doReturn and doThrow in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of doReturn with Mockito

What will be the output of the following JUnit test using Mockito's doReturn?

JUnit

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

class Service {
    String getData() { return "real"; }
}

public class ServiceTest {
    @Test
    void testDoReturn() {
        Service mockService = mock(Service.class);
        doReturn("mocked").when(mockService).getData();
        String result = mockService.getData();
        assertEquals("mocked", result);
        System.out.println(result);
    }
}

A Throws NullPointerException

B real

C null

D mocked

💡 Hint

doReturn sets the return value for the mocked method call.

The doReturn method in Mockito sets the return value of the mocked method getData() to "mocked". So when mockService.getData() is called, it returns "mocked" instead of the real method's "real".

❓ Predict Output

intermediate

Effect of doThrow in Mockito

What happens when the following JUnit test runs with Mockito's doThrow?

JUnit

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

class Service {
    void perform() {}
}

public class ServiceTest {
    @Test
    void testDoThrow() {
        Service mockService = mock(Service.class);
        doThrow(new IllegalStateException("fail")).when(mockService).perform();
        assertThrows(IllegalStateException.class, () -> mockService.perform());
    }
}

A Test passes because perform() throws IllegalStateException

B Test fails because perform() returns normally

C Test fails with NullPointerException

D Test fails with AssertionError due to wrong exception

💡 Hint

doThrow makes the mocked method throw the specified exception.

The doThrow method configures the mock to throw an IllegalStateException when perform() is called. The test expects this exception, so it passes.

❓ assertion

advanced

Correct assertion for doThrow exception message

Which assertion correctly verifies that a mocked method throws an exception with message "error occurred" using doThrow?

JUnit

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

class Service {
    void execute() {}
}

public class ServiceTest {
    @Test
    void testExceptionMessage() {
        Service mockService = mock(Service.class);
        doThrow(new RuntimeException("error occurred")).when(mockService).execute();
        // Which assertion below is correct?
    }
}

A assertTrue(() -> mockService.execute().equals("error occurred"));

B RuntimeException ex = assertThrows(RuntimeException.class, () -> mockService.execute()); assertEquals("error occurred", ex.getMessage());

C assertThrows(RuntimeException.class, () -> mockService.execute(), "error occurred");

D assertThrowsMessage(RuntimeException.class, "error occurred", () -> mockService.execute());

💡 Hint

Use assertThrows to capture the exception and then check its message.

Option B correctly captures the exception thrown by execute() and asserts that its message matches "error occurred". Options B and C use non-existent or incorrect assertion signatures. Option B is invalid because execute() returns void and cannot be compared to a string.

🔧 Debug

advanced

Why does doReturn fail with spy on final method?

Given a spy on a class with a final method, why does the following code throw an exception?

JUnit

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

class FinalClass {
    final String finalMethod() { return "real"; }
}

public class SpyTest {
    @Test
    void testDoReturnOnFinal() {
        FinalClass spy = spy(new FinalClass());
        doReturn("mocked").when(spy).finalMethod();
        String result = spy.finalMethod();
    }
}

A The spy object is null, causing NullPointerException.

B doReturn requires the method to be static, finalMethod is not static.

C Mockito cannot mock or stub final methods by default, so doReturn fails.

D doReturn only works with interfaces, not classes.

💡 Hint

Mockito has limitations with final methods unless configured.

By default, Mockito cannot mock or stub final methods because of JVM restrictions. Attempting to use doReturn on a final method causes an exception. To mock final methods, additional configuration or tools are needed.

🧠 Conceptual

expert

Difference between doReturn().when() and when().thenReturn()

Which statement correctly explains the difference between doReturn().when() and when().thenReturn() in Mockito?

A <code>doReturn().when()</code> can stub methods on spies without calling the real method, while <code>when().thenReturn()</code> calls the real method on spies during stubbing.

B <code>when().thenReturn()</code> is used only for void methods, <code>doReturn().when()</code> is for non-void methods.

C <code>doReturn().when()</code> is deprecated and should not be used; <code>when().thenReturn()</code> is the modern replacement.

D Both behave identically in all cases; the difference is only stylistic.

💡 Hint

Consider how spies behave when stubbing methods.

when().thenReturn() calls the real method when used on spies, which can cause side effects or exceptions. doReturn().when() does not call the real method during stubbing, making it safer for spies.