verify() for interaction verification in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this JUnit test using Mockito's verify()?

Consider the following JUnit test code snippet using Mockito. What will be the test result after execution?

JUnit

import static org.mockito.Mockito.*;
import org.junit.jupiter.api.Test;

class Service {
    void process() {}
}

public class VerifyTest {
    @Test
    void testVerifyCalledOnce() {
        Service service = mock(Service.class);
        service.process();
        verify(service).process();
    }
}

A Test passes successfully

B Test fails with WantedButNotInvoked exception

C Test fails with TooManyActualInvocations exception

D Test fails with NullPointerException

💡 Hint

Mockito's verify() checks if the method was called exactly once by default.

The test mocks the Service class, calls process() once, then verifies that process() was called exactly once. Since it was called once, the verification passes and the test passes.

❓ assertion

intermediate

Which verify() assertion checks that a method was never called?

In Mockito, which verify() usage correctly asserts that the method process() was never called on the mock?

JUnit

Service service = mock(Service.class);
// no call to service.process() here

A verify(service, times(0)).process();

B verifyNoMoreInteractions(service);

C verify(service).process();

D verify(service, never()).process();

💡 Hint

Check the Mockito documentation for verifying zero invocations.

verify(service, never()).process() asserts that process() was never called. times(0) is equivalent but less readable. verify(service).process() expects one call. verifyNoMoreInteractions checks no other calls beyond verified ones.

🔧 Debug

advanced

Why does this verify() call fail with TooManyActualInvocations?

Given the code below, why does the verify() call fail with a TooManyActualInvocations exception?

JUnit

Service service = mock(Service.class);
service.process();
service.process();
verify(service).process();

A Because verify() requires specifying times(2) explicitly

B Because process() was never called

C Because process() was called twice but verify() expects exactly one call by default

D Because the mock was not initialized properly

💡 Hint

Check how many times process() was called versus what verify() expects.

The method process() was called twice, but verify(service).process() expects exactly one call. This mismatch causes the TooManyActualInvocations exception.

🧠 Conceptual

advanced

What is the purpose of verifyNoMoreInteractions() in Mockito?

Select the best description of what verifyNoMoreInteractions(mock) does in Mockito tests.

A It resets the mock to remove all previous interactions

B It checks that no other methods besides those already verified were called on the mock

C It verifies that the mock was never called at all

D It verifies that all methods on the mock were called at least once

💡 Hint

Think about verifying that no unexpected calls happened.

verifyNoMoreInteractions(mock) ensures that after verifying some interactions, no other calls were made on the mock. It helps catch unexpected method calls.

❓ framework

expert

In a JUnit 5 test using Mockito, which verify() usage correctly checks a method was called exactly twice with specific arguments?

Given a mock of Service with method process(String input), which verify() call correctly asserts process("data") was called exactly twice?

JUnit

Service service = mock(Service.class);
service.process("data");
service.process("data");

A verify(service, times(2)).process("data");

B verify(service).process("data"); verify(service).process("data");

C verify(service, atLeast(2)).process("data");

D verify(service, times(1)).process(anyString());

💡 Hint

Check how to specify exact call counts with arguments in verify().

verify(service, times(2)).process("data") asserts the method was called exactly twice with argument "data". Option A verifies two separate calls but does not check call count properly. Option A allows two or more calls, not exactly two. Option A checks only one call with any string.

Practice

1. What is the main purpose of verify() in JUnit testing with mocks?

easy

A. To create a mock object

B. To check if a method was called on a mock object

C. To assert the return value of a method

D. To initialize test data before tests run

Solution

1. Step 1: Understand the role of verify()

   verify() is used to check interactions with mock objects, specifically if certain methods were called.

2. Step 2: Differentiate from other mock-related actions

   Creating mocks is done by mock(), asserting return values is done by assertEquals(), and initializing data is unrelated to verify().

