies this map during computation.- Specified by:
computeIfPresentin 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
Attempts to compute a mapping for the specified key and its current mapped value (ornullif there is no current mapping). For example, to either create or append aStringmsg to a value mapping:
(Methodmap.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
ConcurrentModificationExceptionif it is detected that the remapping function modifies this map during computation.- Specified by:
computein 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
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 msgto 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
ConcurrentModificationExceptionif it is detected that the remapping function modifies this map during computation.- Specified by:
mergein 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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clone
Returns a shallow copy of thisHashMapinstance: the keys and values themselves are not cloned.- Overrides:
clonein classAbstractMap<K,V> - Returns:
- a shallow copy of this map
- See Also:
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newHashMap
Creates a new, empty HashMap suitable for the expected number of mappings. The returned map uses the default load factor of 0.75, and its initial capacity is generally large enough so that the expected number of mappings can be added without resizing the map.- Type Parameters:
K- the type of keys maintained by the new mapV- the type of mapped values- Parameters:
numMappings- the expected number of mappings- Returns:
- the newly created map
- Throws:
IllegalArgumentException- if numMappings is negative- Since:
- 19
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