🔁 Java Iterator – Complete Guide for Traversing Lists, Sets & Maps

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🧲 Introduction – Why Java Iterator Is Useful

When working with Java Collections like ArrayList, HashSet, or HashMap, you often need a standard way to traverse through elements — regardless of the collection type. That’s where the Java Iterator interface becomes indispensable.

A Java Iterator allows sequential access to elements in a forward-only direction, making it ideal for generic, type-safe iteration.

By the end of this article, you’ll learn:

✅ What an Iterator is and how it works
✅ How to iterate over lists, sets, and maps
✅ How to safely remove elements during iteration
✅ How Iterator compares with for-each and ListIterator

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🔑 What Is an Iterator in Java?

🔁 Iterator is an interface in java.util that provides methods to traverse and manipulate collections in a uniform way.

It supports:

• Forward traversal
• Safe removal during iteration

📦 Iterator Methods

Method	Description
hasNext()	Checks if another element exists
next()	Returns the next element
remove()	Removes the last element returned by next() (optional)

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🧪 Basic Syntax – Using Iterator with ArrayList

import java.util.*;

public class IteratorDemo {
    public static void main(String[] args) {
        List<String> colors = new ArrayList<>();
        colors.add("Red");
        colors.add("Green");
        colors.add("Blue");

        Iterator<String> it = colors.iterator();

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

✅ Explanation:

• iterator() returns an Iterator for the list
• hasNext() checks if another element is present
• next() fetches the next element

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🚫 Removing Elements with Iterator (Safely)

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

while (it.hasNext()) {
    Integer num = it.next();
    if (num % 2 == 0) {
        it.remove(); // ✅ Safe removal
    }
}
System.out.println(numbers); // [1, 3, 5]

⚠️ Do not use list.remove() directly inside a loop — it throws ConcurrentModificationException.

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🧰 Using Iterator with Other Collections

🧩 Set Example

Set<String> set = new HashSet<>(Set.of("A", "B", "C"));
Iterator<String> it = set.iterator();

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

✅ Works exactly like with lists

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🗺️ Iterating Over Map with Iterator

Map<Integer, String> map = new HashMap<>();
map.put(1, "One");
map.put(2, "Two");

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

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

✅ entrySet() gives a Set of key-value pairs that can be iterated

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🔄 Iterator vs For-each Loop

Feature	Iterator	for-each loop
Direction	Forward only	Forward only
Can remove elements?	✅ Yes (iterator.remove())	❌ No
Generic Collection Type	✅ Yes	✅ Yes
Safe in concurrent mods	✅ Safer	❌ May cause ConcurrentModificationException
Complexity	Slightly more verbose	Simpler syntax

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🧠 Advanced: ListIterator (Bi-directional)

ListIterator<String> listIt = colors.listIterator();

while (listIt.hasNext()) {
    System.out.println("Forward: " + listIt.next());
}

while (listIt.hasPrevious()) {
    System.out.println("Backward: " + listIt.previous());
}

✅ ListIterator supports:

• Backward traversal (hasPrevious(), previous())
• Add, replace, or remove during iteration

📘 Only works with List collections

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💡 Best Practices for Java Iterator

✅ Use iterator.remove() instead of collection.remove() during iteration
✅ Prefer for-each for read-only traversal
✅ Use ListIterator only when bidirectional traversal is required
✅ Avoid modifying collection structure outside the iterator loop
✅ Use Java 8 forEachRemaining() to consume the rest of the iteration

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✅ Summary

• Iterator is a universal tool for traversing collections in Java
• Provides safe, forward-only access with optional removal
• Works across List, Set, and Map structures
• Choose ListIterator for bi-directional iteration on lists
• Always use iterator.remove() for modifying elements during iteration

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❓ FAQs – Java Iterator

❓ What is the difference between Iterator and ListIterator?

• Iterator: works for all collections, forward only
• ListIterator: works only for List, supports bidirectional traversal and add/replace

❓ Can I use Iterator on a Map?

Yes. Use map.entrySet().iterator() to iterate through key-value pairs.

❓ What happens if I remove an element directly inside a loop?

You’ll get a ConcurrentModificationException unless you use the iterator’s remove() method.

❓ Is Iterator thread-safe?

No. Use ConcurrentHashMap, CopyOnWriteArrayList, or external synchronization for thread safety.

❓ Can Iterator be reused?

No. Once fully traversed, you must call collection.iterator() again to restart iteration.

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