K - the type of keys maintained by this map
V - the type of mapped values
Serializable, Cloneable, Map<K,V>
LinkedHashMap, PrinterStateReasons
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.
Map.Entry<K,V>Constructors
Constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75).
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.
Constructs a new HashMap with the same mappings as the specified Map.
void
Removes all of the mappings from this map.
Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned.
Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping).
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 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
Returns true if this map contains a mapping for the specified key.
boolean
Returns true if this map maps one or more keys to the specified value.
Returns a
Set
view of the mappings contained in this map.
Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
boolean
Returns true if this map contains no key-value mappings.
Returns a
Set
view 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.
Creates a new, empty HashMap suitable for the expected number of mappings.
Associates the specified value with the specified key in this map.
void
Copies all of the mappings from the specified map to this map.
Removes the mapping for the specified key from this map if present.
int
Returns the number of key-value mappings in this map.
Returns a
Collection
view of the values contained in this map.
public HashMap(int initialCapacity, float loadFactor)
Constructs an empty HashMap with the specified initial capacity and load factor.
HashMap with an initial capacity that accommodates an expected number of mappings, use newHashMap.
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)
Constructs an empty HashMap with the specified initial capacity and the default load factor (0.75).
HashMap with an initial capacity that accommodates an expected number of mappings, use newHashMap.
initialCapacity - the initial capacity.
IllegalArgumentException - if the initial capacity is negative.
public HashMap()
Constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75).
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.
m - the map whose mappings are to be placed in this map
NullPointerException - if the specified map is null
public int size()
Returns the number of key-value mappings in this map.
public boolean isEmpty()
Returns true if this map contains no key-value mappings.
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.
Returns true if this map contains a mapping for the specified key.
containsKey in interface Map<K,V>
containsKey in class AbstractMap<K,V>
key - The key whose presence in this map is to be tested
true if this map contains a mapping for the specified key.
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.
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
key, or null if there was no mapping for key. (A null return can also indicate that the map previously associated null with key.)
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.
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
Removes the mapping for the specified key from this map if present.
remove in interface Map<K,V>
remove in class AbstractMap<K,V>
key - key whose mapping is to be removed from the map
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 clear()
Removes all of the mappings from this map. The map will be empty after this call returns.
Returns true if this map maps one or more keys to the specified value.
containsValue in interface Map<K,V>
containsValue in class AbstractMap<K,V>
value - value whose presence in this map is to be tested
true if this map maps one or more keys to the specified value
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
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
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.
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.
computeIfAbsent in interface Map<K,V>
key - key with which the specified value is to be associated
mappingFunction - the mapping function to compute a value
ConcurrentModificationException - if it is detected that the mapping function modified this map
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.
computeIfPresent in interface Map<K,V>
key - key with which the specified value is to be associated
remappingFunction - the remapping function to compute a value
ConcurrentModificationException - if it is detected that the remapping function modified this map
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.
compute in interface Map<K,V>
key - key with which the specified value is to be associated
remappingFunction - the remapping function to compute a value
ConcurrentModificationException - if it is detected that the remapping function modified 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. 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.
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 remapping function to recompute a value if present
ConcurrentModificationException - if it is detected that the remapping function modified this map
Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned.
clone in class AbstractMap<K,V>
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.
K - the type of keys maintained by the new map
V - the type of mapped values
numMappings - the expected number of mappings
IllegalArgumentException - if numMappings is negative
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