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.