Internal implementation of Hashtable in java

Internal implementation of Hashtable in java

Hashtable is extends Dictionary abstract class and implements Map, Cloneable, and Serializable interfaces, and it is legacy class which is introduced in java 1.0

public class Hashtable<K,V> extends Dictionary<K,V> implements Map<K,V>, Cloneable, Serializable

Hashtable is synchronized and thread safe, that most of the methods of Hashtable are synchronized.

Few methods in Hashtable

public synchronized int size()

public synchronized V get(Object paramObject)

public synchronized V put(K paramK, V paramV)

public synchronized V remove(Object paramObject)

public synchronized void putAll(Map<? extends K, ? extends V> paramMap)

Hashtable allows only unique elements, which mean it does not allow duplicate key’s but allows duplicate values.

HashTable does not allow the null key’s and null values, if any case we pass the null key and value into Hashtable then it will throw NullPointerException

Hashtable table=new Hashtable();

table.put(101, "emp1");

table.put(102, "emp2");

table.put(null, "emp3"); //java.lang.NullPointerException

Hashtable follows the hashing technique same like HashMap, while storing the elements first of fall it will check hashcode of key and then find the exact bucket to store the elements.

Till your confusing check onceHashMap internal implementation

Note : Initial capacity of Hashtable is 11

We have two options to iterate the Hashtable elements by using Iterator and Enumaration interfaces. Hashtable is fail-fast iterator, while iterating elements by using Iterator interface. If we perform any changes like add, remove elements then we will get ConcurrentModificationException

In the same case Enumaration is not fail-fast iterator.

Example for fail-fast iterator

import java.util.Hashtable;

import java.util.Iterator;

import java.util.Map.Entry;

public class HashtableTest {

       public static void main(String[] args) {

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

              table.put(101, "emp1");   

              table.put(102, "emp2");          

              table.put(103, "emp3");

              Iterator itr=table.entrySet().iterator();             

              while(itr.hasNext()){                   

                     Entry entry=(Entry) itr.next();

                     System.out.println(entry.getKey()+"...."+entry.getValue());

                     table.put(104, "emp4"); //adding element       

              }

       }

}

Insertion order not preserved and it is based on hashcode of key’s

Hashtable is the best choice for if our frequent operation is search operation.