JUnit Test - ParameterizedTests - 'No implicit conversion of type java.lang.Integer to type [Ljava.lang.Integer;'

Question

Using JUnit5. This doesn't seem to be behaving correctly to me, I'm likely missing something.

    @ParameterizedTest
    @MethodSource
    void testFunc(Integer arr []) {
        for(Integer i : arr){
            System.out.print(i + " ");
        }
        System.out.println("\n");
    }

    static Stream<Arguments> testFunc() {
        return Stream.of(
                Arguments.of(new Integer [] {12, 42, 52, 1234, 12, 425, 4}),
                Arguments.of(new Integer [] {12, 42, 52, 1234, 12, 425}),
                Arguments.of(new Integer [] {12})
        );
    }

Produces the error:

org.junit.jupiter.api.extension.ParameterResolutionException: Error converting parameter at index 0: No implicit conversion to convert object of type java.lang.Integer to type [Ljava.lang.Integer;

I've also tried the above code using int instead of Integer, but this works correctly.

This works with no error:

    public static void main(String args[]){
        test(new Integer []{12, 42, 52, 1234, 12, 425, 4});
    }

    static void test(Integer[] arr) {
        for(Integer a : arr){
            System.out.println(a);
        }
    }

Answer 1

You're getting this error, because the method declaration of Arguments.of() is the following:

public static Arguments of(Object... arguments)

Object... indicates a varargs (variable arguments) parameter, so when writing:

Arguments.of(1, 2, 3)

at compile time, java will desuger this into an array:

Arguments.of(new Object[]{1, 2, 3})

The problem that you're having can be shown by the following:

Integer[] ints = new Integer[]{1, 2, 3};
Object[] objs = ints; // works

So when you write:

Arguments.of(new Integer[]{12, 42, 52, 1234, 12, 425, 4})

Then java will just pass this array directly into the method, it would effectively behave the same as both of the following:

Arguments.of(new Object[]{12, 42, 52, 1234, 12, 425, 4})
Arguments.of(12, 42, 52, 1234, 12, 425, 4)

To overcome this problem, you need to cast your Integer[] array directly to Object, that way it will be wrapped inside another array:

Arguments.of((Object) new Integer[]{12, 42, 52, 1234, 12, 425, 4})

Will become:

Arguments.of(new Object[]{new Integer[]{12, 42, 52, 1234, 12, 425, 4}})

As you've noted it works correctly with int[] because the following doesn't work:

int[] ints = new int[]{1, 2, 3};
Object[] objs = ints; // int[] cannot be assigned to Object[]