Java Collection Framework Hashtable

Java Hashtable is an implementation of hash table which stores elements in key-value pair. It does not allow null key and null values. It is synchronized version of HashMap.

It extends Dictionary class and implements Map interface. Declaration of this class is given below.

Hashtable Declaration

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

Important Points

• It contains values based on the key.
• It contains unique elements.
• It doesn't allow null key or value.
• It is synchronized.
• The initial default capacity of Hashtable is 11.

In this tutorial, we will learn to create a Hashtable, add new elements to it, traverse its elements etc.

Hashtable Constructors

Example: Creating Hashtable

Lets take an example to create hashtable that takes elements of string and integer type pair. Initially it is empty so if we print it, it shows empty braces. We will learn to add elements in next example.

  
import java.util.*;

class Demo
{
  public static void main(String args[])
  {
    // Creating Hashtable
    Hashtable<String,Integer> hashtable = new Hashtable<String,Integer>();

    // Displaying Hashtable
    System.out.println(hashtable);
    
  }
}
  

{}

Hashtable Methods

Adding Elements to Hashtable

To insert elements into the hashtable, we are using put() method that adds new elements. It takes two argument: first is key and second is value.

  
import java.util.*;

class Demo
{
  public static void main(String args[])
  {
    // Creating Hashtable
    Hashtable<String,Integer> hashtable = new Hashtable<String,Integer>();
    // Adding elements
    hashtable.put("a",100);
    hashtable.put("b",200);
    hashtable.put("c",300);
    hashtable.put("d",400);
    // Displaying Hashtable
    System.out.println(hashtable);
    
  }
}
  

{b=200, a=100, d=400, c=300}

Example: No Null Allowed

Since hashtable does not allow null key or value then forcing to insert the null will throw an error. See the below example.

  
import java.util.*;

class Demo
{
  public static void main(String args[])
  {
    // Creating Hashtable
    Hashtable<String,Integer> hashtable = new Hashtable<String,Integer>();
    // Adding elements
    hashtable.put("a",100);
    hashtable.put("b",200);
    hashtable.put("c",300);
    hashtable.put("d",400);
    hashtable.put(null, 0); // error: no null allowed
    // Displaying Hashtable
    System.out.println(hashtable);
    
  }
}
  

Exception in thread "main" java.lang.NullPointerException

Example: Search for a key or value

Hashtable provides various methods such as contains(), containsKey() etc to search for an element in the hashtable. Contains() method search for specified value while containsKey() method search for specified key.

  
import java.util.*;

class Demo
{
  public static void main(String args[])
  {
    // Creating Hashtable
    Hashtable<String,Integer> hashtable = new Hashtable<String,Integer>();
    // Adding elements
    hashtable.put("a",100);
    hashtable.put("b",200);
    hashtable.put("c",300);
    hashtable.put("d",400);
    // Displaying Hashtable
    System.out.println(hashtable);
    // Search for a value
    boolean val = hashtable.contains(400);
    System.out.println("is 400 present: "+val);
    // Search for a key
    val = hashtable.containsKey("d");
    System.out.println("is d present: "+val);   
  }
}
  

{b=200, a=100, d=400, c=300} is 400 present: true is d present: true

Example: Traverse Hashtable

We can traverse the hashtable elements using loop to access its elements and to get list of all the available elements.

  
import java.util.*;

class Demo
{
  public static void main(String args[])
  {
    // Creating Hashtable
    Hashtable<String,Integer> hashtable = new Hashtable<String,Integer>();
    // Adding elements
    hashtable.put("a",100);
    hashtable.put("b",200);
    hashtable.put("c",300);
    hashtable.put("d",400);
    // Traversing Hashtable
    for(Map.Entry<String, Integer> m : hashtable.entrySet()) {
      System.out.println(m.getKey()+" "+m.getValue());
    }   
  }
}
  

b 200 a 100 d 400 c 300