Implementing ArrayList Algorithms

Every pattern from Lesson 9.5 (sum/average, min/max, search, property checks, duplicates, reversing) applies to ArrayList too — just swap [i]/.length for get(i)/.size().

Min / Max / Average

public static double average(ArrayList<Double> nums)
{
    double sum = 0;
    for (int i = 0; i < nums.size(); i++)
    {
        sum += nums.get(i);
    }
    return sum / nums.size();
}

Duplicates — Use .equals(), Not ==

For ArrayList<Integer> (or any object type), comparing with == checks identity, not value — use .equals() instead:

for (int i = 0; i < nums.size(); i++)
{
    for (int j = i + 1; j < nums.size(); j++)
    {
        if (nums.get(i).equals(nums.get(j))) // .equals(), not ==
        {
            return true;
        }
    }
}

Rotating (In Place, with get/set)

public static void rotateRight(ArrayList<Double> nums)
{
    double last = nums.get(nums.size() - 1);
    for (int i = nums.size() - 1; i > 0; i--)
    {
        nums.set(i, nums.get(i - 1));
    }
    nums.set(0, last);
}

Parallel ArrayLists

Same idea as parallel arrays (Ch.9.1) — two lists kept in sync by index:

ArrayList<String> autoNames = new ArrayList<String>();
ArrayList<Double> autoTimes = new ArrayList<Double>();
// autoTimes.get(i) is the recorded time for autoNames.get(i)

Common Pitfalls

• Comparing objects with == instead of .equals(). This is the single most common ArrayList-specific mistake — nums.get(i) == nums.get(j) checks whether they're the same object, which is unreliable even for boxed Integers.
• Using array syntax ([]) out of habit. nums[i] doesn't compile on an ArrayList — it's always get/set.
• Forgetting .size() changes as you add/remove. A loop bound computed once before a series of removals can end up stale.

Key Takeaways

• Every array algorithm pattern (accumulator, min/max, search, property checks, duplicates, rotate, reverse) transfers directly to ArrayList — just swap [i]/.length for get(i)/.size().
• Compare ArrayList elements with .equals(), not ==, when the element type is an object (including boxed numbers like Integer).
• Parallel ArrayLists keep related data in sync by index, same as parallel arrays.

Derived from csawesome-2026: unit-4-data-collections/4.10-implementing-arraylist-algorithms.md (CSA 4.10)