Hashtable In Java With Example

Here we will see Hashtable in details.

Hashtable In Java

 

Let’s see some points and examples for Hashtable.

  • Hashtable is a legacy class which extends Dictionary class and implements Map, Cloneable, Serializable interface.
  • Properties, UIDefaults are the child class of Hashtable.
  • Hashtable stores the elements in key and value format.
  • In Hashtable, the key can’t be duplicate, but the value can be duplicate.
  • In Hashtable, null is not allowed as key or value. If we try to add null as a key or value, there is no compilation error but during the execution of the program, we will get NullPointerException.
  • Hashtable store the elementsĀ in an unordered way.
  • Hashtable is synchronized.

 

Example 1 –

package hashtable;

import java.util.Hashtable;
import java.util.Iterator;
import java.util.Map;

public class HashtableExample1 {
public static void main(String[] args) {
	Map<Integer, String> hashtableObject = new Hashtable<>();
	 
	hashtableObject.put(2, "ram");
	hashtableObject.put(3, "mohan");
	hashtableObject.put(4, "shyam");
	hashtableObject.put(5, "Rakesh");

	// we will iterate using Iterator
	Iterator<Map.Entry<Integer, String>> iterator = hashtableObject
			.entrySet().iterator();

	while (iterator.hasNext()) {
		Map.Entry<Integer, String> entry = iterator.next();
		System.out.println("Key is = " + entry.getKey() + ", Value is = "
				+ entry.getValue());
	}
}
}

Output is –

Key is = 5, Value is = Rakesh
Key is = 4, Value is = shyam
Key is = 3, Value is = mohan
Key is = 2, Value is = ram

Example 2 – inserting null as key in Hashtable.

package hashtable;

import java.util.Hashtable;
import java.util.Iterator;
import java.util.Map;

public class HashtableExample1 {
public static void main(String[] args) {
	Map<Integer, String> hashtableObject = new Hashtable<>();
	 
	hashtableObject.put(2, "ram");
	hashtableObject.put(3, "mohan");
	hashtableObject.put(null, "shyam");
	hashtableObject.put(5, "Rakesh");

	// we will iterate hashtable using for each loop
	for (Integer key : hashtableObject.keySet()) {
		System.out.println("Key is : " + key + "   Value is : " + hashtableObject.get(key));
	}
}
}

Output is –


Exception in thread “main” java.lang.NullPointerException
at java.util.Hashtable.put(Unknown Source)
at hashtable.HashtableExample1.main(HashtableExample1.java:13)

Example 3 – Adding null as value.


package hashtable;

import java.util.Hashtable;
import java.util.Iterator;
import java.util.Map;

public class HashtableExample1 {
public static void main(String[] args) {
	Map<Integer, String> hashtableObject = new Hashtable<>();
	 
	hashtableObject.put(2, "ram");
	hashtableObject.put(3, "mohan");
	hashtableObject.put(4, null);
	hashtableObject.put(5, "Rakesh");

	// we will iterate hashtable using for each loop
	for (Integer key : hashtableObject.keySet()) {
		System.out.println("Key is : " + key + "   Value is : " + hashtableObject.get(key));
	}
}
}

Output is –

Exception in thread “main” java.lang.NullPointerException
at java.util.Hashtable.put(Unknown Source)
at hashtable.HashtableExample1.main(HashtableExample1.java:13)

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