javaType errorMedium

Raw type wildcard warning in generic list casting

Time: 0:00

BugFind the bug and fix the code

What it should do

Sum list of integers safely. Example: sumGenericIntegerList([10, 20]) -> 30

What happens instead

Unchecked cast warning and ClassCastException when raw types bypass generics

import java.util.List;

public class Main {
    public static int sumGenericIntegerList(List rawList) {
        // Raw type unchecked iteration triggers ClassCastException if list contains non-integers
        int sum = 0;
        for (Object obj : rawList) {
            sum += (Integer) obj;
        }
        return sum;
    }

    public static void main(String[] args) {
        System.out.println(sumGenericIntegerList(List.of(10, 20)));
    }
}

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 type errors pattern

Totals that concatenate, sorts that put 10 before 9, comparisons that refuse to run.

Learn the type errors pattern Step through your own code

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import java.util.List; public class Main { public static int sumGenericIntegerList(List rawList) { // Raw type unchecked iteration triggers ClassCastException if list contains non-integers int sum = 0; for (Object obj : rawList) { sum += (Integer) obj; } return sum; } public static void main(String[] args) { System.out.println(sumGenericIntegerList(List.of(10, 20))); } }

Raw type wildcard warning in generic list casting

What it should do

What happens instead

More type errors to practise

Raw type wildcard warning in generic list casting

What it should do

What happens instead

More type errors to practise