javaLogic errorMedium

Stream toList produces unmodifiable list on mutation

Time: 0:00

BugFind the bug and fix the code

What it should do

Filter even numbers and append bonus value. Example: appendItemToStreamList([2, 4], 99) -> [2, 4, 99]

What happens instead

java.lang.UnsupportedOperationException thrown when calling .add() on Stream.toList() result

import java.util.List;

public class Main {
    public static List<Integer> appendItemToStreamList(List<Integer> nums, int bonus) {
        // Stream.toList() in Java 16+ returns an unmodifiable list
        List<Integer> evens = nums.stream().filter(n -> n % 2 == 0).toList();
        evens.add(bonus); // Throws UnsupportedOperationException
        return evens;
    }

    public static void main(String[] args) {
        System.out.println(appendItemToStreamList(List.of(2, 4), 99));
    }
}

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

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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

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import java.util.List; public class Main { public static List<Integer> appendItemToStreamList(List<Integer> nums, int bonus) { // Stream.toList() in Java 16+ returns an unmodifiable list List<Integer> evens = nums.stream().filter(n -> n % 2 == 0).toList(); evens.add(bonus); // Throws UnsupportedOperationException return evens; } public static void main(String[] args) { System.out.println(appendItemToStreamList(List.of(2, 4), 99)); } }

Stream toList produces unmodifiable list on mutation

What it should do

What happens instead

More logic errors to practise

Stream toList produces unmodifiable list on mutation

What it should do

What happens instead

More logic errors to practise