waba.util
Class Hashtable

java.lang.Object
  |
  +--waba.util.Hashtable

public class Hashtable
extends java.lang.Object

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 be String, because currently on SuperWaba only string objects implements hashCode method.

An instance of Hashtable has two parameters that affect its efficiency: its capacity and its load factor. The load factor should be between 0.0 and 1.0. 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. Larger load factors use memory more efficiently, at the expense of larger expected time per lookup.

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", Convert.toString(1));
     numbers.put("two", Convert.toString(2));
     numbers.put("three", Convert.toString(3));
 

To retrieve a number, use the following code:

     String n = (String)numbers.get("two");
     if (n != null) {
         System.out.println("two = " + Convert.toInt(n));
     }
 
Important notes (added by guich)

There are no default constructor to decrease memory usage. Also,

thanks to Arthur van Hoff


Constructor Summary
Hashtable(int initialCapacity)
          Constructs a new, empty hashtable with the specified initial capacity and default load factor of 0.75f.
Hashtable(int initialCapacity, float loadFactor)
          Constructs a new, empty hashtable with the specified initial capacity and the specified load factor.
 
Method Summary
 void clear()
          Clears this hashtable so that it contains no keys.
 java.lang.Object get(java.lang.String key)
          Returns the value to which the specified key is mapped in this hashtable.
 Vector getKeys()
          Return a Vector of the keys in the Hashtable.
 java.lang.Object put(java.lang.String key, java.lang.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.
 java.lang.Object remove(java.lang.String key)
          Removes the key (and its corresponding value) from this hashtable.
 int size()
          Returns the number of keys in this hashtable.
 java.lang.String toString()
          Returns a rather long string representation of this hashtable.
 
Methods inherited from class java.lang.Object
hashCode
 

Constructor Detail

Hashtable

public Hashtable(int initialCapacity)
Constructs a new, empty hashtable with the specified initial capacity and default load factor of 0.75f.
Parameters:
initialCapacity - the initial capacity of the hashtable.

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 - a number between 0.0 and 1.0.
Method Detail

clear

public void clear()
Clears this hashtable so that it contains no keys.

get

public java.lang.Object get(java.lang.String key)
Returns the value to which the specified key is mapped in this hashtable.
Parameters:
key - a key in the hashtable.
Returns:
the value to which the key is mapped in this hashtable; null if the key is not mapped to any value in this hashtable.
Since:
JDK1.0
See Also:
java.util.Hashtable#put(java.lang.Object, java.lang.Object)

getKeys

public Vector getKeys()
Return a Vector of the keys in the Hashtable. Added ds@120.

put

public java.lang.Object put(java.lang.String key,
                            java.lang.Object value)
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.

Parameters:
key - the hashtable key.
value - the value.
Returns:
the previous value of the specified key in this hashtable, or null if it did not have one.
Since:
JDK1.0
See Also:
java.lang.Object#equals(java.lang.Object), java.util.Hashtable#get(java.lang.Object)

rehash

protected void rehash()
Rehashes the contents of the hashtable into a hashtable with a larger capacity. This method is called automatically when the number of keys in the hashtable exceeds this hashtable's capacity and load factor.
Since:
JDK1.0

remove

public java.lang.Object remove(java.lang.String 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 null if the key did not have a mapping.

size

public int size()
Returns the number of keys in this hashtable.
Returns:
the number of keys in this hashtable.

toString

public java.lang.String toString()
Returns a rather long string representation of this hashtable.
Overrides:
toString in class java.lang.Object
Returns:
a string representation of this hashtable.
Since:
JDK1.0