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Java™ Platform Standard Ed. 6 |
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java.util
Class Hashtable<K,V>
java.lang.Object
java.util.Dictionary<K,V>
java.util.Hashtable<K,V>
- All Implemented Interfaces:
- Serializable, Cloneable, Map<K,V>
- Direct Known Subclasses:
- Properties, UIDefaults
public class Hashtable<K,V>extends Dictionary<K,V>implements Map<K,V>, Cloneable, Serializable
This class implements a hashtable, 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 aConcurrentModificationException. 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.
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.
- Since:
- JDK1.0
- See Also:
Object.equals(java.lang.Object),Object.hashCode(),rehash(),Collection,Map,HashMap,TreeMap, Serialized Form
| Constructor Summary | |
|---|---|
Hashtable() Constructs a new, empty hashtable with a default initial capacity (11) and load factor (0.75). |
|
Hashtable(int initialCapacity) 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. |
|
Hashtable(Map<? extends K,? extends V> t) Constructs a new hashtable with the same mappings as the given Map. |
|
| Method Summary | |
|---|---|
void |
clear() Clears this hashtable so that it contains no keys. |
Object |
clone() Creates a shallow copy of this hashtable. |
boolean |
contains(Object value) Tests if some key maps into the specified value in this hashtable. |
boolean |
containsKey(Object key) Tests if the specified object is a key in this hashtable. |
boolean |
containsValue(Object value) Returns true if this hashtable maps one or more keys to this value. |
Enumeration<V> |
elements() Returns an enumeration of the values in this hashtable. |
Set<Map.Entry<K,V>> |
entrySet() Returns a Set view of the mappings contained in this map. |
boolean |
equals(Object o) Compares the specified Object with this Map for equality, as per the definition in the Map interface. |
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. |
int |
hashCode() Returns the hash code value for this Map as per the definition in the Map interface. |
boolean |
isEmpty() Tests if this hashtable maps no keys to values. |
Enumeration<K> |
keys() Returns an enumeration of the keys in this hashtable. |
Set<K> |
keySet() Returns a Set view of the keys contained in this map. |
V |
put(K key, V value) Maps the specified key to the specified value in this hashtable. |
void |
putAll(Map<? extends K,? extends V> t) Copies all of the mappings from the specified map to this hashtable. |
protected void |
rehash() Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently. |
V |
remove(Object key) Removes the key (and its corresponding value) from this hashtable. |
int |
size() Returns the number of keys in this hashtable. |
String |
toString() 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). |
Collection<V> |
values() Returns a Collection view of the values contained in this map. |
| Methods inherited from class java.lang.Object |
|---|
finalize, getClass, notify, notifyAll, wait, wait, wait |
| Constructor Detail |
|---|
Hashtable
public Hashtable(int initialCapacity,
float loadFactor)
- Constructs a new, empty hashtable with the specified initial capacity and the specified load factor.
- Parameters:
initialCapacity- the initial capacity of the hashtable.loadFactor- the load factor of the hashtable.- Throws:
IllegalArgumentException- if the initial capacity is less than zero, or if the load factor is nonpositive.
Hashtable
public Hashtable(int initialCapacity)
- Constructs a new, empty hashtable with the specified initial capacity and default load factor (0.75).
- Parameters:
initialCapacity- the initial capacity of the hashtable.- Throws:
IllegalArgumentException- if the initial capacity is less than zero.
Hashtable
public Hashtable()
- Constructs a new, empty hashtable with a default initial capacity (11) and load factor (0.75).
Hashtable
public Hashtable(Map<? extends K,? extends V> t)
- 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).
- Parameters:
t- the map whose mappings are to be placed in this map.- Throws:
NullPointerException- if the specified map is null.- Since:
- 1.2
| Method Detail |
|---|
size
public int size()
- Returns the number of keys in this hashtable.
- Returns:
- the number of keys in this hashtable.
isEmpty
public boolean isEmpty()
- Tests if this hashtable maps no keys to values.
- Returns:
trueif this hashtable maps no keys to values;falseotherwise.
keys
public Enumeration<K> keys()
- Returns an enumeration of the keys in this hashtable.
- Specified by:
keysin classDictionary<K,V>
- Returns:
- an enumeration of the keys in this hashtable.
