Java Notes

Iterator in Java

Complete guide to Iterator, ListIterator, and Iterable interfaces in Java — internal working, fail-fast behavior, custom iterators, and enhanced for-each loop mechanics.

What is an Iterator?

An Iterator is a design pattern (and Java interface) that provides a way to traverse a collection sequentially without exposing its underlying structure. Whether you're iterating over an ArrayList, HashSet, or TreeMap, the Iterator provides a uniform interface.

java example

public interface Iterator<E> {
    boolean hasNext();    // Is there another element?
    E next();            // Return the next element
    default void remove();  // Remove the last returned element (optional)
    default void forEachRemaining(Consumer<? super E> action);  // Java 8+
}

Basic Usage

java example

import java.util.*;

public class IteratorBasics {
    public static void main(String[] args) {
        List<String> fruits = new ArrayList<>(Arrays.asList(
            "Apple", "Banana", "Cherry", "Date", "Elderberry"
        ));

        // Get an iterator from the collection
        Iterator<String> it = fruits.iterator();

        // Traverse using hasNext() and next()
        while (it.hasNext()) {
            String fruit = it.next();
            System.out.println(fruit);
        }
    }
}

Output:

Example

Apple Banana Cherry Date Elderberry

Iterator Methods in Detail

hasNext()

Returns true if the iteration has more elements. Does not advance the cursor.

java example

Iterator<Integer> it = Arrays.asList(1, 2, 3).iterator();
System.out.println(it.hasNext());  // true
System.out.println(it.hasNext());  // still true — doesn't advance!
System.out.println(it.next());     // 1 — NOW it advances

next()

Returns the next element and advances the cursor. Throws NoSuchElementException if no more elements.

java example

Iterator<Integer> it = Arrays.asList(1).iterator();
it.next();  // 1
it.next();  // NoSuchElementException!

remove()

Removes the last element returned by next(). Can only be called once per next() call.

Example

List<Integer> numbers = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5, 6)); Iterator<Integer> it = numbers.iterator(); while (it.hasNext()) { int num = it.next(); if (num % 2 == 0) { it.remove(); // Safely removes even numbers } } System.out.println(numbers); // [1, 3, 5]

Rules for remove():

Must call next() before remove()
Cannot call remove() twice without calling next() in between
Throws IllegalStateException if violated

java example

Iterator<String> it = list.iterator();
// it.remove();        // IllegalStateException — no next() called yet
it.next();
it.remove();           // OK
// it.remove();        // IllegalStateException — can't remove twice

forEachRemaining() (Java 8+)

Processes all remaining elements with a Consumer:

java example

Iterator<String> it = Arrays.asList("A", "B", "C", "D").iterator();
it.next();  // Skip "A"
it.forEachRemaining(s -> System.out.print(s + " "));
// Output: B C D

Internal Working — How ArrayList's Iterator Works

Example

// Simplified version of ArrayList's internal iterator private class Itr implements Iterator<E> { int cursor; // Index of next element to return int lastRet = -1; // Index of last element returned (-1 if none) int expectedModCount = modCount; // Snapshot of structural modifications public boolean hasNext() { return cursor != size; } public E next() { checkForComodification(); int i = cursor; if (i >= size) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; cursor = i + 1; return (E) elementData[lastRet = i]; } public void remove() { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); ArrayList.this.remove(lastRet); cursor = lastRet; lastRet = -1; expectedModCount = modCount; // Reset after legitimate modification } final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); } }

Key Points:

cursor tracks the position of the next element
lastRet tracks the last returned element (for remove())
expectedModCount detects structural modifications

Fail-Fast Mechanism

What is Fail-Fast?

Iterators from most Java collections are fail-fast — they immediately throw ConcurrentModificationException if the collection is structurally modified (add/remove) during iteration by any means OTHER than the iterator's own remove() method.

