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java.lang.Objectjava.util.Hashtable<K,V>
@TransactionType(value=SUPPORTS) public class Hashtable<K,V>
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
efficiency: its capacity and its load factor. The load factor
in the CLDC implementation of the hashtable class is always 75 percent. When
the number of entries in the hashtable exceeds the product of the load factor
and the current capacity, the capacity is increased by calling the
rehash
method.
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 numbers = new Hashtable(); numbers.put("one", new Integer(1)); numbers.put("two", new Integer(2)); numbers.put("three", new Integer(3));
To retrieve a number, use the following code:
Integer n = (Integer) numbers.get("two"); if (n != null) { System.out.println("two = " + n); }
This class is thread-safe (synchronzied).
Object.equals(java.lang.Object)
,
Object.hashCode()
,
rehash()
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). |
Method Summary | |
---|---|
void |
clear()
Clears this hashtable so that it contains no keys. |
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. |
Enumeration<V> |
elements()
Returns an enumeration of the values in this hashtable. |
boolean |
equals(Object o)
Compares the specified object with this hashtable for equality. |
V |
get(K key)
Returns the value to which the specified key is mapped in this hashtable. |
int |
hashCode()
Returns the hash code value for this hashtable. |
boolean |
isEmpty()
Tests if this hashtable maps no keys to values. |
Enumeration<K> |
keys()
Returns an enumeration of the keys in this hashtable. |
V |
put(K key,
V value)
Maps the specified key to the specified value
in this hashtable. |
protected void |
rehash()
Rehashes the contents of the hashtable into a hashtable with a larger capacity. |
V |
remove(K 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 rather long string representation of this hashtable. |
Methods inherited from class java.lang.Object |
---|
getClass, notify, notifyAll, wait, wait, wait |
Constructor Detail |
---|
public Hashtable(int initialCapacity)
initialCapacity
- the initial capacity of the hashtable.
IllegalArgumentException
- if the initial capacity is less than zeropublic Hashtable()
Method Detail |
---|
public int size()
public boolean isEmpty()
true
if this hashtable maps no keys to values;
false
otherwise.public Enumeration<K> keys()
Enumeration
,
elements()
public Enumeration<V> elements()
Enumeration
,
keys()
public boolean contains(Object value)
containsKey
method.
value
- a value to search for.
true
if some key maps to the value
argument in this hashtable; false
otherwise.
NullPointerException
- if the value is null
.containsKey(java.lang.Object)
public boolean containsKey(Object key)
key
- possible key.
true
if the specified object is a key in this
hashtable; false
otherwise.contains(java.lang.Object)
public V get(K key)
key
- a key in the hashtable.
null
if the key is not mapped to any value in this
hashtable.put(java.lang.Object, java.lang.Object)
protected void rehash()
public V put(K key, V value)
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.
key
- the hashtable key.value
- the value.
null
if it did not have one.
NullPointerException
- if the key or value is null
.Object.equals(java.lang.Object)
,
get(java.lang.Object)
public V remove(K key)
key
- the key that needs to be removed.
null
if the key did not have a mapping.public void clear()
public String toString()
toString
in class Object
public boolean equals(Object o)
equals
in class Object
o
- object to be compared for equality with this hashtable
Boolean.hashCode()
,
Hashtable
public int hashCode()
Object.hashCode()
.
hashCode
in class Object
Object.equals(Object)
,
equals(Object)
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