- See Also:
Enumeration,elements(),keySet(),Map
elements
public Enumeration<V> elements()
- Returns an enumeration of the values in this hashtable. Use the Enumeration methods on the returned object to fetch the elements sequentially.
- Specified by:
elementsin classDictionary<K,V>
- Returns:
- an enumeration of the values in this hashtable.
- See Also:
Enumeration,keys(),values(),Map
contains
public boolean contains(Object value)
- Tests if some key maps into the specified value in this hashtable. This operation is more expensive than the
containsKeymethod.Note that this method is identical in functionality to
containsValue, (which is part of theMapinterface in the collections framework). - Parameters:
value- a value to search for- Returns:
trueif and only if some key maps to thevalueargument in this hashtable as determined by the equals method;falseotherwise.- Throws:
NullPointerException- if the value isnull
containsValue
public boolean containsValue(Object value)
- Returns true if this hashtable maps one or more keys to this value.
Note that this method is identical in functionality to
contains(which predates theMapinterface). - Specified by:
containsValuein interfaceMap<K,V>
- Parameters:
value- value whose presence in this hashtable is to be tested- Returns:
- true if this map maps one or more keys to the specified value
- Throws:
NullPointerException- if the value isnull- Since:
- 1.2
containsKey
public boolean containsKey(Object key)
- Tests if the specified object is a key in this hashtable.
- Specified by:
containsKeyin interfaceMap<K,V>
- Parameters:
key- possible key- Returns:
trueif and only if the specified object is a key in this hashtable, as determined by the equals method;falseotherwise.- Throws:
NullPointerException- if the key isnull- See Also:
contains(Object)
get
public V get(Object key)
- Returns the value to which the specified key is mapped, or
nullif this map contains no mapping for the key.More formally, if this map contains a mapping from a key
kto a valuevsuch that(key.equals(k)), then this method returnsv; otherwise it returnsnull. (There can be at most one such mapping.) - Parameters:
key- the key whose associated value is to be returned- Returns:
- the value to which the specified key is mapped, or
nullif this map contains no mapping for the key - Throws:
NullPointerException- if the specified key is null- See Also:
put(Object, Object)
rehash
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.
put
public V put(K key, V value)
- Maps the specified
keyto the specifiedvaluein this hashtable. Neither the key nor the value can benull.The value can be retrieved by calling the
getmethod with a key that is equal to the original key. - Parameters:
key- the hashtable keyvalue- the value- Returns:
- the previous value of the specified key in this hashtable, or
nullif it did not have one - Throws:
NullPointerException- if the key or value isnull- See Also:
Object.equals(Object),get(Object)
remove
public V remove(Object key)
- Removes the key (and its corresponding value) from this hashtable. This method does nothing if the key is not in the hashtable.
- Parameters:
key- the key that needs to be removed- Returns:
- the value to which the key had been mapped in this hashtable, or
nullif the key did not have a mapping - Throws:
NullPointerException- if the key isnull
putAll
public void putAll(Map<? extends K,? extends V> t)
- 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.
- Parameters:
t- mappings to be stored in this map- Throws:
NullPointerException- if the specified map is null- Since:
- 1.2
clear
public void clear()
clone
public Object clone()
- 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 clone of the hashtable
- See Also:
Cloneable
toString
public String toString()
- 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 string representation of this hashtable
keySet
public Set<K> keySet()
- Returns a
Setview 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
- Since:
- 1.2
entrySet
public Set<Map.Entry<K,V>> entrySet()
- Returns a
Setview 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
- Since:
- 1.2
values
public Collection<V> values()
- Returns a
Collectionview 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 theIterator.remove, Collection.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
- Since:
- 1.2
equals
public boolean equals(Object o)
- Compares the specified Object with this Map for equality, as per the definition in the Map interface.
- Parameters:
o- object to be compared for equality with this hashtable- Returns:
- true if the specified Object is equal to this Map
- Since:
- 1.2
- See Also:
Map.equals(Object)
hashCode
public int hashCode()
- Returns the hash code value for this Map as per the definition in the Map interface.
- Returns:
- a hash code value for this object.
- Since:
- 1.2
- See Also:
Map.hashCode()
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Java™ Platform Standard Ed. 6 |
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