public class Hashtable extends Object
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);
}
Object.equals(java.lang.Object),
Object.hashCode(),
rehash()| Constructor and Description |
|---|
Hashtable()
Constructs a new, empty hashtable with a default capacity and load
factor.
|
Hashtable(int initialCapacity)
Constructs a new, empty hashtable with the specified initial
capacity.
|
| Modifier and Type | Method and Description |
|---|---|
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 |
elements()
Returns an enumeration of the values in this hashtable.
|
Object |
get(Object key)
Returns the value to which the specified key is mapped in this hashtable.
|
boolean |
isEmpty()
Tests if this hashtable maps no keys to values.
|
Enumeration |
keys()
Returns an enumeration of the keys in this hashtable.
|
Object |
put(Object key,
Object 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.
|
Object |
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 rather long string representation of this hashtable.
|
public Hashtable(int initialCapacity)
initialCapacity - the initial capacity of the hashtable.IllegalArgumentException - if the initial capacity is less
than zeropublic Hashtable()
public int size()
public boolean isEmpty()
true if this hashtable maps no keys to values;
false otherwise.public Enumeration keys()
Enumeration,
elements()public Enumeration 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 Object get(Object 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 Object put(Object key, Object 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 Object remove(Object key)
key - the key that needs to be removed.null if the key did not have a mapping.public void clear()