K - the type of keys maintained by this map
V - the type of mapped values
Serializable, Cloneable, Map<K,V>
Properties, UIDefaults
This class implements a hash table, which maps keys to values. Any non-
null
object can be used as a key or as a value.
To successfully store and retrieve objects from a hashtable, the objects used as keys must implement the hashCode method and the equals method.
An instance of Hashtable 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. Note that the hash table is open: in the case of a "hash collision", a single bucket stores multiple entries, which must be searched sequentially. The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. The initial capacity and load factor parameters are merely hints to the implementation. The exact details as to when and whether the rehash method is invoked are implementation-dependent.
Generally, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the time cost to look up an entry (which is reflected in most Hashtable operations, including get and put).
The initial capacity controls a tradeoff between wasted space and the need for rehash operations, which are time-consuming. No rehash operations will ever occur if the initial capacity is greater than the maximum number of entries the Hashtable will contain divided by its load factor. However, setting the initial capacity too high can waste space.
If many entries are to be made into a Hashtable, creating it with a sufficiently large capacity may allow the entries to be inserted more efficiently than letting it perform automatic rehashing as needed to grow the table.
This example creates a hashtable of numbers. It uses the names of the numbers as keys:
Hashtable<String, Integer> numbers
= new Hashtable<String, Integer>();
numbers.put("one", 1);
numbers.put("two", 2);
numbers.put("three", 3);
To retrieve a number, use the following code:
Integer n = numbers.get("two");
if (n != null) {
System.out.println("two = " + n);
}
The iterators returned by the iterator method of the collections returned by all of this class's "collection view methods" are fail-fast: if the Hashtable 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. The Enumerations returned by Hashtable's keys and elements methods are not fail-fast; if the Hashtable is structurally modified at any time after the enumeration is created then the results of enumerating are undefined.
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.
As of the Java 2 platform v1.2, this class was retrofitted to implement the Map interface, making it a member of the Java Collections Framework. Unlike the new collection implementations, Hashtable is synchronized. If a thread-safe implementation is not needed, it is recommended to use HashMap in place of Hashtable. If a thread-safe highly-concurrent implementation is desired, then it is recommended to use ConcurrentHashMap in place of Hashtable.
Constructors
Constructs a new, empty hashtable with a default initial capacity (11) and load factor (0.75).
Constructs a new, empty hashtable with the specified initial capacity and default load factor (0.75).
Hashtable(int initialCapacity, float loadFactor)
Constructs a new, empty hashtable with the specified initial capacity and the specified load factor.
Constructs a new hashtable with the same mappings as the given Map.
void
Clears this hashtable so that it contains no keys.
Creates a shallow copy of this hashtable.
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
Tests if some key maps into the specified value in this hashtable.
boolean
Tests if the specified object is a key in this hashtable.
boolean
Returns true if this hashtable maps one or more keys to this value.
Returns an enumeration of the values in this hashtable.
Returns a
Set
view of the mappings contained in this map.
boolean
Compares the specified Object with this Map for equality, as per the definition in the Map interface.
Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
int
Returns the hash code value for this Map as per the definition in the Map interface.
boolean
Tests if this hashtable maps no keys to values.
Returns an enumeration of the keys in this hashtable.
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.
Maps the specified key to the specified value in this hashtable.
void
Copies all of the mappings from the specified map to this hashtable.
protected void
Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently.
Removes the key (and its corresponding value) from this hashtable.
int
Returns the number of keys in this hashtable.
Returns a string representation of this Hashtable object in the form of a set of entries, enclosed in braces and separated by the ASCII characters " , " (comma and space).
Returns a
Collection
view of the values contained in this map.
public Hashtable(int initialCapacity, float loadFactor)
Constructs a new, empty hashtable with the specified initial capacity and the specified load factor.
initialCapacity - the initial capacity of the hashtable.
loadFactor - the load factor of the hashtable.
IllegalArgumentException - if the initial capacity is less than zero, or if the load factor is nonpositive.
public Hashtable(int initialCapacity)
Constructs a new, empty hashtable with the specified initial capacity and default load factor (0.75).
initialCapacity - the initial capacity of the hashtable.
IllegalArgumentException - if the initial capacity is less than zero.
public Hashtable()
Constructs a new, empty hashtable with a default initial capacity (11) and load factor (0.75).
Constructs a new hashtable with the same mappings as the given Map. The hashtable is created with an initial capacity sufficient to hold the mappings in the given Map and a default load factor (0.75).
t - 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 keys in this hashtable.
public boolean isEmpty()
Tests if this hashtable maps no keys to values.
Returns an enumeration of the keys in this hashtable. Use the Enumeration methods on the returned object to fetch the keys sequentially. If the hashtable is structurally modified while enumerating over the keys then the results of enumerating are undefined.
keys in class Dictionary<K,V>
Returns an enumeration of the values in this hashtable. Use the Enumeration methods on the returned object to fetch the elements sequentially. If the hashtable is structurally modified while enumerating over the values then the results of enumerating are undefined.
elements in class Dictionary<K,V>
Tests if some key maps into the specified value in this hashtable. This operation is more expensive than the
containsKey
method.
Note that this method is identical in functionality to containsValue, (which is part of the Map interface in the collections framework).
value - a value to search for
true if and only if some key maps to the value argument in this hashtable as determined by the equals method; false otherwise.
NullPointerException - if the value is null
Note that this method is identical in functionality to contains (which predates the Map interface).
containsValue in interface Map<K,V>
value - value whose presence in this hashtable is to be tested
true if this map maps one or more keys to the specified value
NullPointerException - if the value is null
Tests if the specified object is a key in this hashtable.
containsKey in interface Map<K,V>
key - possible key
true if and only if the specified object is a key in this hashtable, as determined by the equals method; false otherwise.
NullPointerException - if the key is null
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.equals(k)), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)
get in interface Map<K,V>
get in class Dictionary<K,V>
key - the key whose associated value is to be returned
null if this map contains no mapping for the key
NullPointerException - if the specified key is null
protected void rehash()
Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently. This method is called automatically when the number of keys in the hashtable exceeds this hashtable's capacity and load factor.
Maps the specified
key
to the specified
value
in this hashtable. Neither the key nor the value can be
null
.
The value can be retrieved by calling the get method with a key that is equal to the original key.
put in interface Map<K,V>
put in class Dictionary<K,V>
key - the hashtable key
value - the value
null if it did not have one
NullPointerException - if the key or value is null
Removes the key (and its corresponding value) from this hashtable. This method does nothing if the key is not in the hashtable.
remove in interface Map<K,V>
remove in class Dictionary<K,V>
key - the key that needs to be removed
null if the key did not have a mapping
NullPointerException - if the key is null
Copies all of the mappings from the specified map to this hashtable. These mappings will replace any mappings that this hashtable had for any of the keys currently in the specified map.
putAll in interface Map<K,V>
t - mappings to be stored in this map
NullPointerException - if the specified map is null
public void clear()
Clears this hashtable so that it contains no keys.
Creates a shallow copy of this hashtable. All the structure of the hashtable itself is copied, but the keys and values are not cloned. This is a relatively expensive operation.
Returns a string representation of this Hashtable object in the form of a set of entries, enclosed in braces and separated by the ASCII characters " , " (comma and space). Each entry is rendered as the key, an equals sign =, and the associated element, where the toString method is used to convert the key and element to strings.
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 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
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.
Compares the specified Object with this Map for equality, as per the definition in the Map interface.
public int hashCode()
Returns the hash code value for this Map as per the definition in the Map interface.
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 the mapping function modified 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 the remapping function modified 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 the remapping function modified 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 the remapping function modified 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
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