Hashtable Class

Hashtable class implements Map interface. Hashtable stores the data in the form of key-value pairs, where key and value are objects. Hashcode of each key is computed internally and this value of hash code is used as an index at which the value is stored in the Hashtable. This is how the association between each key-value pair is formed.

Some important features of Hashtable

Null values cannot be stored in the keys or values.
Hashtable is very similar to HashMap but Hashtable is synchronized and thread-safe while HashMap is not synchronized.
Hashtable elements are ordered on the basis of hash code value of each key(which is computed internally) & hence order of key-value pairs may appear different as compared to their insertion order.

A simple constructor of Hashtable class

Hashtable()
This constructor creates an empty Hashtable. Example -

Hashtable <Integer,Integer> ht = new Hashtable <Integer,Integer>();

This constructor creates a Hashtable to hold key-value pair of Integer objects.

Some methods in a Hashtable.

As Hashtable implements Map interface, so some methods are inherited from Map.

Methods	Description
void clear()	Removes all the key/value pairs from the Hashtable.
int size()	Returns the total number of key-value pairs in a Hashtable.
boolean isEmpty()	Returns true if Hashtable has no key-value pair in it.
V get(Object key)	Returns the value associated with a specified key.
put(K key, V value)	Puts the specified key and its associated value in the Hashtable.
Set<Map.Entry<K,V>> entrySet()	Returns the Set containing Map.Entry objects.

Hashtable example

In this example we will create a Hashtable to store objects in the form of key-value pair, where key is a String object and value is an Integer object. For a given a key, we can find its value. And in this example, we will also obtain a value on the basis of a particular key.

import java.util.*;

class HashtableDemo
{
public static void main(String... ar)
{
Hashtable <String,Integer> hm = new Hashtable <String,Integer>();

hm.put("Max", 1000);
hm.put("John", 4000);
hm.put("Tom", 2000);
hm.put("Ana", 6000);
hm.put("Rick", 5000);

System.out.println(hm);

System.out.println("Value at the key, Tom is "+ hm.get("Tom"));
System.out.println("Value at the key, Ana is "+ hm.get("Ana"));

}

}

Output is :

{Max=1000, Tom=2000, John=4000, Rick=5000, Ana=6000}
Value at the key Tom is 2000
Value at the key Ana is 6000

Program Analysis

In the code above, we have created a generic Hashtable to store objects in the form of String and Integer objects.
Method put() is used to put the values of Key-Value pair by specifying the Key and its Value.
Method get() is used to obtain a Value associated with a particular Key in a Hashtable.
On the first call, method get() has returned the Value(2000) associated with the Key "Tom".
On the second call, method get() has returned Value(6000) associated with the Key "Ana".

Iterating key-value pairs in a HashMap using entrySet() method

In this example we will create another Hashtable to store objects in the form of key-value pair, where key is a String object and value is an Integer object. We will use entrySet() method to get a Set view of this Hashtable. We will iterate over each map entry(key-value pair) using for-each loop. Let's see the code -:

import java.util.*;

class HashtableDemo
{
public static void main(String... ar)
{
Hashtable <String,Integer> hm = new Hashtable <String,Integer>();

hm.put("Max", 1000);
hm.put("John", 4000);
hm.put("Tom", 2000);
hm.put("Ana", 6000);
hm.put("Rick", 5000);

System.out.println("Iterating Hashtable using Map.Entry in a for-each loop");
Set<Map.Entry<String,Integer>> set = hm.entrySet();


for(Map.Entry<String,Integer> mapE : set)
{
	System.out.print(mapE.getKey() + " : ");
	System.out.println(mapE.getValue());
}

}
}

Output is :

Iterating Hashtable using Map.Entry in a for-each loop
Max : 1000
Tom : 2000
John : 4000
Rick : 5000
Ana : 6000

Program Analysis

In the code above, we have created a generic Hashtable to store objects in the form of String and Integer objects.
We have called a method, entrySet() which returns a Set containing the values in the Hashtable. These entries are in the form of objects of type Map.Entry.
Inside for-each loop we have called methods of Map.Entry like getKey() and getValue() to read each key-value pair stored in the Hashtable.
Method getKey() of Map.Entry returns the key information of a particular pair, while the method getValue() returns the value associated with the key .

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