ACSA Exam Prep
Unit 4 · Data Collections · Topic 4.9

ArrayList Traversals

Skill 2.BSkill 3.DSkill 4.A

Big idea: Loop over an ArrayList with get(i) or an enhanced for. Removing while you loop is where bugs hide, because every removal shifts the elements after it.

Learning objectives
  • Develop code used to traverse the elements of an ArrayList and determine the results of these traversals.

Two ways to loop

Code
ArrayList<String> words = new ArrayList<String>();
words.add("sun");
words.add("moon");
words.add("star");
for (int i = 0; i < words.size(); i++) {
  System.out.print(words.get(i).length() + " ");
}
System.out.println();
for (String w : words) {
  System.out.print(w.substring(0, 1));
}
System.out.println();
Output
3 4 4
sms

Removing safely

After remove(i), the next element slides into index i. If the loop then does i++, that element is skipped. Two fixes: only increment when you don't remove, or loop backward.

Code
ArrayList<Integer> a = new ArrayList<Integer>(List.of(2, 4, 4, 5));
for (int i = 0; i < a.size(); i++) {
  if (a.get(i) % 2 == 0) a.remove(i);    // buggy
}
System.out.println(a);
ArrayList<Integer> b = new ArrayList<Integer>(List.of(2, 4, 4, 5));
for (int i = b.size() - 1; i >= 0; i--) {
  if (b.get(i) % 2 == 0) b.remove(i);    // backward: safe
}
System.out.println(b);
Output
[4, 5]
[5]

Don't change size in an enhanced for

Adding or removing elements inside an enhanced for loop over the same list can throw a ConcurrentModificationException.

Code
ArrayList<String> list = new ArrayList<String>(List.of("a", "b", "c"));
for (String s : list) {
  if (s.equals("a")) list.remove(0);
}
Output
throws ConcurrentModificationException
Key points
  • Traverse an ArrayList with indexed loops, enhanced for loops, or recursion.
  • Removing during traversal needs care so you don't skip elements.
  • An invalid index throws IndexOutOfBoundsException.
  • Don't add or remove inside an enhanced for; it can throw ConcurrentModificationException.
Watch out
  • Forward loop + remove(i) + i++ skips the element that slides into position i.
Exam tip
  • List.of(...) appears here only to build sample lists quickly; you won't need it on the exam.
Quick check
Why does looping backward make removal safe?
Show answer
Removing index i only shifts elements after i, which the loop has already visited.
Practice 4.9 (10 questions)