How to Use JUnit 5 @MethodSource for Parameterized Tests in a Nested Class?

How can I implement JUnit 5 @MethodSource in a nested class for parameterized tests?

@MethodSource("cards")

To use `@MethodSource` in a nested class while adhering to the JUnit 5 framework's requirements, you can follow these approaches. The core challenge lies in the need for the method providing the arguments to be `static` and the compatibility of nested classes. Below are step-by-step solutions to effectively implement this.

@Nested
static class Cost {
    static Stream<Arguments> cards() {
        return Stream.of(
            Arguments.of(Card.common(0, Color.RED), 0),
            Arguments.of(Card.choseColor(), 50)
        );
    }

    @MethodSource("cards")
    @ParameterizedTest
    void cardCost(Card card, int cost) {
        assertThat(card.cost()).isEqualTo(cost);
    }
}

Causes

• The `@MethodSource` annotation requires the method supplying the stream of arguments to be static.
• Non-static inner classes in Java cannot have static methods, which leads to a conflict when attempting to use `@MethodSource`.
• If the inner class is made static, JUnit does not recognize it, hindering test execution.

Solutions

• Move the parameter source method to a separate static method in the enclosing class. This enables it to be accessible without breaking the rules of static inner classes.
• Declare the nested class as static which allows the use of static methods inside it while still being recognized by JUnit.

Mistake: Declaring the cards() method as non-static in a static nested class.

Solution: Always declare the argument-providing method as static in a nested class.

Mistake: Forgetting to configure JUnit to discover nested tests correctly.

Solution: Ensure the nested class is properly annotated with @Nested and maintain a static context for parameters.