ParameterResolver extension in JUnit - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to implement the ParameterResolver interface.

JUnit

public class MyParameterResolver implements ParameterResolver {
    @Override
    public boolean supportsParameter(ParameterContext parameterContext, ExtensionContext extensionContext) throws ParameterResolutionException {
        return parameterContext.getParameter().getType() == [1];
    }

    @Override
    public Object resolveParameter(ParameterContext parameterContext, ExtensionContext extensionContext) throws ParameterResolutionException {
        return "Hello, JUnit!";
    }
}

Drag options to blanks, or click blank then click option'

AObject.class

Bint.class

CList.class

DString.class

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong class type in supportsParameter.

Forgetting to use .class after the type.

The supportsParameter method should check if the parameter type is String.class to provide a String value.

2fill in blank

medium

Complete the test method to use the ParameterResolver extension.

JUnit

@ExtendWith(MyParameterResolver.class)
public class MyTest {
    @Test
    void testWithParameter([1] message) {
        assertEquals("Hello, JUnit!", message);
    }
}

Drag options to blanks, or click blank then click option'

Aint

BList<String>

CString

DObject

Attempts:

3 left

💡 Hint

Common Mistakes

Using a parameter type not supported by the resolver.

Forgetting to annotate the test class with @ExtendWith.

The test method parameter must be String to match the ParameterResolver's supported type.

3fill in blank

hard

Fix the error in the supportsParameter method to correctly check for Integer type.

JUnit

@Override
public boolean supportsParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
    return parameterContext.getParameter().getType() == [1];
}

Drag options to blanks, or click blank then click option'

AInteger.class

Bint.class

CLong.class

DDouble.class

Attempts:

3 left

💡 Hint

Common Mistakes

Using int.class instead of Integer.class.

Confusing primitive and wrapper types.

Use Integer.class to check for the Integer wrapper type, not int.class which is primitive.

4fill in blank

hard

Fill both blanks to correctly implement resolveParameter returning a fixed integer value.

JUnit

@Override
public Object resolveParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
    return [1];
}

@Override
public boolean supportsParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
    return parameterContext.getParameter().getType() == [2];
}

Drag options to blanks, or click blank then click option'

A42

BString.class

CInteger.class

D100

Attempts:

3 left

💡 Hint

Common Mistakes

Returning a String instead of an Integer.

Checking for String.class instead of Integer.class.

The resolver returns 42 as an Integer, so supportsParameter must check for Integer.class.

5fill in blank

hard

Fill all three blanks to create a ParameterResolver that provides a List<String> with one element "test".

JUnit

public class ListParameterResolver implements ParameterResolver {
    @Override
    public boolean supportsParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
        return parameterContext.getParameter().getType() == [1];
    }

    @Override
    public Object resolveParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
        return [2];
    }
}

// Usage in test:
@ExtendWith(ListParameterResolver.class)
public class ListTest {
    @Test
    void testListParameter([3] list) {
        assertEquals(1, list.size());
        assertEquals("test", list.get(0));
    }
}

Drag options to blanks, or click blank then click option'

AList.class

BList.of("test")

CList<String>

DArrayList.class

Attempts:

3 left

💡 Hint

Common Mistakes

Using ArrayList.class instead of List.class in supportsParameter.

Returning an empty list or wrong list content.

Using raw List instead of List in test method.

The supportsParameter checks for List.class, resolveParameter returns List.of("test"), and the test method parameter is List.

Practice

1. What is the main purpose of a ParameterResolver in JUnit 5?

easy

A. To define the order of test execution

B. To generate test reports after execution

C. To create mock objects for tests

D. To automatically provide values for test method parameters

Solution

1. Step 1: Understand ParameterResolver role

   A ParameterResolver is designed to supply values to parameters of test methods automatically, so tests don't need manual setup for those parameters.

2. Step 2: Compare with other options

   Options A, C, and D relate to other testing features but not to parameter injection.

3. Final Answer:

   To automatically provide values for test method parameters -> Option D

4. Quick Check:

   ParameterResolver = automatic parameter injection [OK]

Hint: ParameterResolver injects parameters automatically [OK]

Common Mistakes:

• Confusing ParameterResolver with test ordering
• Thinking it creates mocks automatically
• Assuming it generates reports

2. Which two methods must a class implement to be a valid ParameterResolver in JUnit 5?

easy

A. supportsParameter and resolveParameter

B. beforeEach and afterEach

C. initialize and cleanup

D. createParameter and validateParameter

Solution

1. Step 1: Recall ParameterResolver interface methods

   The ParameterResolver interface requires implementing supportsParameter to check if a parameter is supported, and resolveParameter to provide the value.

