Iterator and safe removal

What you will learn

• Walk a list with an Iterator using hasNext, next and remove
• Explain why list.remove inside a loop throws ConcurrentModificationException
• Spot the skipped item when a forward index loop removes
• Remove items safely with Iterator.remove, removeIf or a backwards loop

Topics in this lesson

1. What an Iterator is
2. Removing inside a loop goes wrong
3. The index loop skips items
4. Safe ways to remove
5. Removing objects: removeIf or an Iterator

Key points

• An Iterator walks a list with hasNext(), next() and remove(); a for-each loop uses one behind the scenes.
• Changing the list directly inside a loop throws ConcurrentModificationException; a forward index loop that removes skips the next item.
• Remove with iterator.remove(), removeIf(...) or a backwards index loop.
• Call next() before remove(), and check hasNext() before next().

Interview questions this lesson answers

• What is an Iterator?
• Why do you get ConcurrentModificationException when you remove from a list inside a for-each loop?
• How do you remove items safely while looping?
• Does ConcurrentModificationException mean you have a threading problem?
• Follow-up: does removing inside a for-each loop always throw?
• What is the difference between iterator.remove() and list.remove(x) inside a loop?
• What does removeIf do and what does it return?