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Collections Framework > Legacy Classes and Interfaces >
Siva Nookala - 14 Apr 2016
Hashtable implements a hash table. It stores key/value pairs in a hash table i.e. keys are mapped to values. Key and value can be any non-null object.

Hashtable has two parameters : initial_Capacity and load_Factor . Initial_Capacity is the capacity when the hash table is created. load_Factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased.
When the number of entries in the Hashtableexceeds the product of the load Factor and the current capacity, the capacity is increased by calling the rehash()method.

Constructors of the Hashtable class are :
Constructors Description
Hashtable()
The default constructor. It creates a new Hashtable with default initial capacity (11) and load factor (0.75).
Hashtable(int initialCapacity)
Creates a new, empty Hashtablewith the given initial capacity and default load factor (0.75).
Hashtable(int initialCapacity, float loadFactor)
Creates a new, empty Hashtablewith the given initial capacity and load factor.
Hashtable(Map t)
Creates a new Hashtable, initialized with the elements in the given Map.
Methods Description
boolean containsKey(Object key)
Returns true if some key equal to key exists within the hash table. Returns false if the key isn't found.
containsValue(Object value)
Returns true Returns true if some value equal to value exists within the hash table. Returns false if the value isn't found.

Here is an example program using Hashtable class.
Hashtable Demo
import java.util.Hashtable;

class HashtableTest
{
    public static void main(String arg[])
    {
        Hashtable<Integer, String> ranktable = new Hashtable<>(); // LINE A
        Enumeration<Integer> e;
        String playerName;
        Integer rank;
        ranktable.put(4, "Del Potro"); // LINE B
        ranktable.put(1, "Rafael Nadal");
        ranktable.put(2, "Novak Djokovic");
        ranktable.put(5, "David Ferrer");
        ranktable.put(3, "Stanislas Wawrinka");
        e = ranktable.keys();
        while (e.hasMoreElements()) // LINE C
        {
            rank = e.nextElement();
            playerName = ranktable.get(rank);
            System.out.println(rank + ": " + playerName);
        }    
    }
}
OUTPUT

5: David Ferrer
4: Del Potro
3: Stanislas Wawrinka
2: Novak Djokovic
1: Rafael Nadal

DESCRIPTION

At LINE A a new Hashtable is created with Integer as key and String as value.
From LINE B elements are added into the Hastable.
At LINE C the Hashtable is iterated to display key-value pair with Enumeration.

THINGS TO TRY
  • Try to iterate over the Hashtable using keySet() method.
  • Replace Stanislas Wawrinka with Roger Ferrer and display the elements in the Hashtable. See that we can have duplicate values for two different keys.
  • Check whether 10 is a key in the given Hashtable or not. If it is, replace it with Jo-Wilfried Tsonga.
  • Change the rank (key) of David Ferrer to 6, and see the output.

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