🔁 Java Iterator Methods – Full Guide with Examples (2025)

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🧲 Introduction – Why Java Iterator Methods Matter

In Java, the Iterator is a fundamental interface used to traverse collections like ArrayList, HashSet, and more. When working with collections, especially during looping, removal, or conditional filtering, Iterator methods allow:

• ✅ Safe traversal of elements
• ✅ Element removal without ConcurrentModificationException
• ✅ Compatibility with all Java Collections Framework classes

📘 All Collection types in java.util package provide an iterator() method.

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🔑 What Is a Java Iterator?

An Iterator is an object that enables sequential access to elements in a collection without exposing its structure.

import java.util.Iterator;
Iterator<Type> it = collection.iterator();

✅ Commonly used with: List, Set, Queue, etc.

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📋 Core Java Iterator Methods

🧩 Method	📘 Purpose
hasNext()	Checks if another element exists
next()	Returns the next element
remove()	Removes the last element returned by next()

⚠️ Note: remove() can only be called once per next(), or it throws IllegalStateException.

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✅ Java Iterator Method Examples

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✅ 1. hasNext() + next() – Traverse Elements

import java.util.*;

public class IteratorExample {
    public static void main(String[] args) {
        List<String> languages = Arrays.asList("Java", "Python", "C++");
        Iterator<String> it = languages.iterator();

        while (it.hasNext()) {
            System.out.println(it.next());
        }
    }
}

Output:

Java
Python
C++

✅ Explanation:

• hasNext() checks for availability
• next() returns and moves to the next item

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✅ 2. remove() – Safe Removal While Iterating

List<Integer> numbers = new ArrayList<>(List.of(1, 2, 3, 4, 5));
Iterator<Integer> it = numbers.iterator();

while (it.hasNext()) {
    if (it.next() % 2 == 0) {
        it.remove();
    }
}

System.out.println(numbers);  // Output: [1, 3, 5]

✅ Explanation: Removes even numbers safely while iterating.
⚠️ Don’t use list.remove(item) inside a loop — it causes ConcurrentModificationException.

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✅ 3. Enhanced For-Loop vs Iterator

for (String lang : languages) {
    System.out.println(lang);
}

✅ Cleaner but read-only: Cannot safely remove items with enhanced for-loop.

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🧪 Java ListIterator (Advanced)

If you need bidirectional traversal, use ListIterator:

List<String> cities = new ArrayList<>(List.of("London", "Paris", "Tokyo"));
ListIterator<String> li = cities.listIterator();

while (li.hasNext()) {
    System.out.println(li.next());
}

while (li.hasPrevious()) {
    System.out.println(li.previous());
}

✅ ListIterator Methods:

🧩 Method	📘 Purpose
hasPrevious()	Checks if a previous element exists
previous()	Returns the previous element
add(E)	Adds element at current cursor position
set(E)	Replaces last element returned
nextIndex() / previousIndex()	Get element indexes

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📌 Summary Table – Iterator vs ListIterator

🔧 Feature	🔁 Iterator	🔁 ListIterator
Traverse Forward	✅ Yes	✅ Yes
Traverse Backward	❌ No	✅ Yes
Remove Element	✅ Yes	✅ Yes
Add/Replace Element	❌ No	✅ Yes
Works with All Collections	✅ Yes	❌ Only List types

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💡 Best Practices with Iterators

• ✅ Use iterator.remove() for safe element deletion
• ✅ Prefer enhanced for-each loop for read-only operations
• ⚠️ Avoid calling remove() without calling next() first
• ✅ Use ListIterator only when modification or reverse traversal is needed

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❓FAQs on Java Iterator Methods

❓ Can I use Iterator with HashMap?

Yes. Use it on entrySet() or keySet():

Iterator<Map.Entry<String, Integer>> it = map.entrySet().iterator();

❓ What happens if I call next() when no element exists?

It throws a NoSuchElementException.

❓ Can I use remove() twice in a row?

No. You must call next() before each remove().

❓ What’s the difference between Iterator and Enumeration?

Iterator is more modern and supports removal.
Enumeration is read-only and legacy (used in old APIs like Vector).

❓ Is Iterator thread-safe?

No. Use ConcurrentHashMap or synchronized wrappers for thread safety.

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