when().thenReturn() stubbing in JUnit - Test Execution Trace

Test Overview

This test uses Mockito's when().thenReturn() to stub a method call. It verifies that the stubbed method returns the expected value when called.

Test Code - JUnit with Mockito

JUnit

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

import org.junit.jupiter.api.Test;
import java.util.List;

public class ListStubTest {

    @Test
    public void testWhenThenReturnStubbing() {
        // Create mock
        List<String> mockedList = mock(List.class);

        // Stub method
        when(mockedList.get(0)).thenReturn("first element");

        // Call stubbed method
        String result = mockedList.get(0);

        // Verify returned value
        assertEquals("first element", result);
    }
}

Execution Trace - 4 Steps

Step	Action	System State	Assertion	Result
1	Create mock object of List interface using Mockito.mock()	mockedList is a mock List object with no behavior defined yet	—	PASS
2	Stub the get(0) method of mockedList to return "first element" using when().thenReturn()	mockedList.get(0) is now stubbed to return "first element"	—	PASS
3	Call mockedList.get(0) to get the stubbed value	mockedList.get(0) returns "first element" as stubbed	Check that returned value equals "first element"	PASS
4	Assert that the returned value is "first element" using assertEquals	Assertion compares expected "first element" with actual result	assertEquals("first element", result) passes	PASS

Failure Scenario

Failing Condition: The stub was not set correctly or method called with different argument

Execution Trace Quiz - 3 Questions

What does when(mockedList.get(0)).thenReturn("first element") do?

A It calls the real get(0) method on the list

B It tells the mock to return "first element" when get(0) is called

C It verifies that get(0) was called

D It throws an exception when get(0) is called

Key Result

Use when().thenReturn() to define exact return values for mock methods. This helps isolate tests by controlling dependencies' behavior.

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()