Java.Collections.What is the difference between Enumeration and Iterator.

Great follow-up, Stanley! Enumeration and Iterator are both used to traverse elements in a collection, but they come from different generations of the Java Collection Framework, and they have key differences in capability, safety, and flexibility.

Let’s compare them clearly:

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🧾 1. Definition & Origin

Feature	Enumeration	Iterator
Introduced in	JDK 1.0	JDK 1.2 (with Collection Framework)
Package	java.util.Enumeration	java.util.Iterator
Type	Interface for legacy classes	Interface for modern collections

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🔄 2. Used With

Enumeration	Iterator
Legacy classes like:	All modern collections like:
– Vector	– ArrayList, HashSet, LinkedList
– Stack	– HashMap, TreeSet, etc.
– Hashtable

⚙️ 3. Method Comparison

Feature	Enumeration	Iterator
Check next	hasMoreElements()	hasNext()
Get next	nextElement()	next()
Remove element	❌ Not supported	✅ remove() method supported

➡️ Iterator is more powerful because it allows element removal during iteration (safely, via iterator.remove()).

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🛡️ 4. Fail-Fast Behavior

Behavior	Enumeration	Iterator
Fail-fast?	❌ No	✅ Yes (throws ConcurrentModificationException)

➡️ Enumeration is not fail-fast: it doesn’t detect concurrent modifications.
➡️ Iterator is fail-fast: detects structure changes and fails early.

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🧪 Example

✅ Using Enumeration (old way)

Vector<String> vector = new Vector<>();
vector.add("Java");
vector.add("Python");

Enumeration<String> e = vector.elements();
while (e.hasMoreElements()) {
    System.out.println(e.nextElement());
}

✅ Using Iterator (modern way)

List<String> list = new ArrayList<>();
list.add("Java");
list.add("Python");

Iterator<String> it = list.iterator();
while (it.hasNext()) {
    String val = it.next();
    if (val.equals("Python")) {
        it.remove(); // ✅ Safe removal
    }
}

🔚 Summary Table

Feature	Enumeration	Iterator
Legacy vs Modern	Legacy	Modern
Remove support	❌ Not supported	✅ Supported
Fail-fast	❌ No	✅ Yes
Read-only	Yes	No (read + remove)
Usage Today	Rare (legacy only)	Standard

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✅ When to use what?

• Use Enumeration only when working with legacy classes (Vector, Hashtable).
• Prefer Iterator for modern code — it’s safer, supports removal, and is fail-fast.