THE PACKAGE JAVA.UTIL

java.util.Hashtable

21.5

21.5 The Class

java.util.Hashtable

. . . never did they seem to have new experiences in common . . . and the things

they had for dissection college, contemporary personality, and the like 

they had hashed and rehashed for many a frugal conversational meal.

 F. Scott Fitzgerald,

This Side of Paradise

 (1920)

The class

Hashtable

 implements the abstract class

Dictionary

 ( 21.4), with

some additional functionality.

public class Hashtable extends Dictionary implements Cloneable {

public Hashtable(int initialCapacity, float loadFactor);

public Hashtable(int initialCapacity);

public Hashtable();

public String toString();

public Object clone();

public int size();

public boolean isEmpty();

public Object get(Object key)

throws NullPointerException;

public Object put(Object key, Object value)

throws NullPointerException;

public Object remove(Object key)

throws NullPointerException;

public Enumeration keys();

public Enumeration elements();

public boolean contains(Object value);

public boolean containsKey(Object key);

protected void rehash();

public void clear();

}

A

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, using 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 in a

Hashtable

, creating it with a sufficiently large

capacity may allow the entries to be inserted more efficiently than letting it per 

form automatic rehashing as needed to grow the table.

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