verify() for interaction verification in JUnit

Introduction

We use verify() to check if a specific action happened in our code, like a button click or a method call. It helps us confirm that parts of our program work together correctly.

When you want to check if a method was called on a mock object during a test.

When you need to confirm that a user interaction triggered the right backend call.

When you want to ensure a service was called exactly once or a certain number of times.

When you want to test that no unexpected interactions happened with a mock.

When you want to verify the order of method calls between objects.

Syntax

JUnit

verify(mockObject).methodName(arguments);

mockObject is the fake object you created to watch interactions.

methodName(arguments) is the method you expect to be called with specific arguments.

Examples

Check if login was called with username "user1" and password "pass123".

JUnit

verify(userService).login("user1", "pass123");

Check if sendEmail was called exactly two times with any string argument.

JUnit

verify(emailSender, times(2)).sendEmail(anyString());

Check that deleteUser was never called with any integer argument.

JUnit

verify(database, never()).deleteUser(anyInt());

Sample Program

This test creates a fake UserRepository and checks that when register is called, the saveUser method is called with the username "alice".

JUnit

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

class UserServiceTest {

    interface UserRepository {
        void saveUser(String username);
    }

    class UserService {
        private UserRepository repo;
        UserService(UserRepository repo) { this.repo = repo; }
        void register(String username) {
            repo.saveUser(username);
        }
    }

    @Test
    void testRegisterCallsSaveUser() {
        UserRepository mockRepo = mock(UserRepository.class);
        UserService service = new UserService(mockRepo);

        service.register("alice");

        verify(mockRepo).saveUser("alice");
    }
}

Important Notes

Always create mocks for the objects you want to watch interactions on.

If verify() fails, the test will fail and tell you which method call was missing or incorrect.

You can use times(n), never(), and other options to control how many times a method should be called.

Summary

verify() checks if a method was called on a mock object during a test.

It helps confirm that parts of your code interact as expected.

Use it to check method calls, call counts, and argument values.

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