AP CSA 4.9 Exercise 2: Traversing ArrayLists Applied Practice

Unit 4: Data Collections · Lesson 4.9 · Exercise 2

Traversing ArrayLists: Applied Practice

Six scenarios on the one traversal bug that only ArrayLists have: changing the collection while you are walking it.

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How this one is different

The exercise-1 editor asks you to traverse an ArrayList. These six are mostly about removal, because removing during a loop is the mistake this topic exists to teach.

Applied Practice

6 questions · scenario driven · every answer is recorded for your teacher

Question 1 of 6Analyze
A forward loop over [2, 4, 6, 8] removes every even value using an index that always increments.
What actually happens?
Incorrect. The list shrinks as it goes; running off the end is possible but is not what makes this wrong.
Incorrect. Removals do occur. They are simply incomplete.
Incorrect. The skipping means several survive the pass.
Correct. Correct. The index advances while the data slides underneath it, so every other match is missed.
Question 2 of 6Apply
A program must remove every element matching a condition, using an index-based loop.
What is the simplest correct fix?
Incorrect. The enhanced for is worse here: modifying during it throws.
Incorrect. That compounds the skipping rather than fixing it.
Correct. Correct. Traversing backward is the other standard fix, since removal only shifts elements the backward walk has already passed.
Incorrect. Removing twice removes the wrong elements.
Question 3 of 6Analyze
A program calls list.remove(...) inside an enhanced for loop over that same list.
What is the likely result?
Incorrect. This is precisely the case the exception exists to forbid.
Correct. Correct, and the loud failure is better than the index loop's silent skipping.
Incorrect. The index loop fails quietly; this one fails noisily. That difference is the point.
Incorrect. It compiles. The failure happens at run time.
Question 4 of 6Apply
A backward loop removes matching elements from an ArrayList.
Why does traversing backward avoid the skipping problem?
Correct. Correct. The shift lands entirely in territory already covered, so nothing unvisited moves.
Incorrect. Direction has no meaningful speed effect.
Incorrect. Nothing is reversed. Only the visiting order changes.
Incorrect. Removal always shifts the same way. Direction changes whether that matters.
Question 5 of 6Evaluate
A loop is written as for (int i = 0; i < list.size(); i++) and the body sometimes removes an element.
What is true about the bound?
Incorrect. It does not throw, which is exactly what makes the bug hard to spot.
Incorrect. It is an ordinary method call and entirely legal there.
Incorrect. The condition is a live expression, re-evaluated each pass. Caching it in a variable is what creates a stale bound.
Correct. Correct, and it is why this loop skips elements rather than crashing. The bound is fine; the increment is not.
Question 6 of 6Apply
A method must build a NEW list of the elements that pass a test, leaving the original untouched.
Which approach is cleanest?
Incorrect. That leaves nulls in the original, corrupting it and inviting NullPointerException.
Incorrect. That destroys the original, which the requirement forbids.
Correct. Correct, and it sidesteps the entire category of bug this topic is about.
Incorrect. Sorting does not partition by an arbitrary test and reorders the data.

Where to go next

Try the Lesson 4.9 coding exercise Pick another Unit 4 lesson

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