javaLogic errorMedium

Unexpected Mutation of Nested ArrayLists

Time: 0:00

BugFind the bug and fix the code

What it should do

Create a function that returns a copy of a nested ArrayList without affecting the original. Example: f([[1, 2], [3, 4]]) -> [[1, 2], [3, 4]]

What happens instead

Modifying the copied nested ArrayList unexpectedly alters the original nested ArrayList.

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<ArrayList<Integer>> original = new ArrayList<>();
        original.add(new ArrayList<>());
        original.get(0).add(1);
        original.get(0).add(2);
        ArrayList<ArrayList<Integer>> copy = copyNestedArrayLists(original);
        copy.get(0).set(0, 99);
        System.out.println(original); // Expect [[1, 2]] but get [[99, 2]]
    }

    public static ArrayList<ArrayList<Integer>> copyNestedArrayLists(ArrayList<ArrayList<Integer>> original) {
        ArrayList<ArrayList<Integer>> copy = new ArrayList<>(original);
        return copy;
    }
}

Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).

⏱️0:00

Hints

🏆 0 hints = Flawless Solve

Think & Test Cooldown

2:00 remaining

Take a moment to read the bug symptom and trace the code with Run & Check before unlocking Hint 1.

Hint 2

Unlocks after Hint 1

Hint 3

Unlocks after Hint 2

This bug belongs to the logic errors pattern

The wrong operator, an inverted condition, a formula the right way round but backwards.

Learn the logic errors pattern Step through your own code

<iframe src="https://trybughunt.com/embed/d68f5e81-10aa-4f19-b75f-74d01efca874" title="Debugging challenge: Unexpected Mutation of Nested ArrayLists" width="100%" height="580" loading="lazy" style="border:1px solid #d4d4d8;border-radius:12px;max-width:760px"></iframe> <p style="font-size:14px">Debugging challenge from <a href="https://trybughunt.com/challenges/d68f5e81-10aa-4f19-b75f-74d01efca874">TryBugHunt</a></p>

import java.util.ArrayList; public class Main { public static void main(String[] args) { ArrayList<ArrayList<Integer>> original = new ArrayList<>(); original.add(new ArrayList<>()); original.get(0).add(1); original.get(0).add(2); ArrayList<ArrayList<Integer>> copy = copyNestedArrayLists(original); copy.get(0).set(0, 99); System.out.println(original); // Expect [[1, 2]] but get [[99, 2]] } public static ArrayList<ArrayList<Integer>> copyNestedArrayLists(ArrayList<ArrayList<Integer>> original) { ArrayList<ArrayList<Integer>> copy = new ArrayList<>(original); return copy; } }

Unexpected Mutation of Nested ArrayLists

What it should do

What happens instead

More logic errors to practise

Unexpected Mutation of Nested ArrayLists

What it should do

What happens instead

More logic errors to practise