when().thenReturn() stubbing in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of when().thenReturn() stubbed method call

Given the following JUnit test code using Mockito, what will be the output of the test assertion?

JUnit

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

interface Calculator {
    int add(int a, int b);
}

public class CalculatorTest {
    @Test
    void testAdd() {
        Calculator calc = mock(Calculator.class);
        when(calc.add(2, 3)).thenReturn(10);
        int result = calc.add(2, 3);
        assertEquals(10, result);
    }
}

A Test fails because the stub returns 5 for add(2, 3)

B Test passes because the stub returns 10 for add(2, 3)

C Test fails with NullPointerException because add is not implemented

D Test fails because the stub returns 0 for add(2, 3)

💡 Hint

Remember that when().thenReturn() sets the return value for the mocked method call.

The stub sets add(2, 3) to return 10, so the assertion assertEquals(10, result) passes.

❓ assertion

intermediate

Correct assertion for stubbed method return

You stub a method call with when().thenReturn() to return "Hello". Which assertion correctly verifies this behavior?

JUnit

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

interface Greeter {
    String greet();
}

public class GreeterTest {
    @Test
    void testGreet() {
        Greeter greeter = mock(Greeter.class);
        when(greeter.greet()).thenReturn("Hello");
        String message = greeter.greet();
        // Which assertion is correct here?
    }
}

A assertEquals("Hello", message);

B assertTrue(message == "Hello");

C assertNull(message);

D assertFalse(message.equals("Hello"));

💡 Hint

Use assertEquals to compare string values in tests.

assertEquals("Hello", message) correctly checks that the stubbed method returns "Hello".

🔧 Debug

advanced

Identify the error in stubbing with when().thenReturn()

What is wrong with the following stubbing code snippet?

JUnit

import static org.mockito.ArgumentMatchers.anyInt;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;

import java.util.List;

List<String> list = mock(List.class);
when(list.get(0)).thenReturn("first");
when(list.get(anyInt())).thenReturn("any");
String result = list.get(0);

A The code returns null because mocks do not support stubbing

B The code throws a NullPointerException because of anyInt() usage

C The second when() overrides the first, so list.get(0) returns "any" instead of "first"

D The code causes a compile error because thenReturn() is missing a semicolon

💡 Hint

Consider the order and specificity of stubbing calls.

The second stub with anyInt() matches all indices including 0, overriding the first stub. So list.get(0) returns "any".

🧠 Conceptual

advanced

Behavior of unstubbed method calls on mocks

If you create a mock object and stub only one method with when().thenReturn(), what happens when you call another method on the mock that was not stubbed?

A The unstubbed method returns the default value for its return type (e.g., null, 0, false)

B The unstubbed method throws an exception immediately

C The unstubbed method returns the last stubbed value

D The unstubbed method causes the test to fail automatically

💡 Hint

Think about how Mockito handles unstubbed method calls by default.

By default, unstubbed methods on mocks return default values: null for objects, 0 for numbers, false for booleans.

❓ framework

expert

Correct usage of when().thenReturn() with chained method calls

Given a mock object with a method that returns another mock, how do you correctly stub a chained method call using when().thenReturn()?

JUnit

interface Service {
    Repository getRepository();
}
interface Repository {
    String findName();
}

Service service = mock(Service.class);
Repository repo = mock(Repository.class);
// How to stub service.getRepository().findName() to return "Data"?

A when(service.getRepository().findName()).thenReturn("Data");

B

when(service.getRepository()).thenReturn(null);
when(repo.findName()).thenReturn("Data");

C

when(service.getRepository().findName()).thenReturn(null);
when(repo.findName()).thenReturn("Data");

D

when(service.getRepository()).thenReturn(repo);
when(repo.findName()).thenReturn("Data");

💡 Hint

You must stub each method in the chain separately if the intermediate method returns a mock.

You first stub service.getRepository() to return the mock repo, then stub repo.findName() to return "Data".