Why Choosing the right test double in JUnit? - Purpose & Use Cases

The Big Idea

What if you could test complex parts without waiting or risking real failures?

The Scenario

Imagine testing a car's navigation system by manually driving through every possible route to check if directions are correct.

The Problem

This manual testing is slow, tiring, and prone to mistakes. You might miss some routes or get inconsistent results because of traffic or weather changes.

The Solution

Using the right test double lets you simulate parts of the system, like fake GPS signals or mocked traffic data, so you can test navigation logic quickly and reliably without real driving.

Before vs After

✗ Before

RealGPS gps = new RealGPS();
Navigation nav = new Navigation(gps);
nav.start(); // drives real routes

✓ After

MockGPS gps = new MockGPS();
Navigation nav = new Navigation(gps);
nav.start(); // simulates routes instantly

What It Enables

It enables fast, repeatable, and controlled tests that catch bugs early without relying on real-world conditions.

Real Life Example

When testing a payment system, using a stub for the bank API lets you simulate approvals or declines instantly, avoiding costly real transactions.

Key Takeaways

Manual testing of complex parts is slow and unreliable.

Choosing the right test double simulates dependencies effectively.

This leads to faster, safer, and more accurate tests.

Practice

1. Which test double should you use in JUnit when you want to replace a real object and verify that certain methods were called during the test?

easy

A. Mock

B. Stub

C. Fake

D. Spy

Solution

1. Step 1: Understand the purpose of mocks

   Mocks are used to fake behavior and verify interactions, such as method calls.

2. Step 2: Compare with other test doubles

   Stubs provide fixed responses, fakes have working implementations, spies record calls but mocks are designed for verification.

3. Final Answer:

   Mock -> Option A

4. Quick Check:

   Verify calls = Mock [OK]

Hint: Mocks verify calls; stubs just return data [OK]

Common Mistakes:

• Confusing stubs with mocks
• Using fake when verification is needed
• Thinking spies are the same as mocks

2. Which of the following is the correct way to create a stub in JUnit using Mockito?

easy

A. spy(new Service());

B. when(service.getData()).thenReturn("data");

C. mock(Service.class);

D. verify(service).getData();

Solution

1. Step 1: Identify stub creation syntax

   Stubs return fixed data; in Mockito, use when(...).thenReturn(...) to stub a method.

2. Step 2: Check other options

   Verify checks calls, mock creates mocks, spy wraps real objects; only when().thenReturn() stubs.

3. Final Answer:

   when(service.getData()).thenReturn("data"); -> Option B

4. Quick Check:

   Stub method = when().thenReturn() [OK]

Hint: Stubs use when().thenReturn() syntax [OK]

Common Mistakes:

• Using verify() to stub methods
• Confusing mock() with stub setup
• Using spy() instead of stub

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

List<String> list = mock(List.class);
when(list.size()).thenReturn(5);
assertEquals(5, list.size());

medium

A. Test passes because size() returns 5

B. Test fails because size() returns 0 by default

C. Test throws NullPointerException

D. Test fails due to syntax error

Solution

1. Step 1: Analyze mock behavior

   The list is mocked, and size() is stubbed to return 5.

2. Step 2: Check assertion

   assertEquals(5, list.size()) compares 5 with stubbed return 5, so it passes.

3. Final Answer:

   Test passes because size() returns 5 -> Option A

4. Quick Check:

   Stubbed method returns set value [OK]

Hint: Stubbed methods return set values, so assertion passes [OK]

Common Mistakes:

• Assuming default mock returns 0
• Expecting real method call on mock
• Confusing mock with spy behavior

4. Identify the error in this JUnit test using Mockito:

@Test
void testServiceCall() {
  Service service = mock(Service.class);
  service.process();
  verify(service.process());
}

medium

A. Mock creation syntax is wrong

B. Missing stub for process() method

C. process() method cannot be mocked

D. Incorrect use of verify; should be verify(service).process();

Solution

1. Step 1: Check verify syntax

   verify() expects the mock object, then method call separately: verify(service).process();

2. Step 2: Identify error in code

   Current code calls verify(service.process()), which is incorrect syntax and causes compile error.

3. Final Answer:

   Incorrect use of verify; should be verify(service).process(); -> Option D

4. Quick Check:

   verify syntax = verify(mock).method() [OK]

Hint: verify() needs mock object, then method call separately [OK]

Common Mistakes:

• Calling verify with method call inside
• Forgetting to mock the object
• Confusing stub and verify usage

5. You want to test a payment processing system. The payment gateway is an external service that charges real money. Which test double should you use in your JUnit tests to avoid real charges but still verify that the payment method was called correctly?

hard

A. Use a stub to return a fixed success response

B. Use a fake with a simple in-memory implementation

C. Use a mock to verify the payment method call

D. Use a spy to wrap the real payment gateway

Solution

1. Step 1: Understand test goal

   We want to avoid real charges but verify the payment method was called.

2. Step 2: Choose test double type

   Mocks fake behavior and verify calls, perfect for checking method calls without real effects.

3. Step 3: Compare other options

   Stubs return fixed data but don't verify calls; fakes have real logic; spies call real methods, risking real charges.

4. Final Answer:

   Use a mock to verify the payment method call -> Option C

5. Quick Check:

   Verify calls without real effect = Mock [OK]

Hint: Mocks verify calls without real side effects [OK]

Common Mistakes:

• Using spy risks real external calls
• Using stub misses call verification
• Choosing fake adds unnecessary complexity