java.util Hashtable

java.util
Hashtable

Declaration
public class Hashtable
java.lang.Object
|
+--java.util.Hashtable
Description
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 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);
}
Since: JDK1.0, CLDC 1.0
See Also: java.lang.Object.equals(Object), java.lang.Object.hashCode(),
rehash()


Constructors
Hashtable()
Hashtable(int initialCapacity)
Methods
void clear()
boolean contains(java.lang.Object value)

Hashtable(int)


boolean containsKey(java.lang.Object key)
Enumeration elements()
java.lang.Object get(java.lang.Object key)
boolean isEmpty()
Enumeration keys()
java.lang.Object put(java.lang.Object key, java.lang.Object value)
protected void rehash()
java.lang.Object remove(java.lang.Object key)
int size()
java.lang.String toString()




Methods inherited from class Object
equals(Object), getClass(), hashCode(), notify(), notifyAll(), wait(), wait(), wait()


Constructors
Hashtable(int)
Declaration:
public Hashtable(int initialCapacity)
Description:
Constructs a new, empty hashtable with the specified initial capacity.
Parameters:
initialCapacity - the initial capacity of the hashtable.
Throws:
java.lang.IllegalArgumentException - if the initial capacity is less than zero
Since: JDK1.0
Hashtable()
Declaration:
public Hashtable()
Description:
Constructs a new, empty hashtable with a default capacity and load factor.
Since: JDK1.0


Methods
size()
Declaration:
public int size()
Description:
Returns the number of keys in this hashtable.
Returns: the number of keys in this hashtable.
Since: JDK1.0
isEmpty()
Declaration:
public boolean isEmpty()
Description:
Tests if this hashtable maps no keys to values.
Returns: true if this hashtable maps no keys to values; false otherwise.
Since: JDK1.0
keys()
Declaration:
public java.util.Enumeration keys()
Description:
Returns an enumeration of the keys in this hashtable.
Returns: an enumeration of the keys in this hashtable.
Since: JDK1.0
See Also: Enumeration, elements()
elements()
Declaration:
public java.util.Enumeration elements()
Description:
Returns an enumeration of the values in this hashtable. Use the Enumeration methods on the returned object
to fetch the elements sequentially.
Returns: an enumeration of the values in this hashtable.
Since: JDK1.0
See Also: Enumeration, keys()
contains(Object)
Declaration:
public boolean contains(java.lang.Object value)

containsKey(Object)

Description:
Tests if some key maps into the specified value in this hashtable. This operation is more expensive than the
containsKey method.
Parameters:
value - a value to search for.
Returns: true if some key maps to the value argument in this hashtable; false otherwise.
Throws:
java.lang.NullPointerException - if the value is null.
Since: JDK1.0
See Also: containsKey(Object)
containsKey(Object)
Declaration:
public boolean containsKey(java.lang.Object key)
Description:
Tests if the specified object is a key in this hashtable.
Parameters:
key - possible key.
Returns: true if the specified object is a key in this hashtable; false otherwise.
Since: JDK1.0
See Also: contains(Object)
get(Object)
Declaration:
public java.lang.Object get(java.lang.Object key)
Description:
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: put(Object, Object)
rehash()
Declaration:
protected void rehash()
Description:
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


put(Object, Object)
Declaration:
public java.lang.Object put(java.lang.Object key, java.lang.Object value)
Description:
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.
Throws:
java.lang.NullPointerException - if the key or value is null.
Since: JDK1.0
See Also: java.lang.Object.equals(Object), get(Object)
remove(Object)
Declaration:
public java.lang.Object remove(java.lang.Object key)
Description:
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.
Since: JDK1.0
clear()
Declaration:
public void clear()
Description:
Clears this hashtable so that it contains no keys.
Since: JDK1.0
toString()
Declaration:
public java.lang.String toString()
Description:
Returns a rather long string representation of this hashtable.
Overrides: toString in class Object
Returns: a string representation of this hashtable.

toString()
278
Since: JDK1.0