How to test void methods

Testing void methods in JUnit is a common task—especially when you're testing side effects, like updating state, writing to logs, saving to a database, etc.

Here’s a comprehensive guide on how to test void methods using JUnit 5, with or without Mockito.

✅ 1. Test Side Effects Directly (No Mockito)

If the void method changes state or causes observable behavior, assert those outcomes.

🔹 Example: Testing a void method that changes internal state

public class Counter {
    private int count = 0;

    public void increment() {
        count++;
    }

    public int getCount() {
        return count;
    }
}

🧪 JUnit Test

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class CounterTest {

    @Test
    void testIncrement() {
        Counter counter = new Counter();
        counter.increment();

        assertEquals(1, counter.getCount());
    }
}

✅ 2. Use Mockito to Verify Method Calls (void method interactions)

🔹 Example: You want to ensure a void method was called (e.g., logger.log())

public class UserService {
    private Logger logger;

    public UserService(Logger logger) {
        this.logger = logger;
    }

    public void createUser(String name) {
        // Do some user creation logic
        logger.log("User created: " + name); // void method
    }
}

🧪 JUnit + Mockito Test

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

class UserServiceTest {

    @Test
    void testCreateUserLogsMessage() {
        Logger mockLogger = mock(Logger.class);
        UserService service = new UserService(mockLogger);

        service.createUser("Alice");

        verify(mockLogger).log("User created: Alice");
    }
}

✅ 3. Use doNothing() / doThrow() for void methods with side effects

This is useful when mocking void methods that might throw exceptions (e.g., emailService.send()).

doNothing().when(mockEmailService).sendEmail(anyString());

Or:

doThrow(new RuntimeException("SMTP down")).when(mockEmailService).sendEmail("[email protected]");

✅ 4. Capture Arguments Passed to void Methods

Use ArgumentCaptor to check what was passed to a void method.

import org.mockito.ArgumentCaptor;

@Test
void testLogContent() {
    Logger mockLogger = mock(Logger.class);
    UserService service = new UserService(mockLogger);

    service.createUser("Bob");

    ArgumentCaptor<String> captor = ArgumentCaptor.forClass(String.class);
    verify(mockLogger).log(captor.capture());

    assertTrue(captor.getValue().contains("Bob"));
}

📌 Summary