Known Direct Subclasses
LinkedHashMap<K, V>
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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 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.
Public Constructor Summary
HashMap(int initialCapacity, float loadFactor)
Constructs an empty HashMap with the specified initial
capacity and load factor.
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HashMap(int initialCapacity)
Constructs an empty HashMap with the specified initial
capacity and the default load factor (0.75).
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HashMap()
Constructs an empty HashMap with the default initial capacity
(16) and the default load factor (0.75).
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Public Method Summary
void |
clear()
Removes all of the mappings from this map.
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Object |
clone()
Returns a shallow copy of this HashMap instance: the keys and
values themselves are not cloned.
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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 (or
null 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
to
null ), attempts to compute its value using the given mapping
function and enters it into this map unless null . |
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.
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boolean | |
boolean | |
Set<Entry<K, V>> | |
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.
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V | |
V |
getOrDefault(Object key, V defaultValue)
Returns the value to which the specified key is mapped, or
defaultValue if this map contains no mapping for the key. |
boolean |
isEmpty()
Returns true if this map contains no key-value mappings.
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Set<K> | |
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.
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V |
put(K key, V value)
Associates the specified value with the specified key in this map.
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void | |
V |
putIfAbsent(K key, V value)
If the specified key is not already associated with a value (or is mapped
to
null ) associates it with the given value and returns
null , else returns the current value. |
boolean | |
V | |
boolean |
replace(K key, V oldValue, V newValue)
Replaces the entry for the specified key only if currently
mapped to the specified value.
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V |
replace(K key, V value)
Replaces the entry for the specified key only if it is
currently mapped to some value.
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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.
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int |
size()
Returns the number of key-value mappings in this map.
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Collection<V> |
values()
Returns a
Collection view of the values contained in this map. |
Inherited Method Summary
Public Constructors
public HashMap (int initialCapacity, float loadFactor)
Constructs an empty HashMap with the specified initial capacity and load factor.
Parameters
initialCapacity | the initial capacity |
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loadFactor | the load factor |
Throws
IllegalArgumentException | if the initial capacity is negative or the load factor is nonpositive |
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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. |
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Throws
IllegalArgumentException | if the initial capacity is negative. |
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public HashMap ()
Constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75).
public 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 | the map whose mappings are to be placed in this map |
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Throws
NullPointerException | if the specified map is null |
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Public Methods
public void clear ()
Removes all of the mappings from this map. The map will be empty after this call returns.
public Object clone ()
Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned.
Returns
- a shallow copy of this map
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 (or null
if there is no current mapping). For
example, to either create or append a String
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.
Parameters
key | key with which the specified value is to be associated |
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remappingFunction | the remapping function to compute a value |
Returns
- the new value associated with the specified key, or null if none
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
to null
), attempts to compute its value using the given mapping
function and enters it into this map unless null
.
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.
Parameters
key | key with which the specified value is to be associated |
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mappingFunction | 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
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.
Parameters
key | key with which the specified value is to be associated |
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remappingFunction | the remapping function to compute a value |
Returns
- the new value associated with the specified key, or null if none
public boolean containsKey (Object key)
Returns true if this map contains a mapping for the specified key.
Parameters
key | The key whose presence in this map is to be tested |
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Returns
- true if this map contains a mapping for the specified key.
public boolean containsValue (Object value)
Returns true if this map maps one or more keys to the specified value.
Parameters
value | value whose presence in this map is to be tested |
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Returns
- true if this map maps one or more keys to the specified value
public 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
- a set view of the mappings contained in this map
public 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 | The action to be performed for each entry |
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public 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 | the key whose associated value is to be returned |
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Returns
- the value to which the specified key is mapped, or
null
if this map contains no mapping for the key
See Also
public V getOrDefault (Object key, V defaultValue)
Returns the value to which the specified key is mapped, or
defaultValue
if this map contains no mapping for the key.
Parameters
key | the key whose associated value is to be returned |
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defaultValue | the default mapping of the key |
Returns
- the value to which the specified key is mapped, or
defaultValue
if this map contains no mapping for the key
public boolean isEmpty ()
Returns true if this map contains no key-value mappings.
Returns
- true if this map contains no key-value mappings
public 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
- a set view of the keys contained in this map
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 is null
. This
method may be of use when combining multiple mapped values for a key.
For example, to either create or append a String 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.
Parameters
key | key with which the resulting value is to be associated |
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value | 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 key |
remappingFunction | 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
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.
Parameters
key | key with which the specified value is to be associated |
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value | value to be associated with the specified key |
Returns
- 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.)
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.
Parameters
m | mappings to be stored in this map |
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Throws
NullPointerException | if the specified map is null |
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public V putIfAbsent (K key, V value)
If the specified key is not already associated with a value (or is mapped
to null
) associates it with the given value and returns
null
, else returns the current value.
Parameters
key | key with which the specified value is to be associated |
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value | value to be associated with the specified key |
Returns
- the previous value associated with the specified key, or
null
if there was no mapping for the key. (Anull
return can also indicate that the map previously associatednull
with the key, if the implementation supports null values.)
public boolean remove (Object key, Object value)
Removes the entry for the specified key only if it is currently mapped to the specified value.
Parameters
key | key with which the specified value is associated |
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value | value expected to be associated with the specified key |
Returns
true
if the value was removed
public V remove (Object key)
Removes the mapping for the specified key from this map if present.
Parameters
key | key whose mapping is to be removed from the map |
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Returns
- 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.)
public 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 | key with which the specified value is associated |
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oldValue | value expected to be associated with the specified key |
newValue | value to be associated with the specified key |
Returns
true
if the value was replaced
public V replace (K key, V value)
Replaces the entry for the specified key only if it is currently mapped to some value.
Parameters
key | key with which the specified value is associated |
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value | value to be associated with the specified key |
Returns
- the previous value associated with the specified key, or
null
if there was no mapping for the key. (Anull
return can also indicate that the map previously associatednull
with the key, if the implementation supports null values.)
public 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 | the function to apply to each entry |
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public int size ()
Returns the number of key-value mappings in this map.
Returns
- the number of key-value mappings in this map
public 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
- a view of the values contained in this map