Topic 4.6 — Traversing ArrayLists

Goal: traverse an ArrayList using indexed loops, use size() correctly, and handle modification patterns safely.

The big idea

Traversing an ArrayList looks a lot like traversing an array—but you use size() and get(i) instead of length and [i].

If you’re reading or updating by index, an indexed for loop is your best friend.

for (int i = 0; i < list.size(); i++) {
  // use list.get(i)
}

int sum = 0;
for (int i = 0; i < nums.size(); i++) {
  sum += nums.get(i);
}

This is the ArrayList version of the classic “accumulator” pattern.

Update elements in-place by index.

// Replace negatives with 0
for (int i = 0; i < nums.size(); i++) {
  if (nums.get(i) < 0) {
    nums.set(i, 0);
  }
}

Track the index where the best value lives.

int maxIndex = 0;
for (int i = 1; i < nums.size(); i++) {
  if (nums.get(i) > nums.get(maxIndex)) {
    maxIndex = i;
  }
}

Removing shifts elements left, so you can accidentally skip items if you go forward.

// ❌ Risky: may skip elements after a remove
for (int i = 0; i < list.size(); i++) {
  if (list.get(i) == 0) {
    list.remove(i);
  }
}

Traversing from the end avoids skipping because indexes after i don’t shift “past” you.

for (int i = list.size() - 1; i >= 0; i--) {
  if (list.get(i) == 0) {
    list.remove(i);
  }
}

Mistake	Why it’s wrong	Fix
`list.length`	ArrayLists don’t have `length`	Use `list.size()`
`list[i]`	ArrayLists don’t use bracket indexing	Use `list.get(i)`
`i <= list.size()`	Last valid index is `size()-1`	Use `i < list.size()`
Remove while looping forward	Shifts elements; may skip checks	Loop backward or adjust index

What’s the correct loop condition for traversing: i < list.size() or i <= list.size()?
How do you read the element at index 3?
How do you replace the element at index 3?
Why is removing while looping forward risky?
Write a backward loop that removes all zeros.