3. Final Answer:

   To check if a method was called on a mock object -> Option B

4. Quick Check:

   verify() checks method calls = B [OK]

Hint: Remember: verify() confirms method calls on mocks [OK]

Common Mistakes:

• Confusing verify() with mock() creation
• Using verify() to check return values
• Thinking verify() initializes test data

2. Which of the following is the correct syntax to verify that method save() was called exactly once on a mock object repository?

easy

A. verify(repository).times(1).save();

B. verify(repository).save(1);

C. verify(repository, times(1)).save();

D. verify(repository).calledOnce(save());

Solution

1. Step 1: Recall correct verify() syntax

   The correct syntax uses verify(mockObject, times(n)).method() to check method call count.

2. Step 2: Match options to syntax

   verify(repository, times(1)).save(); matches the correct syntax. Options B, C, and D use invalid or non-existent methods.

3. Final Answer:

   verify(repository, times(1)).save(); -> Option C

4. Quick Check:

   verify(mock, times(1)).method() = A [OK]

Hint: Use verify(mock, times(1)).method() to check single call [OK]

Common Mistakes:

• Writing times(1) after method call
• Passing call count as method argument
• Using non-existent verify methods

3. Given the following code snippet, what will happen when the test runs?

MyService service = mock(MyService.class);
service.process();
verify(service, times(2)).process();

medium

A. Test fails because process() was called only once

B. Test passes because verify() ignores call count

C. Test fails due to syntax error in verify()

D. Test passes because process() was called twice

Solution

1. Step 1: Count method calls on mock

   The method process() is called once on service.

2. Step 2: Check verify() expectation

   The verify expects process() to be called twice (times(2)), but it was called once.

3. Final Answer:

   Test fails because process() was called only once -> Option A

4. Quick Check:

   verify times(2) but called once = D [OK]

Hint: Count actual calls before verify times(n) check [OK]

Common Mistakes:

• Assuming verify ignores call count
• Confusing times(2) with atLeast(2)
• Thinking one call satisfies times(2)

4. Identify the error in this test code snippet:

List<String> list = mock(List.class);
verify(list).add("item");
list.add("item");

medium

A. No error, code is correct

B. Missing times() argument in verify()

C. Incorrect mock creation syntax

D. verify() is called before the method call on the mock

Solution

1. Step 1: Understand verify() checks past calls

   verify() checks if a method was called before it is invoked.

2. Step 2: Check order of calls

   Here, verify() is called before list.add("item"), so no call happened yet to verify.

3. Final Answer:

   verify() is called before the method call on the mock -> Option D

4. Quick Check:

   verify() must follow method call [OK]

Hint: Always call method before verify() to check it [OK]

Common Mistakes:

• Calling verify() before method invocation
• Assuming verify() can predict future calls
• Ignoring call order in tests

5. You want to verify that a method updateStatus(String status) was called exactly once with argument "ACTIVE" on a mock userService. Which code snippet correctly achieves this?

hard

A. verify(userService, times(1)).updateStatus("ACTIVE");

B. verify(userService).updateStatus(anyString());

C. verify(userService, times(1)).updateStatus();

D. verify(userService, times(1)).updateStatus(anyString());

Solution

1. Step 1: Use verify() with times(1) for exact call count

   To check exactly one call, use times(1) in verify.

2. Step 2: Match method call with exact argument

   Pass the exact argument "ACTIVE" inside the method call in verify.

3. Step 3: Evaluate options

   verify(userService, times(1)).updateStatus("ACTIVE"); matches both call count and argument. verify(userService).updateStatus(anyString()); allows any string, not exact. verify(userService, times(1)).updateStatus(); misses argument. verify(userService, times(1)).updateStatus(anyString()); uses times(1) but anyString() matches any argument, not specifically "ACTIVE".

4. Final Answer:

   verify(userService, times(1)).updateStatus("ACTIVE"); -> Option A

5. Quick Check:

   verify with times(1) and exact arg = C [OK]

Hint: Use times(1) and exact argument in verify() [OK]

Common Mistakes:

• Using anyString() instead of exact argument
• Omitting times(1) for call count
• Calling verify() without arguments