Custom argument providers in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a JUnit test using a custom argument provider

Given the following JUnit 5 test code using a custom argument provider, what will be the output when the test runs?

JUnit

import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.Arguments;
import org.junit.jupiter.params.provider.ArgumentsProvider;
import org.junit.jupiter.params.provider.ArgumentsSource;
import java.util.stream.Stream;
import static org.junit.jupiter.api.Assertions.assertTrue;

class CustomArgProviderTest {
    static class EvenNumberProvider implements ArgumentsProvider {
        @Override
        public Stream<Arguments> provideArguments(org.junit.jupiter.api.extension.ExtensionContext context) {
            return Stream.of(2, 4, 6, 8).map(Arguments::of);
        }
    }

    @ParameterizedTest
    @ArgumentsSource(EvenNumberProvider.class)
    void testEvenNumbers(int number) {
        assertTrue(number % 2 == 0);
    }
}

A All tests pass successfully

B Test fails because the argument provider returns odd numbers

C Test fails due to a runtime exception in the argument provider

D Test fails because the test method signature is incorrect

💡 Hint

Check what numbers the custom argument provider returns and what the assertion checks.

The custom argument provider returns only even numbers (2, 4, 6, 8). The test asserts that each number is even using 'assertTrue(number % 2 == 0)'. Since all provided numbers are even, all assertions pass and the test succeeds.

❓ assertion

intermediate

Assertion outcome with a custom argument provider

Consider a custom argument provider that supplies the following integers: 1, 3, 5, 7. The test method asserts that the number is less than 5. What will be the assertion outcomes for each input?

JUnit

import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.Arguments;
import org.junit.jupiter.params.provider.ArgumentsProvider;
import org.junit.jupiter.params.provider.ArgumentsSource;
import java.util.stream.Stream;
import static org.junit.jupiter.api.Assertions.assertTrue;

class LessThanFiveProviderTest {
    static class OddNumberProvider implements ArgumentsProvider {
        @Override
        public Stream<Arguments> provideArguments(org.junit.jupiter.api.extension.ExtensionContext context) {
            return Stream.of(1, 3, 5, 7).map(Arguments::of);
        }
    }

    @ParameterizedTest
    @ArgumentsSource(OddNumberProvider.class)
    void testLessThanFive(int number) {
        assertTrue(number < 5);
    }
}

A Tests for 1, 3, and 5 pass; test for 7 fails

B Tests for 1 and 3 pass; tests for 5 and 7 fail

C All tests fail

D All tests pass

💡 Hint

Check which numbers satisfy the condition number < 5.

The numbers 1 and 3 are less than 5, so their assertions pass. The numbers 5 and 7 are not less than 5, so their assertions fail.

🔧 Debug

advanced

Identify the error in a custom argument provider implementation

What is the cause of the runtime error in the following custom argument provider code?

JUnit

import org.junit.jupiter.params.provider.Arguments;
import org.junit.jupiter.params.provider.ArgumentsProvider;
import java.util.stream.Stream;

public class NullProvider implements ArgumentsProvider {
    @Override
    public Stream<Arguments> provideArguments(org.junit.jupiter.api.extension.ExtensionContext context) {
        return null;
    }
}

A The provideArguments method returns null instead of a Stream, causing a NullPointerException

B The class does not implement ArgumentsProvider correctly

C The method signature is incorrect

D The Stream returned contains null elements

💡 Hint

Check what the provideArguments method returns and what JUnit expects.

JUnit expects provideArguments to return a non-null Stream of Arguments. Returning null causes a NullPointerException at runtime.

❓ framework

advanced

Correct usage of custom argument provider in JUnit 5

Which of the following is the correct way to use a custom argument provider in a JUnit 5 parameterized test?

A

@ParameterizedTest
@CsvSource({"a", "b"})
void testMethod(String input) { /* test code */ }

B

@Test
@ArgumentsSource(MyProvider.class)
void testMethod(String input) { /* test code */ }

C

@ParameterizedTest
@ValueSource(strings = {"a", "b"})
void testMethod(String input) { /* test code */ }

D

@ParameterizedTest
@ArgumentsSource(MyProvider.class)
void testMethod(String input) { /* test code */ }

💡 Hint

Custom argument providers require a specific annotation and test type.

Custom argument providers must be used with @ParameterizedTest and @ArgumentsSource annotations. Option D correctly uses both. Option D uses @Test which is not for parameterized tests. Options C and D use built-in sources, not custom providers.

🧠 Conceptual

expert

Understanding lifecycle and state in custom argument providers

A custom argument provider maintains internal state by storing a counter that increments each time provideArguments is called. What is the expected behavior of this counter across multiple test method invocations in JUnit 5?

A The argument provider is a singleton and the counter is shared across all test classes

B The same instance is reused for all test methods, so the counter accumulates across all tests

C A new instance of the argument provider is created for each test method invocation, so the counter resets each time

D JUnit 5 does not allow stateful argument providers and throws an error

💡 Hint

Consider how JUnit 5 manages instances of argument providers during test execution.

JUnit 5 creates a new instance of the argument provider for each test method invocation, so any internal state like a counter will reset each time provideArguments is called.