How It Works

Every collection maintains an internal modCount (modification count) that increments on structural changes. When you create an iterator, it captures the current modCount as expectedModCount. Before each next() call, it checks if they still match.

java example

List<String> list = new ArrayList<>(Arrays.asList("A", "B", "C", "D"));

// WRONG — Modifying through the list during iteration
for (String s : list) {
    if (s.equals("B")) {
        list.remove(s);  // ConcurrentModificationException!
    }
}

Why This Happens with for-each

The enhanced for-each loop is syntactic sugar for Iterator:

java example

// This:
for (String s : list) { ... }

// Compiles to:
Iterator<String> it = list.iterator();
while (it.hasNext()) {
    String s = it.next();  // Checks modCount here — throws CME!
    ...
}

Safe Alternatives

java example

// 1. Use Iterator.remove()
Iterator<String> it = list.iterator();
while (it.hasNext()) {
    if (it.next().equals("B")) {
        it.remove();  // Safe!
    }
}

// 2. Use removeIf() (Java 8+)
list.removeIf(s -> s.equals("B"));

// 3. Collect and remove later
List<String> toRemove = new ArrayList<>();
for (String s : list) {
    if (s.equals("B")) toRemove.add(s);
}
list.removeAll(toRemove);

// 4. Use CopyOnWriteArrayList (fail-safe)
List<String> cowList = new CopyOnWriteArrayList<>(list);
for (String s : cowList) {
    cowList.remove(s);  // No CME — iterates over snapshot
}

ListIterator — Bidirectional Iteration

ListIterator extends Iterator with bidirectional traversal and modification capabilities. Only available for List implementations.

java example

public interface ListIterator<E> extends Iterator<E> {
    boolean hasNext();
    E next();
    boolean hasPrevious();
    E previous();
    int nextIndex();
    int previousIndex();
    void remove();
    void set(E e);    // Replace last returned element
    void add(E e);    // Insert element before next()
}

Example

java example

List<String> colors = new ArrayList<>(Arrays.asList("Red", "Green", "Blue"));
ListIterator<String> lit = colors.listIterator();

// Forward traversal
while (lit.hasNext()) {
    int index = lit.nextIndex();
    String color = lit.next();
    System.out.println(index + ": " + color);
}
// Output: 0: Red, 1: Green, 2: Blue

// Backward traversal
while (lit.hasPrevious()) {
    String color = lit.previous();
    System.out.println(color);
}
// Output: Blue, Green, Red

Modifying During Iteration

Example

List<Integer> nums = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5)); ListIterator<Integer> lit = nums.listIterator(); while (lit.hasNext()) { int val = lit.next(); if (val % 2 == 0) { lit.set(val * 10); // Replace: 2→20, 4→40 } if (val == 3) { lit.add(99); // Insert 99 after 3 } } System.out.println(nums); // [1, 20, 3, 99, 40, 5]

Starting from a Specific Position

java example

// Start iteration from index 2
ListIterator<String> lit = list.listIterator(2);

Iterator vs ListIterator

Feature	Iterator	ListIterator
Direction	Forward only	Forward and backward
Available for	Any Collection	Only List
Methods	hasNext, next, remove	+ hasPrevious, previous, set, add
Index access	No	nextIndex(), previousIndex()
Modification	remove() only	remove(), set(), add()

The Iterable Interface

Any class implementing Iterable<T> can be used in a for-each loop:

java example

public interface Iterable<T> {
    Iterator<T> iterator();
    default void forEach(Consumer<? super T> action) { ... }
    default Spliterator<T> spliterator() { ... }
}

Custom Iterable Example

Example

public class NumberRange implements Iterable<Integer> { private final int start; private final int end; public NumberRange(int start, int end) { this.start = start; this.end = end; } @Override public Iterator<Integer> iterator() { return new Iterator<Integer>() { private int current = start; @Override public boolean hasNext() { return current <= end; } @Override public Integer next() { if (!hasNext()) throw new NoSuchElementException(); return current++; } }; } } // Usage — works in for-each loop! NumberRange range = new NumberRange(1, 5); for (int num : range) { System.out.print(num + " "); } // Output: 1 2 3 4 5

Spliterator (Java 8+)

Spliterator (Splitable Iterator) is designed for parallel processing:

java example

public interface Spliterator<T> {
    boolean tryAdvance(Consumer<? super T> action);
    Spliterator<T> trySplit();  // Split into two halves
    long estimateSize();
    int characteristics();
}

