when().thenReturn() stubbing in JUnit - Build an Automation Script

Stub a method using when().thenReturn() in JUnit with Mockito

Preconditions (2)

Step 1: Create a mock object of the service class using Mockito.mock()

Step 2: Use Mockito.when() to specify the method call to stub

Step 3: Use thenReturn() to specify the value to return when the method is called

Step 4: Call the stubbed method on the mock object

Step 5: Verify that the returned value matches the stubbed value using an assertion

✅ Expected Result: The stubbed method returns the specified value and the assertion passes

Automation Requirements - JUnit 5 with Mockito

Assertions Needed:

Assert that the stubbed method returns the expected value

Best Practices:

Use Mockito.mock() to create mock objects

Use when().thenReturn() for stubbing methods

Use assertions from JUnit Jupiter (Assertions.assertEquals)

Keep test methods focused and clear

Use @Test annotation for test methods

Automated Solution

JUnit

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

import org.junit.jupiter.api.Test;

class UserService {
    public String getUserRole(String username) {
        // Imagine this calls a database or external service
        return "Unknown";
    }
}

public class UserServiceTest {

    @Test
    void testGetUserRoleStub() {
        // Create mock
        UserService mockService = mock(UserService.class);

        // Stub method
        when(mockService.getUserRole("admin")).thenReturn("Administrator");

        // Call stubbed method
        String role = mockService.getUserRole("admin");

        // Assert returned value
        assertEquals("Administrator", role);
    }
}

This test creates a mock of UserService using Mockito. Then it stubs the getUserRole method to return "Administrator" when called with "admin". The test calls the stubbed method and asserts the returned value matches the stubbed value. This shows how when().thenReturn() works for stubbing methods in unit tests.

Common Mistakes - 3 Pitfalls

Not creating a mock object before stubbing

Using when() with real method calls instead of mock methods

Forgetting to import static Mockito methods

Bonus Challenge

Now add data-driven testing with 3 different usernames and expected roles using parameterized tests

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