Spy objects in JUnit - Test Execution Trace

Test Overview

This test uses a spy object to verify that a method is called with the correct argument and that the method returns the expected value.

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.ArrayList;
import java.util.List;

public class SpyTest {

    @Test
    public void testSpyOnList() {
        List<String> list = spy(new ArrayList<>());

        doReturn("hello").when(list).get(0);

        String result = list.get(0);

        verify(list).get(0);
        assertEquals("hello", result);
    }
}

Execution Trace - 7 Steps

Step	Action	System State	Assertion	Result
1	Test starts	JUnit test runner initialized	-	PASS
2	Create spy object of List interface	Spy object wrapping List.class created	-	PASS
3	Stub get(0) method to return "hello"	Spy configured to return "hello" when get(0) is called	-	PASS
4	Call get(0) on spy object	Method get(0) invoked on spy	Return value is "hello"	PASS
5	Verify get(0) was called on spy	Mockito verifies method invocation	get(0) was called exactly once	PASS
6	Assert returned value equals "hello"	JUnit assertion checks equality	result equals "hello"	PASS
7	Test ends	Test completes successfully	-	PASS

Failure Scenario

Failing Condition: The spy's get(0) method is not stubbed or returns a different value

Execution Trace Quiz - 3 Questions

What does the spy object do in this test?

A It wraps the real List and allows method call verification

B It creates a completely fake List with no real behavior

C It automatically returns null for all methods

D It prevents any method calls on the List

Key Result

Using spy objects lets you check if methods are called and control their return values, which helps test interactions without losing real behavior.