Key Characteristics

Characteristic	Meaning
ORDERED	Elements have a defined encounter order
DISTINCT	No duplicates (like Set)
SORTED	Elements are in sorted order
SIZED	estimateSize() returns exact count
NONNULL	No null elements
IMMUTABLE	Source cannot be modified
CONCURRENT	Source can be safely modified concurrently
SUBSIZED	Split children are also SIZED

Usage with Streams

java example

List<String> words = Arrays.asList("Hello", "World", "Java", "Streams");
Spliterator<String> spliterator = words.spliterator();

// Characteristics
System.out.println(spliterator.estimateSize());       // 4
System.out.println(spliterator.hasCharacteristics(Spliterator.ORDERED));  // true

// Split for parallel processing
Spliterator<String> secondHalf = spliterator.trySplit();
// spliterator now covers first half, secondHalf covers second half

Iteration Performance

Collection	Iterator Creation	next()	hasNext()
-----------	:-:	:-:	:-:
ArrayList	O(1)	O(1)	O(1)
LinkedList	O(1)	O(1)	O(1)
HashSet	O(1)	O(1) amortized	O(1) amortized
TreeSet	O(log n)	O(log n) amortized	O(1)
HashMap (entrySet)	O(capacity)	O(1) amortized	O(1) amortized

Note: HashSet/HashMap iterators traverse the internal bucket array, skipping empty buckets. Iteration time is proportional to capacity (not size), so keep load factor reasonable.

Common Mistakes

1. Calling next() Without hasNext()

java example

// WRONG — may throw NoSuchElementException
while (true) {
    String item = it.next();  // Crashes when exhausted
}

// CORRECT
while (it.hasNext()) {
    String item = it.next();
}

2. Calling next() Twice Per Iteration

java example

// WRONG — skips elements and may crash
while (it.hasNext()) {
    System.out.println(it.next());
    process(it.next());  // Calls next() AGAIN!
}

// CORRECT
while (it.hasNext()) {
    String item = it.next();
    System.out.println(item);
    process(item);
}

3. Using for-each When You Need to Remove

java example

// WRONG
for (String s : list) {
    if (condition) list.remove(s);  // CME!
}

// CORRECT
Iterator<String> it = list.iterator();
while (it.hasNext()) {
    if (condition) it.remove();
}

4. Reusing an Exhausted Iterator

java example

Iterator<String> it = list.iterator();
while (it.hasNext()) { it.next(); }  // Exhausted

// WRONG — it's already exhausted!
while (it.hasNext()) { ... }  // Won't enter loop

// CORRECT — get a fresh iterator
it = list.iterator();

Interview Questions

Q1: What is the difference between Iterator and Enumeration?

A: Enumeration (legacy) has hasMoreElements() and nextElement() — no remove(). Iterator is the modern replacement with remove() support and fail-fast behavior.

Q2: What is a fail-fast iterator?

A: An iterator that throws ConcurrentModificationException if the underlying collection is structurally modified during iteration (except through the iterator's own methods).

Q3: How can you remove elements during iteration?

A: Use Iterator.remove(), ListIterator.remove(), removeIf() (Java 8+), or use a fail-safe collection like CopyOnWriteArrayList.

Q4: What is the difference between Iterator and ListIterator?

A: Iterator is forward-only and works on any Collection. ListIterator supports bidirectional traversal, index queries, and add/set operations, but only works on Lists.

Q5: Can you get ConcurrentModificationException in a single-threaded program?

A: Yes! It's not about threads — it's about modifying the collection directly (not through the iterator) during iteration.

Q6: What is a Spliterator?

A: A parallel-capable iterator introduced in Java 8. It can split a data source into two halves for divide-and-conquer parallel processing in streams.

Q7: Why should you prefer Iterator over index-based loop for LinkedList?

A: Because list.get(i) on LinkedList is O(n) (traverses from head each time), making an indexed loop O(n²). Iterator traverses in O(n) total.

Q8: What is the difference between Iterator and Spliterator?

A: Iterator is sequential and single-threaded. Spliterator (Java 8+) supports parallel traversal — it can split the data source into multiple parts for parallel processing in streams. Spliterator also provides characteristics (ORDERED, SORTED, SIZED, etc.) and estimateSize() for optimization.

