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.

Practice

1. What is the main purpose of a custom argument provider in JUnit?

easy

A. To generate test reports

B. To create new test methods automatically

C. To supply test data to parameterized tests in a custom way

D. To replace the @Test annotation

Solution

1. Step 1: Understand the role of argument providers

   Argument providers supply data to parameterized tests, allowing tests to run multiple times with different inputs.

2. Step 2: Identify the purpose of custom argument providers

   Custom argument providers let you define your own way to supply test data beyond built-in sources.

3. Final Answer:

   To supply test data to parameterized tests in a custom way -> Option C

4. Quick Check:

   Custom argument providers = supply test data [OK]

Hint: Custom providers supply test data uniquely for parameterized tests [OK]

Common Mistakes:

• Confusing argument providers with test method creators
• Thinking they replace @Test annotation
• Assuming they generate reports

2. Which interface must you implement to create a custom argument provider in JUnit?

easy

A. ArgumentsSource

B. ArgumentsProvider

C. TestTemplateInvocationContextProvider

D. TestInstancePostProcessor

Solution

1. Step 1: Recall JUnit interfaces for argument providers

   The interface to implement for custom argument providers is ArgumentsProvider.

2. Step 2: Differentiate from other interfaces

   ArgumentsSource is an annotation, not an interface. The others serve different purposes.

3. Final Answer:

   ArgumentsProvider -> Option B

4. Quick Check:

   Implement ArgumentsProvider for custom data [OK]

Hint: Implement ArgumentsProvider interface for custom data supply [OK]

Common Mistakes:

• Confusing ArgumentsSource annotation with interface
• Choosing unrelated interfaces
• Not overriding provideArguments method

3. Given this custom argument provider code snippet, what will be the output when the parameterized test runs?

public class MyProvider implements ArgumentsProvider {
    @Override
    public Stream<Arguments> provideArguments(ExtensionContext context) {
        return Stream.of(Arguments.of("apple"), Arguments.of("banana"));
    }
}

@ParameterizedTest
@ArgumentsSource(MyProvider.class)
void testFruits(String fruit) {
    System.out.println(fruit);
}

medium

A. apple\nbanana (each on separate lines)

B. apple banana

C. Compilation error due to missing imports

D. Runtime error due to wrong method signature

Solution

1. Step 1: Analyze the provideArguments method

   The method returns a stream with two Arguments: "apple" and "banana".

2. Step 2: Understand parameterized test execution

   The test runs twice, once with "apple" and once with "banana", printing each on its own line.

3. Final Answer:

   apple\nbanana (each on separate lines) -> Option A

4. Quick Check:

   Stream of Arguments runs test twice [OK]

Hint: Each argument runs test once, prints on separate lines [OK]

Common Mistakes:

• Assuming output is on one line
• Thinking code has syntax errors without imports
• Confusing method signature correctness

4. Identify the error in this custom argument provider implementation:

public class BadProvider implements ArgumentsProvider {
    public Stream<Arguments> provideArguments() {
        return Stream.of(Arguments.of(1), Arguments.of(2));
    }
}

medium

A. Method provideArguments must have ExtensionContext parameter

B. Missing @Override annotation

C. Arguments.of() cannot take integers

D. Class must extend ArgumentsSource

Solution

1. Step 1: Check method signature requirements

   The provideArguments method must override the interface method and accept ExtensionContext as a parameter.

2. Step 2: Identify the missing parameter

   The code's provideArguments method lacks the ExtensionContext parameter, causing a signature mismatch.

3. Final Answer:

   Method provideArguments must have ExtensionContext parameter -> Option A

4. Quick Check:

   Correct method signature requires ExtensionContext [OK]

Hint: provideArguments must accept ExtensionContext parameter [OK]

Common Mistakes:

• Ignoring method signature parameters
• Thinking @Override is mandatory but not fixing signature
• Confusing Arguments.of() usage

5. You want to create a custom argument provider that supplies pairs of integers where the second is always double the first. Which implementation correctly achieves this?

public class DoubleProvider implements ArgumentsProvider {
    @Override
    public Stream<Arguments> provideArguments(ExtensionContext context) {
        // Fill in the blank
    }
}

hard

A. return Stream.of(Arguments.of(1, 3), Arguments.of(2, 5), Arguments.of(3, 7));

B. return Stream.of(Arguments.of(1, 1), Arguments.of(2, 2), Arguments.of(3, 3));

C. return Stream.empty();

D. return Stream.of(Arguments.of(1, 2), Arguments.of(2, 4), Arguments.of(3, 6));

Solution

1. Step 1: Understand the requirement

   Each pair must have the second integer double the first.

2. Step 2: Evaluate each option

   return Stream.of(Arguments.of(1, 2), Arguments.of(2, 4), Arguments.of(3, 6)); correctly pairs (1,2), (2,4), (3,6). Other options provide pairs like (1,3),(2,5),(3,7) or equal numbers (1,1),(2,2),(3,3), or an empty stream.

3. Final Answer:

   return Stream.of(Arguments.of(1, 2), Arguments.of(2, 4), Arguments.of(3, 6)); -> Option D

4. Quick Check:

   Second integer = 2 * first integer [OK]

Hint: Check if second number is double the first in pairs [OK]

Common Mistakes:

• Providing pairs where second is not double
• Returning empty stream by mistake
• Confusing argument order