structure
Class Hashtable

java.lang.Object
  |
  +--structure.Hashtable

All Implemented Interfaces:

Map

public class Hashtable

extends Object

implements Map

Implements a dictionary as a table of hashed key-value pairs. Collisions are resolved through linear probing. Values used as keys in this structure must have a hashcode method that returns the same value when two keys are "equals". Initially, a table of suggested size is allocated. It will be expanded as the load factor (ratio of pairs to entries) grows.

Example Usage:

To create a hashtable by reading a collection of words and definitions from System.in we could use the following:

 public static void main (String[] argv){
	Hashtable dict = new Hashtable();
	ReadStream r = new ReadStream();
	String word, def;
	System.out.println("Enter a word: ");
	while(!r.eof()){
	    word = r.readLine();
	    System.out.println("Enter a definition: ");
	    def = r.readLine();
	    dict.put(word,def);
	    System.out.println("Enter a word: ");
	}
	System.out.println(dict);
 }
 

See Also:

ChainedHashtable

Field Summary

protected int	capacity The size of the hashtable
protected int	count The number of key-value pairs in table.
protected Association[]	data The data associated with the hashtable.
protected double	loadFactor Load factor that causes rehashing of the table.
protected static Association	reserved A single key-value pair to be used as a token indicating a reserved location in the hashtable.

Constructor Summary

Hashtable()
Construct a hash table that is initially empty.

Hashtable(int initialCapacity)
Construct a hash table that is capable of holding at least initialCapacity values.

Method Summary

void	clear() Remove all key-value pairs from hashtable.
boolean	containsKey(Object key) Returns true iff a specific key appears within the table.
boolean	containsValue(Object value) Returns true if a specific value appears within the table.
Set	entrySet()
protected void	extend()
Object	get(Object key) Get the value associated with a key.
boolean	isEmpty() Determine if table is empty.
Iterator	iterator() Returns a traversal that traverses over the values of the hashtable.
Iterator	keys() Get a traversal over the keys of the hashtable.
Set	keySet()
protected int	locate(Object key)
Object	put(Object key, Object value) Place a key-value pair within the table.
void	putAll(Map other) Put all of the values found in another map into this map, overriding previous key-value associations.
Object	remove(Object key) Remove a key-value pair from the table.
int	size() Return the number of key-value pairs within the table.
String	toString() Generate a string representation of the hash table.
Structure	values()

Methods inherited from class java.lang.Object

, clone, equals, finalize, getClass, hashCode, notify, notifyAll, registerNatives, wait, wait, wait

Methods inherited from interface structure.Map

equals, hashCode

Field Detail

reserved

protected static Association reserved

A single key-value pair to be used as a token indicating a reserved location in the hashtable. Reserved locations are available for insertion, but cause collisions on lookup.

data

protected Association[] data

The data associated with the hashtable.

count

protected int count

The number of key-value pairs in table.

capacity

protected int capacity

The size of the hashtable

loadFactor

protected final double loadFactor

Load factor that causes rehashing of the table.

Constructor Detail

Hashtable

public Hashtable(int initialCapacity)

Construct a hash table that is capable of holding at least initialCapacity values. If that value is approached, it will be expanded appropriately. It is probably best if the capacity is prime. Table is initially empty.

Parameters:

initialCapacity - The initial capacity of the hash table.

Hashtable

public Hashtable()

Construct a hash table that is initially empty.

Method Detail

clear

public void clear()

Remove all key-value pairs from hashtable.

Specified by:

clear in interface Map

Postcondition:

removes all elements from Hashtable

size

public int size()

Return the number of key-value pairs within the table.

Specified by:

size in interface Map

Postcondition:

returns number of elements in hash table

Returns:

The number of key-value pairs currently in table.

isEmpty

public boolean isEmpty()

Determine if table is empty.

Specified by:

isEmpty in interface Map

Postcondition:

returns true iff hash table has 0 elements

Returns:

True if table is empty.

containsValue

public boolean containsValue(Object value)

Returns true if a specific value appears within the table.

Specified by:

containsValue in interface Map

Parameters:

value - The value sought.

Precondition:

value is non-null Object

Postcondition:

returns true iff hash table contains value

Returns:

True iff the value appears within the table.

containsKey

public boolean containsKey(Object key)

Returns true iff a specific key appears within the table.

Specified by:

containsKey in interface Map

Parameters:

key - The key sought.

Precondition:

key is a non-null Object

Postcondition:

returns true if key appears in hash table

Returns:

True iff the key sought appears within table.

iterator

public Iterator iterator()

Returns a traversal that traverses over the values of the hashtable.

Postcondition:

returns traversal to traverse hash table

Returns:

A value traversal, over the values of the table.

get

public Object get(Object key)

Get the value associated with a key.

Specified by:

get in interface Map

Parameters:

key - The key used to find the desired value.

Precondition:

key is non-null Object

Postcondition:

returns value associated with key, or null

Returns:

The value associated with the desired key.

keys

public Iterator keys()

Get a traversal over the keys of the hashtable.

Postcondition:

returns traversal to traverse the keys of hash table;

Returns:

a traversal over the key values appearing within table.

locate

protected int locate(Object key)

put

public Object put(Object key,
                  Object value)

Place a key-value pair within the table.

Specified by:

put in interface Map

Parameters:

key - The key to be added to table.

value - The value associated with key.

Precondition:

key is non-null object

Postcondition:

key-value pair is added to hash table

Returns:

The old value associated with key if previously present.

putAll

public void putAll(Map other)

Put all of the values found in another map into this map, overriding previous key-value associations.

Specified by:

putAll in interface Map

Parameters:

other - is the source mapping

Precondition:

other map is valid

Postcondition:

this hashtable is augmented by the values found in other

remove

public Object remove(Object key)

Remove a key-value pair from the table.

Specified by:

remove in interface Map

Parameters:

key - The key of the key-value pair to be removed.

Precondition:

key is non-null object

Postcondition:

removes key-value pair associated with key

Returns:

The value associated with the removed key.

extend

protected void extend()

Postcondition:

expands the hashtable to reduce loading

entrySet

public Set entrySet()

Specified by:

entrySet in interface Map

Postcondition:

returns a set of Associations associated with this Map

keySet

public Set keySet()

Specified by:

keySet in interface Map

Postcondition:

returns a Set of keys used in this Map

values

public Structure values()

Specified by:

values in interface Map

Postcondition:

returns a Structure that contains the (possibly repeating) values of the range of this map.

toString

public String toString()

Generate a string representation of the hash table.

Overrides:

toString in class Object

Postcondition:

returns a string representation of hash table

Returns:

The string representing the table.