Enumeration Interface In Java

The Enumeration interface in Java is one of the legacy interfaces used to iterate over elements of collections such as Stack, Vector and HashTable. It was introduced in JDK 1.0 and is part of the java.util package

• It is a legacy cursor used before Iterator and ListIterator.
• It supports only forward direction traversal (one-way).
• Enumeration isn’t fail-fast, but Vector and Hashtable are, so modifying them during Enumeration can still cause ConcurrentModificationException.
• Mostly used with Vector, Stack, and Hashtable classes.

Declaration

public interface Enumeration<E>

Where E is the type of elements stored in a collection.

Creating Enumeration Object

Creating an Enumeration Object with the Vector class

Vector v = new Vector();
Enumeration e = v.elements();

import java.util.*;

public class GFG{
    
    public static void main(String[] args){
        
        Vector<String> fruits = new Vector<>();
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Cherry");

        // Getting Enumeration object
        Enumeration<String> e = fruits.elements();

        System.out.println("Elements of Vector:");
        while (e.hasMoreElements()) {
            System.out.println(e.nextElement());
        }
    }
}

Elements of Vector:
Apple
Banana
Cherry

Explanation

• The elements() method of the Vector class returns an Enumeration of the vector’s elements.
• The hasMoreElements() method checks if more elements exist in the collection.
• The nextElement() method retrieves the next element and advances the cursor.
• The iteration continues until all elements are processed.

Methods of Enumeration Interface

• asIterator(): This method returns an Iterator which traverses all the remaining elements covered by this enumeration.
• hasMoreElements(): It returns a boolean indicating whether more elements remain; returns false when all elements are traversed.
• nextElement(): It returns the next element in the enumeration sequence

Example: Using Enumeration with Hashtable

import java.util.*;

public class GFG{
    
    public static void main(String[] args){
        
        Hashtable<Integer, String> map = new Hashtable<>();
        map.put(1, "Java");
        map.put(2, "Python");
        map.put(3, "C++");

        Enumeration<Integer> keys = map.keys();

        System.out.println("Hashtable Keys:");
        while (keys.hasMoreElements()) {
            int key = keys.nextElement();
            System.out.println(key + " -> " + map.get(key));
        }
    }
}

Hashtable Keys:
3 -> C++
2 -> Python
1 -> Java

Working of Enumeration

Let’s understand the working process of the Enumeration cursor step-by-step. Suppose we have a Vector of three elements — Apple, Banana, and Cherry

Step 1: Before Iteration

• The cursor lies before the first element.
• hasMoreElements() -> returns true (next element exists).
• Calling nextElement() returns "Apple" and moves the cursor forward.