Q9: How does fail-fast detection work internally?

A: Collections maintain a modCount field incremented on every structural modification (add, remove). When an iterator is created, it saves the current modCount as expectedModCount. Before each next() call, it checks if modCount != expectedModCount — if true, throws ConcurrentModificationException.

Q10: What is the difference between Iterator.remove() and Collection.remove()?

A: Iterator.remove() removes the last element returned by next() and is safe to call during iteration (updates internal state). Collection.remove() during iteration triggers ConcurrentModificationException because the iterator's state becomes inconsistent with the collection.

Q11: Can you iterate a Map using Iterator?

A: Not directly — Map doesn't extend Iterable. But you can iterate its views: map.keySet().iterator(), map.values().iterator(), or map.entrySet().iterator(). The entrySet iterator gives you both keys and values via Map.Entry.

Q12: What are the limitations of for-each loop compared to Iterator?

A: For-each cannot: (1) remove elements during iteration, (2) traverse backwards (ListIterator can), (3) access the iterator index, (4) iterate multiple collections simultaneously. Use explicit Iterator when you need these capabilities.

Summary

Concept	Key Point
`Iterator`	Forward-only, universal, safe removal
`ListIterator`	Bidirectional, list-only, full modification
`Iterable`	Implement to enable for-each loop
`Spliterator`	Parallel-capable splitting iterator
Fail-fast	Detects concurrent modification via modCount
`remove()`	Only safe way to remove during iteration
`forEachRemaining()`	Process remaining elements with lambda

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Iterator in Java.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

java-master-course, java master course, java, master, course, collections, framework, iterator

Continue learning this concept

PreviousGenerics in Java NextArrayList in Java

Java Notes

Iterator in Java

Complete guide to Iterator, ListIterator, and Iterable interfaces in Java — internal working, fail-fast behavior, custom iterators, and enhanced for-each loop mechanics.

What is an Iterator?

java example

public interface Iterator<E> {
    boolean hasNext();    // Is there another element?
    E next();            // Return the next element
    default void remove();  // Remove the last returned element (optional)
    default void forEachRemaining(Consumer<? super E> action);  // Java 8+
}

Basic Usage

java example

import java.util.*;

public class IteratorBasics {
    public static void main(String[] args) {
        List<String> fruits = new ArrayList<>(Arrays.asList(
            "Apple", "Banana", "Cherry", "Date", "Elderberry"
        ));

        // Get an iterator from the collection
        Iterator<String> it = fruits.iterator();

        // Traverse using hasNext() and next()
        while (it.hasNext()) {
            String fruit = it.next();
            System.out.println(fruit);
        }
    }
}

Output:

Example

Apple Banana Cherry Date Elderberry

Iterator Methods in Detail

hasNext()

Returns true if the iteration has more elements. Does not advance the cursor.

java example

Iterator<Integer> it = Arrays.asList(1, 2, 3).iterator();
System.out.println(it.hasNext());  // true
System.out.println(it.hasNext());  // still true — doesn't advance!
System.out.println(it.next());     // 1 — NOW it advances

next()

Returns the next element and advances the cursor. Throws NoSuchElementException if no more elements.

java example

Iterator<Integer> it = Arrays.asList(1).iterator();
it.next();  // 1
it.next();  // NoSuchElementException!

remove()

Removes the last element returned by next(). Can only be called once per next() call.

Example

Rules for remove():

Must call next() before remove()
Cannot call remove() twice without calling next() in between
Throws IllegalStateException if violated

java example

Iterator<String> it = list.iterator();
// it.remove();        // IllegalStateException — no next() called yet
it.next();
it.remove();           // OK
// it.remove();        // IllegalStateException — can't remove twice

forEachRemaining() (Java 8+)

Processes all remaining elements with a Consumer:

java example

Iterator<String> it = Arrays.asList("A", "B", "C", "D").iterator();
it.next();  // Skip "A"
it.forEachRemaining(s -> System.out.print(s + " "));
// Output: B C D

Internal Working — How ArrayList's Iterator Works

Example

Key Points:

cursor tracks the position of the next element
lastRet tracks the last returned element (for remove())
expectedModCount detects structural modifications

Fail-Fast Mechanism

What is Fail-Fast?

