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 is not fail-fast, so it does not throw ConcurrentModificationException even if the collection is modified during iteration.
• 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();

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

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.

Step 2: After Fetching “Apple”

• Cursor lies between Apple and Banana.
• hasMoreElements() -> returns true.
• Calling nextElement() returns "Banana" and moves cursor forward.

Step 3: After Fetching “Banana”

• Cursor lies between Banana and Cherry.
• hasMoreElements() -> returns true.
• Calling nextElement() returns "Cherry" and moves cursor after the last element.

Step 4: After Fetching “Cherry”

• Cursor lies after the last element.
• hasMoreElements() -> returns false. Iteration ends.

Enumeration vs Iterator