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".

Practice

1. What is the main purpose of when().thenReturn() in JUnit testing?

easy

A. To specify a fake return value for a mocked method call

B. To execute the real method instead of a mock

C. To verify that a method was called

D. To create a new instance of the class under test

Solution

1. Step 1: Understand mocking in tests

   Mocking replaces real objects with fake ones to control behavior during tests.

2. Step 2: Role of when().thenReturn()

   This syntax sets a fake return value for a method call on a mock object.

3. Final Answer:

   To specify a fake return value for a mocked method call -> Option A

4. Quick Check:

   when().thenReturn() = fake return value [OK]

Hint: Remember: when().thenReturn() fakes method results [OK]

Common Mistakes:

• Thinking it calls the real method
• Using it on real objects instead of mocks
• Confusing it with verification methods

2. Which of the following is the correct syntax to stub a method getName() of a mock object userMock to return "Alice"?

easy

A. thenReturn("Alice").when(userMock.getName());

B. when(userMock.getName()).thenReturn("Alice");

C. userMock.when(getName()).thenReturn("Alice");

D. when(userMock.getName() == "Alice");

Solution

1. Step 1: Recall correct when().thenReturn() syntax

   The correct order is when(methodCall).thenReturn(value);

2. Step 2: Check each option

   when(userMock.getName()).thenReturn("Alice"); matches the correct syntax exactly; others are invalid or reversed.

3. Final Answer:

   when(userMock.getName()).thenReturn("Alice"); -> Option B

4. Quick Check:

   Correct syntax = when().thenReturn() [OK]

Hint: Syntax is when(methodCall).thenReturn(value) [OK]

Common Mistakes:

• Reversing when() and thenReturn() order
• Calling when() on the mock object instead of method
• Using equality inside when()

3. Given the code below, what will result print?

List<String> mockList = mock(List.class);
when(mockList.size()).thenReturn(5);
int result = mockList.size();
System.out.println(result);

medium

A. 0

B. null

C. 5

D. Throws NullPointerException

Solution

1. Step 1: Understand stubbing of mockList.size()

   The size() method is stubbed to return 5 whenever called on mockList.

2. Step 2: Check the printed result

   Calling mockList.size() returns 5 as stubbed, so result is 5 and prints 5.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   Stubbed method returns set value [OK]

Hint: Stubbed methods return thenReturn() value, not default [OK]

Common Mistakes:

• Assuming default mock return 0 instead of stubbed 5
• Expecting real method execution
• Confusing null with int return

4. Identify the error in the following stubbing code:

MyService service = new MyService();
when(service.getData()).thenReturn("Mocked Data");

medium

A. Cannot stub a method on a real object, must mock it first

B. Missing semicolon after thenReturn()

C. Incorrect method name getData()

D. thenReturn() cannot return a String

Solution

1. Step 1: Check object type used in when()

   The object service is a real instance, not a mock, so stubbing is invalid.

2. Step 2: Understand mocking requirement

   Stubbing works only on mock objects created by mock() or similar methods.

3. Final Answer:

   Cannot stub a method on a real object, must mock it first -> Option A

4. Quick Check:

   Stub only mocks, not real objects [OK]

Hint: Stub only mock objects, not real instances [OK]

Common Mistakes:

• Trying to stub real objects
• Ignoring need to mock before stubbing
• Assuming thenReturn() syntax error

5. You want to stub a method calculate(int x) of a mock calculatorMock to return x * 2 for any integer input. Which is the correct way to do this using when().thenReturn()?

hard

A. when(calculatorMock.calculate(2)).thenReturn(4);

B. when(calculatorMock.calculate(anyInt())).thenReturn(x * 2);

C. when(calculatorMock.calculate(anyInt())).thenReturn(2);

D. when(calculatorMock.calculate(anyInt())).thenAnswer(invocation -> (int) invocation.getArgument(0) * 2);

Solution

1. Step 1: Understand when().thenReturn() limitation

   thenReturn() returns a fixed value, cannot compute based on input parameter.

2. Step 2: Use thenAnswer() for dynamic return

   thenAnswer() allows lambda to compute return based on method arguments.

3. Final Answer:

   when(calculatorMock.calculate(anyInt())).thenAnswer(invocation -> (int) invocation.getArgument(0) * 2); -> Option D

4. Quick Check:

   Dynamic return needs thenAnswer(), not thenReturn() [OK]

Hint: Use thenAnswer() for dynamic returns, not thenReturn() [OK]

Common Mistakes:

• Trying to compute dynamic values with thenReturn()
• Using undefined variable x in thenReturn()
• Stubbing only one input value instead of anyInt()