How It Works

java example

List<String> list = new ArrayList<>(Arrays.asList("A", "B", "C", "D"));

// WRONG — Modifying through the list during iteration
for (String s : list) {
    if (s.equals("B")) {
        list.remove(s);  // ConcurrentModificationException!
    }
}

Why This Happens with for-each

The enhanced for-each loop is syntactic sugar for Iterator:

java example

// This:
for (String s : list) { ... }

// Compiles to:
Iterator<String> it = list.iterator();
while (it.hasNext()) {
    String s = it.next();  // Checks modCount here — throws CME!
    ...
}

Safe Alternatives

java example

// 1. Use Iterator.remove()
Iterator<String> it = list.iterator();
while (it.hasNext()) {
    if (it.next().equals("B")) {
        it.remove();  // Safe!
    }
}

// 2. Use removeIf() (Java 8+)
list.removeIf(s -> s.equals("B"));

// 3. Collect and remove later
List<String> toRemove = new ArrayList<>();
for (String s : list) {
    if (s.equals("B")) toRemove.add(s);
}
list.removeAll(toRemove);

// 4. Use CopyOnWriteArrayList (fail-safe)
List<String> cowList = new CopyOnWriteArrayList<>(list);
for (String s : cowList) {
    cowList.remove(s);  // No CME — iterates over snapshot
}

ListIterator — Bidirectional Iteration

ListIterator extends Iterator with bidirectional traversal and modification capabilities. Only available for List implementations.

java example

public interface ListIterator<E> extends Iterator<E> {
    boolean hasNext();
    E next();
    boolean hasPrevious();
    E previous();
    int nextIndex();
    int previousIndex();
    void remove();
    void set(E e);    // Replace last returned element
    void add(E e);    // Insert element before next()
}

Example

java example

List<String> colors = new ArrayList<>(Arrays.asList("Red", "Green", "Blue"));
ListIterator<String> lit = colors.listIterator();

// Forward traversal
while (lit.hasNext()) {
    int index = lit.nextIndex();
    String color = lit.next();
    System.out.println(index + ": " + color);
}
// Output: 0: Red, 1: Green, 2: Blue

// Backward traversal
while (lit.hasPrevious()) {
    String color = lit.previous();
    System.out.println(color);
}
// Output: Blue, Green, Red

Modifying During Iteration

Example

Starting from a Specific Position

java example

// Start iteration from index 2
ListIterator<String> lit = list.listIterator(2);

Iterator vs ListIterator

Feature	Iterator	ListIterator
Direction	Forward only	Forward and backward
Available for	Any Collection	Only List
Methods	hasNext, next, remove	+ hasPrevious, previous, set, add
Index access	No	nextIndex(), previousIndex()
Modification	remove() only	remove(), set(), add()

The Iterable Interface

Any class implementing Iterable<T> can be used in a for-each loop:

java example

public interface Iterable<T> {
    Iterator<T> iterator();
    default void forEach(Consumer<? super T> action) { ... }
    default Spliterator<T> spliterator() { ... }
}

Custom Iterable Example

Example

Spliterator (Java 8+)

Spliterator (Splitable Iterator) is designed for parallel processing:

java example

public interface Spliterator<T> {
    boolean tryAdvance(Consumer<? super T> action);
    Spliterator<T> trySplit();  // Split into two halves
    long estimateSize();
    int characteristics();
}

Key Characteristics

Characteristic	Meaning
ORDERED	Elements have a defined encounter order
DISTINCT	No duplicates (like Set)
SORTED	Elements are in sorted order
SIZED	estimateSize() returns exact count
NONNULL	No null elements
IMMUTABLE	Source cannot be modified
CONCURRENT	Source can be safely modified concurrently
SUBSIZED	Split children are also SIZED

Usage with Streams

java example

List<String> words = Arrays.asList("Hello", "World", "Java", "Streams");
Spliterator<String> spliterator = words.spliterator();

// Characteristics
System.out.println(spliterator.estimateSize());       // 4
System.out.println(spliterator.hasCharacteristics(Spliterator.ORDERED));  // true

