Why different doubles serve different purposes in JUnit - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a simple stub for a dependency.

JUnit

MyService service = new MyService();
Dependency dep = new [1]();
service.setDependency(dep);

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AMockDependency

BSpyDependency

CStubDependency

DFakeDependency

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing MockDependency which is used for interaction verification.

A stub provides fixed responses for testing without complex behavior.

2fill in blank

medium

Complete the code to verify that a method was called once using a mock.

JUnit

MockedList list = mock(MockedList.class);
list.add("item");
verify(list, [1]).add("item");

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Atimes(1)

BatLeastOnce()

Cnever()

Dtimes(2)

Attempts:

3 left

💡 Hint

Common Mistakes

Using times(2) which expects two calls.

times(1) verifies the method was called exactly once.

3fill in blank

hard

Fix the error in the spy creation code.

JUnit

List<String> list = new ArrayList<>();
List<String> spyList = [1](list);
spyList.add("test");

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Amock

Bfake

Cstub

Dspy

Attempts:

3 left

💡 Hint

Common Mistakes

Using mock() which does not wrap the real object.

spy() wraps a real object to track interactions while keeping behavior.

4fill in blank

hard

Fill both blanks to create a fake that simulates a database connection and returns a fixed value.

JUnit

class FakeDatabase implements Database {
    public String query(String sql) {
        return [1];
    }
}

Database db = new FakeDatabase();
String result = db.query([2]);

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A"fixed result"

B"SELECT * FROM users"

C"dynamic result"

D"DROP TABLE users"

Attempts:

3 left

💡 Hint

Common Mistakes

Returning dynamic result which is not fixed.

Using DROP TABLE which is destructive.

A fake provides a simple implementation returning fixed data. The query string is a typical SQL command.

5fill in blank

hard

Fill all three blanks to create a mock, set expectation, and verify interaction.

JUnit

List<String> mockList = [1](List.class);
when(mockList.get([2])).thenReturn("hello");
mockList.get(0);
verify(mockList).get([3]);

Drag options to blanks, or click blank then click option'

Amock

B0

Dspy

Attempts:

3 left

💡 Hint

Common Mistakes

Using spy() instead of mock().

Mismatching indexes in when() and verify().

mock() creates the mock object. The index 0 is used for both when() and verify() calls.