K - the type of keys maintained by this map
V - the type of mapped values
public class HashMap<K,V> extends AbstractMap<K ,V> implements Map <K ,V>, Cloneable , Serializable
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.
Object.hashCode() ,
Collection,
Map,
TreeMap,
Hashtable,
Serialized Form
AbstractMap.SimpleEntry <K,V>, AbstractMap.SimpleImmutableEntry <K,V> | Constructor and Description |
|---|
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
Constructs a new
HashMap with the same mappings as the specified
Map.
|
| Modifier and Type | Method and Description |
|---|---|
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.
|
V |
compute(K key, BiFunction
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
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
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
Returns
true if this map contains a mapping for the specified key.
|
boolean |
containsValue(Object
Returns
true if this map maps one or more keys to the specified value.
|
Set |
entrySet()
Returns a
Set view of the mappings contained in this map.
|
void |
forEach(BiConsumer
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
Returns the value to which the specified key is mapped, or
null if this map contains no mapping for the key.
|
V |
getOrDefault(Object
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.
|
Set |
keySet()
Returns a
Set view of the keys contained in this map.
|
V |
merge(K key, V value, BiFunction
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
Copies all of the mappings from the specified map to this map.
|
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.
|
V |
remove(Object
Removes the mapping for the specified key from this map if present.
|
boolean |
remove(Object
Removes the entry for the specified key only if it is currently mapped to the specified value.
|
V |
replace(K key, V value)
Replaces the entry for the specified key only if it is currently mapped to some value.
|
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
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 |
values()
Returns a
Collection view of the values contained in this map.
|
equals, hashCode, toStringpublic HashMap(int initialCapacity,
float loadFactor)
initialCapacity - the initial capacity
loadFactor - the load factor
IllegalArgumentException - if the initial capacity is negative or the load factor is nonpositive
public HashMap(int initialCapacity)
initialCapacity - the initial capacity.
IllegalArgumentException - if the initial capacity is negative.
public HashMap()
public HashMap(Map<? extends K ,? extends V> m)
m - the map whose mappings are to be placed in this map
NullPointerException - if the specified map is null
public int size()
public boolean isEmpty()
public V get(Objectkey)
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.
get in interface
Map<K,V>
get in class
AbstractMap<K,V>
key - the key whose associated value is to be returned
null if this map contains no mapping for the key
put(Object, Object)
public boolean containsKey(Objectkey)
containsKey in interface
Map<K,V>
containsKey in class
AbstractMap<K,V>
key - The key whose presence in this map is to be tested
public V put(K key, V value)
put in interface
Map<K,V>
put in class
AbstractMap<K,V>
key - key with which the specified value is to be associated
value - value to be associated with the specified key
public void putAll(Map<? extends K ,? extends V> m)
putAll in interface
Map<K,V>
putAll in class
AbstractMap<K,V>
m - mappings to be stored in this map
NullPointerException - if the specified map is null
public V remove(Objectkey)
remove in interface
Map<K,V>
remove in class
AbstractMap<K,V>
key - key whose mapping is to be removed from the map
public void clear()
public boolean containsValue(Objectvalue)
containsValue in interface
Map<K,V>
containsValue in class
AbstractMap<K,V>
value - value whose presence in this map is to be tested
public Set<K> keySet()
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.
public Collection<V> values()
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.
public Set<Map .Entry <K ,V>> entrySet()
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.
public V getOrDefault(Objectkey, V defaultValue)
Map
defaultValue if this map contains no mapping for the key.
getOrDefault in interface
Map<K,V>
key - the key whose associated value is to be returned
defaultValue - the default mapping of the key
defaultValue if this map contains no mapping for the key
public V putIfAbsent(K key, V value)
Map
null) associates it with the given value and returns
null, else returns the current value.
putIfAbsent in interface
Map<K,V>
key - key with which the specified value is to be associated
value - value to be associated with the specified key
null if there was no mapping for the key. (A
null return can also indicate that the map previously associated
null with the key, if the implementation supports null values.)
public boolean remove(Objectkey, Object value)
Map
public boolean replace(K key, V oldValue, V newValue)
Map
public V replace(K key, V value)
Map
replace in interface
Map<K,V>
key - key with which the specified value is associated
value - value to be associated with the specified key
null if there was no mapping for the key. (A
null return can also indicate that the map previously associated
null with the key, if the implementation supports null values.)
public V computeIfAbsent(K key, Function<? super K ,? extends V> mappingFunction)
Map
null), attempts to compute its value using the given mapping function and enters it into this map unless
null.
If the function returns null no mapping is recorded. If the 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);
computeIfAbsent in interface
Map<K,V>
key - key with which the specified value is to be associated
mappingFunction - the function to compute a value
public V computeIfPresent(K key, BiFunction<? super K ,? super V ,? extends V> remappingFunction)
Map
If the function returns null, the mapping is removed. If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.
computeIfPresent in interface
Map<K,V>
key - key with which the specified value is to be associated
remappingFunction - the function to compute a value
public V compute(K key, BiFunction<? super K ,? super V ,? extends V> remappingFunction)
Map
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)) (Method
merge() is often simpler to use for such purposes.)
If the function returns null, the mapping is removed (or remains absent if initially absent). If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.
public V merge(K key, V value, BiFunction<? super V ,? super V ,? extends V> remappingFunction)
Map
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 function returns null the mapping is removed. If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.
merge in interface
Map<K,V>
key - key with which the resulting value is to be associated
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 function to recompute a value if present
public void forEach(BiConsumer<? super K ,? super V> action)
Map
public void replaceAll(BiFunction<? super K ,? super V ,? extends V> function)
Map
replaceAll in interface
Map<K,V>
function - the function to apply to each entry