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General

Hash table in java

Hashtable in Java

Hashtable in Java

  • Java Hashtable class implements a Map Interface and extends Dictionary class
  • It is dual value pair collection in Java which stores <key,Value> pair in collection

The important points about Java Hashtable class are:

  • A Hashtable is an array of list. Each list is known as a bucket. The position of the bucket is identified by calling the hashcode() method.
  • A Hashtable contains values based on the key.
  • It contains only unique elements.
  • Value and key should not be NULL.
  • It is synchronized. (Thread safe, can be used in multi-threading environment).

Syntax

  Hashtable<Integer,String> hm=new Hashtable<Integer,String>();  

Example

package Mypkg;
import java.util.Hashtable;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
public class MAPex {
            /**
             * @param args
             */
            public static void main(String[] args) {
                        // TODO Auto-generated method stub
                System.out.println("***********************************");
                Hashtable<Integer,String> hm=new Hashtable<Integer,String>(); 
                hm.put(100,"Besant"); 
                hm.put(600,"Chennai"); 
                hm.put(200,"Technology"); 
                hm.put(103,"Bangalore"); 
                            for(Map.Entry m:hm.entrySet()){ 
                 System.out.println(m.getKey()+"-> "+m.getValue()); 
                } 
                System.out.println("***********************************");
}
}

Output

*********************************

600-> Chennai

103-> Bangalore

200-> Technology

100-> Besant

***********************************

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Methods of Hashtable

Method name Description
void clear() Removes all the key-value pairs.
boolean containsKey(Object key) Returns true, if the Hashtable contains a value for the specified key.
boolean contains value(Object value) Returns true, if the Hashtable maps a value for the specified key.
Enumeration<V> elements() Iterates  the values in the Hashtable.
Object get(Object key) Returns the value to which the specified key is mapped, or null if the map contains no mapping for the key.
Set entrySet() Returns a Sets which contains the key-value pairs of the map.
int hashCode() Returns the hash code value for the Hashtable
boolean isEmpty() Tests if this Hashtable maps no key value pairs.
Enumeration<K> keys() Returns the keys mapped in the Hashtable with the type Enumeration.
Set<K> keySet() Returns the keys mapped in the Hashtable as the type Set.
Value put(K key, V value) Maps the specified key with the specified value in the Hashtable.
void rehash() Increases the size of the Hashtable and re-maps the key values according to the new size.
V replace(K key, V value) Replaces the value of the specified key with the value specified.
int size() Returns the number of key-value pairs in the map.
Value remove(Object key) Removes the key value pair for the specified key from the map if it is present.

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