// Split for parallel processing
Spliterator<String> secondHalf = spliterator.trySplit();
// spliterator now covers first half, secondHalf covers second half

Iteration Performance

Collection	Iterator Creation	next()	hasNext()
-----------	:-:	:-:	:-:
ArrayList	O(1)	O(1)	O(1)
LinkedList	O(1)	O(1)	O(1)
HashSet	O(1)	O(1) amortized	O(1) amortized
TreeSet	O(log n)	O(log n) amortized	O(1)
HashMap (entrySet)	O(capacity)	O(1) amortized	O(1) amortized

Note: HashSet/HashMap iterators traverse the internal bucket array, skipping empty buckets. Iteration time is proportional to capacity (not size), so keep load factor reasonable.

Common Mistakes

1. Calling next() Without hasNext()

java example

// WRONG — may throw NoSuchElementException
while (true) {
    String item = it.next();  // Crashes when exhausted
}

// CORRECT
while (it.hasNext()) {
    String item = it.next();
}

2. Calling next() Twice Per Iteration

java example

// WRONG — skips elements and may crash
while (it.hasNext()) {
    System.out.println(it.next());
    process(it.next());  // Calls next() AGAIN!
}

// CORRECT
while (it.hasNext()) {
    String item = it.next();
    System.out.println(item);
    process(item);
}

3. Using for-each When You Need to Remove

java example

// WRONG
for (String s : list) {
    if (condition) list.remove(s);  // CME!
}

// CORRECT
Iterator<String> it = list.iterator();
while (it.hasNext()) {
    if (condition) it.remove();
}

4. Reusing an Exhausted Iterator

java example

Iterator<String> it = list.iterator();
while (it.hasNext()) { it.next(); }  // Exhausted

// WRONG — it's already exhausted!
while (it.hasNext()) { ... }  // Won't enter loop

// CORRECT — get a fresh iterator
it = list.iterator();

Interview Questions

Q1: What is the difference between Iterator and Enumeration?

A: Enumeration (legacy) has hasMoreElements() and nextElement() — no remove(). Iterator is the modern replacement with remove() support and fail-fast behavior.

Q2: What is a fail-fast iterator?

A: An iterator that throws ConcurrentModificationException if the underlying collection is structurally modified during iteration (except through the iterator's own methods).

Q3: How can you remove elements during iteration?

A: Use Iterator.remove(), ListIterator.remove(), removeIf() (Java 8+), or use a fail-safe collection like CopyOnWriteArrayList.

Q4: What is the difference between Iterator and ListIterator?

A: Iterator is forward-only and works on any Collection. ListIterator supports bidirectional traversal, index queries, and add/set operations, but only works on Lists.

Q5: Can you get ConcurrentModificationException in a single-threaded program?

A: Yes! It's not about threads — it's about modifying the collection directly (not through the iterator) during iteration.

Q6: What is a Spliterator?

A: A parallel-capable iterator introduced in Java 8. It can split a data source into two halves for divide-and-conquer parallel processing in streams.

Q7: Why should you prefer Iterator over index-based loop for LinkedList?

A: Because list.get(i) on LinkedList is O(n) (traverses from head each time), making an indexed loop O(n²). Iterator traverses in O(n) total.

Q8: What is the difference between Iterator and Spliterator?

Q9: How does fail-fast detection work internally?

Q10: What is the difference between Iterator.remove() and Collection.remove()?

Q11: Can you iterate a Map using Iterator?

Q12: What are the limitations of for-each loop compared to Iterator?

Summary

Concept	Key Point
`Iterator`	Forward-only, universal, safe removal
`ListIterator`	Bidirectional, list-only, full modification
`Iterable`	Implement to enable for-each loop
`Spliterator`	Parallel-capable splitting iterator
Fail-fast	Detects concurrent modification via modCount
`remove()`	Only safe way to remove during iteration
`forEachRemaining()`	Process remaining elements with lambda

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Iterator in Java.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

java-master-course, java master course, java, master, course, collections, framework, iterator

Continue learning this concept

PreviousGenerics in Java NextArrayList in Java