java.util.NoSuchElementException at ArrayList$Itr.next

Q: Guard next() with hasNext()

When to use: Use this if you keep a manual iterator because you remove while reading or juggle two cursors over one list.. hasNext() asks whether a next slot exists without moving the cursor. Only advance when it answers true, so the state machine can never overshoot.

Q: Replace manual iteration with enhanced-for

When to use: Use this if you do not mutate the list during the walk and just need every element.. The compiler expands for-each into correct hasNext()/next() pairs, so overshooting becomes impossible by construction.

Q: Use poll()/peek() on queues instead of remove()/element()

When to use: Use this if a Queue or Deque can legitimately be empty at read time.. pollFirst()/peekFirst() are the null-returning twins of removeFirst()/getFirst(): an empty queue yields null instead of an exception, turning a crash path into a branch.

Q: Get a fresh iterator instead of reusing an exhausted one

When to use: Use this if a second pass over the same collection needs its own cursor.. An exhausted iterator never rewinds. Calling collection.iterator() again returns a new cursor positioned before the first element.

Q: Check isEmpty() before getFirst()/getLast() (Java 21)

When to use: Use this if you adopted SequencedCollection accessors on lists or deques.. Java 21 getFirst()/getLast() throw NoSuchElementException on empty collections just like pop(). Guard with isEmpty(), or stay with peekFirst()/pollFirst().

🔴 The Error You're Seeing

Confirm this matches your console output. If it does, you're in the right place.

ERROR LOG

java.util.NoSuchElementException
	at java.base/java.util.ArrayList$Itr.next(ArrayList.java:1053)
	at com.devinhyderabad.batch.FeedMerger.drain(FeedMerger.java:64)

⚡ Quick Fix Works 80% of the time

Guard every next() call with hasNext(), or drop the manual iterator and use enhanced-for.

while (iterator.hasNext()) {
    String value = iterator.next();
    process(value);
}

🧠 Why this Happens

Tap to expand the deep technical explanation

An Iterator is not a copy of the data, it is a cursor holding one integer position. ArrayList$Itr.next() compares that position against size and throws NoSuchElementException when they are equal because there is no element slot left to return. The JVM never rescans the list: cursor position IS the state. Queue types behave identically - ArrayDeque.remove()/element() throw the same exception when head equals tail, which means zero elements. hasNext() is a pure peek that moves nothing, which is exactly why the guard works.

The HITEC City Parking Spot Analogy:

Flipping one more page after the last page of a notebook: your bookmark moved to the end, and another flip hands you nothing.

🔁 How to Reproduce Confirm this is your error

List<Integer> nums = new ArrayList<>(List.of(7)); Iterator<Integer> it = nums.iterator(); it.next(); // consumes the only element it.next(); // throws NoSuchElementException

🛠️ Solutions (5 Ways to Fix)

Solution 1✓ Most common cause

Guard next() with hasNext()

👉 Use this if you keep a manual iterator because you remove while reading or juggle two cursors over one list.

hasNext() asks whether a next slot exists without moving the cursor. Only advance when it answers true, so the state machine can never overshoot.

for (Iterator<String> it = items.iterator(); it.hasNext(); ) {
    String item = it.next();
    if (isJunk(item)) {
        it.remove();
    }
}

Solution 2

Replace manual iteration with enhanced-for

👉 Use this if you do not mutate the list during the walk and just need every element.

The compiler expands for-each into correct hasNext()/next() pairs, so overshooting becomes impossible by construction.

for (String item : items) {
    process(item);
}

Solution 3

Use poll()/peek() on queues instead of remove()/element()

👉 Use this if a Queue or Deque can legitimately be empty at read time.

pollFirst()/peekFirst() are the null-returning twins of removeFirst()/getFirst(): an empty queue yields null instead of an exception, turning a crash path into a branch.

ArrayDeque<Task> queue = new ArrayDeque<>(tasks);
Task next;
while ((next = queue.pollFirst()) != null) {
    run(next);
}

Solution 4

Get a fresh iterator instead of reusing an exhausted one

👉 Use this if a second pass over the same collection needs its own cursor.

An exhausted iterator never rewinds. Calling collection.iterator() again returns a new cursor positioned before the first element.

firstPass.forEach(first::add);          // pass 1 done
for (String s : firstPass) second.add(s); // fresh cursor, not the old one

Solution 5

Check isEmpty() before getFirst()/getLast() (Java 21)

👉 Use this if you adopted SequencedCollection accessors on lists or deques.

Java 21 getFirst()/getLast() throw NoSuchElementException on empty collections just like pop(). Guard with isEmpty(), or stay with peekFirst()/pollFirst().

if (!seq.isEmpty()) {
    var newest = seq.getLast();
}

📋 Version Notes

Java 8

Identical behavior; NoSuchElementException carries no message when thrown by iterator implementations.

Java 21

SequencedCollection adds getFirst()/getLast() which also throw NoSuchElementException when empty; peekFirst()/pollFirst() remain null-safe.

🛡️ How to Prevent This Next Time

Treat next(), remove() and element() as licensed moves: each requires a prior successful guard. Prefer enhanced-for and streams so iterator bookkeeping stays inside the compiler.