2. Step 2: Eliminate incorrect method pairs

   Options A, B, and C list methods unrelated to ParameterResolver interface.

3. Final Answer:

   supportsParameter and resolveParameter -> Option A

4. Quick Check:

   ParameterResolver methods = supportsParameter + resolveParameter [OK]

Hint: Remember: supportsParameter checks, resolveParameter provides [OK]

Common Mistakes:

• Confusing lifecycle methods with resolver methods
• Using incorrect method names
• Assuming initialization methods are required

3. Given this ParameterResolver implementation, what will be printed when the test runs?

class MyResolver implements ParameterResolver {
  public boolean supportsParameter(ParameterContext pc, ExtensionContext ec) {
    return pc.getParameter().getType() == String.class;
  }
  public Object resolveParameter(ParameterContext pc, ExtensionContext ec) {
    return "InjectedValue";
  }
}

@ExtendWith(MyResolver.class)
class TestClass {
  @Test
  void testMethod(String param) {
    System.out.println(param);
  }
}

medium

A. null

B. InjectedValue

C. Compilation error

D. Test fails with exception

Solution

1. Step 1: Analyze supportsParameter method

   It returns true only if the parameter type is String, so the test method's String param is supported.

2. Step 2: Analyze resolveParameter method

   It returns the string "InjectedValue", so this value will be injected into the test method parameter.

3. Final Answer:

   InjectedValue -> Option B

4. Quick Check:

   Parameter injection prints InjectedValue [OK]

Hint: Check supportsParameter return true for parameter type [OK]

Common Mistakes:

• Assuming parameter is null if not supported
• Expecting compilation error due to interface methods
• Thinking test fails without explicit parameter value

4. Identify the error in this ParameterResolver implementation:

class FaultyResolver implements ParameterResolver {
  public boolean supportsParameter(ParameterContext pc, ExtensionContext ec) {
    return pc.getParameter().getType() == Integer.class;
  }
  public Object resolveParameter(ParameterContext pc, ExtensionContext ec) {
    return 42;
  }
}

@ExtendWith(FaultyResolver.class)
class TestClass {
  @Test
  void testMethod(int param) {
    System.out.println(param);
  }
}

medium

A. Missing @Override annotations cause runtime error

B. resolveParameter returns wrong type

C. supportsParameter checks for Integer.class but parameter is primitive int

D. Test method parameter must be Integer, not int

Solution

1. Step 1: Check parameter type in supportsParameter

   The test method parameter is primitive int, but supportsParameter checks for Integer.class, which is the wrapper class. This causes supportsParameter to return false.

2. Step 2: Understand consequence of mismatch

   Because supportsParameter returns false, resolveParameter is never called, so the parameter is not injected, causing test failure or default value usage.

3. Final Answer:

   supportsParameter checks for Integer.class but parameter is primitive int -> Option C

4. Quick Check:

   Primitive vs wrapper mismatch breaks supportsParameter [OK]

Hint: Match primitive and wrapper types carefully in supportsParameter [OK]

Common Mistakes:

• Ignoring difference between int and Integer
• Assuming @Override annotations are mandatory
• Thinking resolveParameter type must differ

5. You want to create a ParameterResolver that injects a unique UUID string into any test method parameter of type String. Which implementation correctly achieves this and ensures a new UUID is generated for each test invocation?

hard

A. In resolveParameter, return UUID.randomUUID().toString() without caching

B. In supportsParameter, cache a UUID and return it for all calls

C. Return a fixed UUID string in resolveParameter for all tests

D. Generate UUID once in constructor and return it in resolveParameter

Solution

1. Step 1: Understand requirement for unique UUID per test

   Each test invocation should get a new UUID string, so the UUID must be generated fresh every time resolveParameter is called.

2. Step 2: Evaluate options for UUID generation

   In resolveParameter, return UUID.randomUUID().toString() without caching generates a new UUID string on each call, fulfilling the requirement. Options B, C, and D cache or fix the UUID, so they do not produce unique values per test.

3. Final Answer:

   In resolveParameter, return UUID.randomUUID().toString() without caching -> Option A

4. Quick Check:

   Generate UUID fresh in resolveParameter for uniqueness [OK]

Hint: Generate new UUID inside resolveParameter method [OK]

Common Mistakes:

• Caching UUID and returning same value every time
• Generating UUID once in constructor
• Returning fixed string instead of UUID