Class HashMap<K,​V>

Type Parameters:
K - the type of keys maintained by this map
V - 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 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.

Since:
1.2
See Also:
Object.hashCode(), Collection, Map, TreeMap, Hashtable, Serialized Form

Nested Classes

Nested classes/interfaces declared in class java.util.AbstractMap

AbstractMap.SimpleEntry<K,​V>, AbstractMap.SimpleImmutableEntry<K,​V>

Nested classes/interfaces declared in interface java.util.Map

Map.Entry<K,​V>

Constructors

Constructor 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<? extends K,​? extends V> m)

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

Methods

Modifier and Type Method 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<? 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.
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<Map.Entry<K,​V>> entrySet()

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

Methods declared in class java.util.AbstractMap

equals, hashCode, toString

Methods declared in class java.lang.Object

finalize, getClass, notify, notifyAll, wait, wait, wait

Methods declared in interface java.util.Map

equals, forEach, getOrDefault, hashCode, putIfAbsent, remove, replace, replace, replaceAll

Constructors

HashMap

public HashMap(int initialCapacity,
               float loadFactor)

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

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

HashMap

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.
Throws:
IllegalArgumentException - if the initial capacity is negative.

HashMap

public HashMap()

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

HashMap

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
Throws:
NullPointerException - if the specified map is null

Methods

size

public int size()

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

Specified by:
size in interface Map<K,​V>
Overrides:
size in class AbstractMap<K,​V>
Returns:
the number of key-value mappings in this map

isEmpty

public boolean isEmpty()

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

Specified by:
isEmpty in interface Map<K,​V>
Overrides:
isEmpty in class AbstractMap<K,​V>
Returns:
true if this map contains no key-value mappings

get

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.

Specified by:
get in interface Map<K,​V>
Overrides:
get in class AbstractMap<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)

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

Specified by:
containsKey in interface Map<K,​V>
Overrides:
containsKey in class AbstractMap<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 interface Map<K,​V>
Overrides:
put in class AbstractMap<K,​V>
Parameters:
key - key with which the specified value is to be associated
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.)

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 interface Map<K,​V>
Overrides:
putAll in class AbstractMap<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 interface Map<K,​V>
Overrides:
remove in class AbstractMap<K,​V>
Parameters:
key - key whose mapping is to be removed from the map
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.)

clear

public void clear()

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

Specified by:
clear in interface Map<K,​V>
Overrides:
clear in class AbstractMap<K,​V>

containsValue

public boolean containsValue(Object value)

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

Specified by:
containsValue in interface Map<K,​V>
Overrides:
containsValue in class AbstractMap<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

keySet

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.

Specified by:
keySet in interface Map<K,​V>
Overrides:
keySet in class AbstractMap<K,​V>
Returns:
a set view of the keys contained in this map

values

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.

Specified by:
values in interface Map<K,​V>
Overrides:
values in class AbstractMap<K,​V>
Returns:
a view of the values contained in this map

entrySet

public Set<Map.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.

Specified by:
entrySet in interface Map<K,​V>
Returns:
a set view of the mappings contained in this map

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 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.

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 interface Map<K,​V>
Parameters:
key - key with which the specified value is to be associated
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
Throws:
ConcurrentModificationException - if it is detected that the mapping function modified this map

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 interface Map<K,​V>
Parameters:
key - key with which the specified value is to be associated
remappingFunction - 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

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 (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))
(Method 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 interface Map<K,​V>
Parameters:
key - key with which the specified value is to be associated
remappingFunction - 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

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 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.

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 interface Map<K,​V>
Parameters:
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 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

clone

public Object clone()

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

Overrides:
clone in class AbstractMap<K,​V>
Returns:
a shallow copy of this map
See Also:
Cloneable

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