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public class gj.util.Hashtable<A, B>

(source file: util/Hashtable.java)
java.lang.Object
   |
   +----gj.util.Dictionary<A, B>
           |
           +----gj.util.Hashtable<A, B>

The pure class interface.
public class Hashtable<A, B>
  extends Dictionary<A, B>
  implements java.io.Serializable, Cloneable
Hashtable class. Maps keys to values. Any object can be used as a key and/or value.

To sucessfully store and retrieve objects from a hash table, the object used as the key must implement the hashCode() and equals() methods.

This example creates a hashtable of numbers. It uses the names of the numbers as keys:

	Hashtable<String, int> numbers = new Hashtable();
	numbers.put("one", 1);
	numbers.put("two", 2);
	numbers.put("three", 3);
 
To retrieve a number use:
	int n = numbers.get("two");
	if (n != null) {
	    System.out.println("two = " + n);
	}
 

See also:
hashCode, equals

Constuctor Index

O Hashtable()
Constructs a new, empty hashtable
O Hashtable(int)
Constructs a new, empty hashtable with the specified initial capacity.
O Hashtable(int, float)
Constructs a new, empty hashtable with the specified initial capacity and the specified

Methods

O clear()
Clears the hash table so that it has no more elements in it.
O clone()
Creates a clone of the hashtable
O contains(B)
Returns true if the specified object is an element of the hashtable. This operation is more
O containsKey(A)
Returns true if the collection contains an element for the key.
O elements()
Returns an enumeration of the elements
O get(A)
Gets the object associated with the specified key in the hashtable.
O isEmpty()
Returns true if the hashtable contains no elements.
O keys()
Returns an enumeration of the hashtable's keys.
O put(A, B)
Puts the specified element into the hashtable, using the specified key
O rehash()
Rehashes the content of the table into a bigger table. This method is called automatically
O remove(A)
Removes the element corresponding to the key
O size()
Returns the number of elements contained in the hashtable.
O toString()
Converts to a rather lengthy String.

Constructors

O 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 number of buckets
loadFactor - a number between 0.0 and 1.0, it defines the threshold for rehashing the hashtable into a bigger one.
Throws:
IllegalArgumentException -If the initial capacity is less than or equal to zero.
IllegalArgumentException -If the load factor is less than or equal to zero.

O Hashtable

public Hashtable(int initialCapacity);
Constructs a new, empty hashtable with the specified initial capacity.

Parameters:
initialCapacity - the initial number of buckets

O Hashtable

public Hashtable();
Constructs a new, empty hashtable. A default capacity and load factor is used. Note that the hashtable will automatically grow when it gets full.

Methods

O size
public int size();
Returns the number of elements contained in the hashtable.

Overrides:
size in class Dictionary

O isEmpty

public boolean isEmpty();
Returns true if the hashtable contains no elements.

Overrides:
isEmpty in class Dictionary

O keys

public synchronized Enumeration<A> keys();
Returns an enumeration of the hashtable's keys.

Overrides:
keys in class Dictionary
See also:
elements, Enumeration

O elements

public synchronized Enumeration<B> elements();
Returns an enumeration of the elements. Use the Enumeration methods on the returned object to fetch the elements sequentially.

Overrides:
elements in class Dictionary
See also:
keys, Enumeration

O contains

public synchronized boolean contains(B value);
Returns true if the specified object is an element of the hashtable. This operation is more expensive than the containsKey() method.

Parameters:
value - the value that we are looking for
Throws:
NullPointerException -If the value being searched for is equal to null.
See also:
containsKey

O containsKey

public synchronized boolean containsKey(A key);
Returns true if the collection contains an element for the key.

Parameters:
key - the key that we are looking for
See also:
contains

O get

public synchronized B get(A key);
Gets the object associated with the specified key in the hashtable.

Parameters:
key - the specified key
Returns:
the element for the key or null if the key is not defined in the hash table.
Overrides:
get in class Dictionary
See also:
put

O rehash

protected void rehash();
Rehashes the content of the table into a bigger table. This method is called automatically when the hashtable's size exceeds the threshold.

O put

public synchronized B put(A key,
                          B value);
Puts the specified element into the hashtable, using the specified key. The element may be retrieved by doing a get() with the same key. The key and the element cannot be null.

Parameters:
key - the specified key in the hashtable
value - the specified element
Returns:
the old value of the key, or null if it did not have one.
Throws:
NullPointerException -If the value of the element is equal to null.
Overrides:
put in class Dictionary
See also:
get

O remove

public synchronized B remove(A key);
Removes the element corresponding to the key. Does nothing if the key is not present.

Parameters:
key - the key that needs to be removed
Returns:
the value of key, or null if the key was not found.
Overrides:
remove in class Dictionary

O clear

public synchronized void clear();
Clears the hash table so that it has no more elements in it.

O clone

public synchronized Object clone();
Creates a clone of the hashtable. A shallow copy is made, the keys and elements themselves are NOT cloned. This is a relatively expensive operation.

Overrides:
clone in class Object

O toString

public synchronized String toString();
Converts to a rather lengthy String.

Overrides:
toString in class Object


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