example: Unit Testing Stubs (Mock Services) - NOT using Mockito

ApiService.java

// main package
public interface ApiService {

    void createNewUser(User user);
}

ApiServiceManager.java

// main package
public class ApiServiceManager {

    private final ApiService apiService;

    public ApiServiceManager(ApiService apiService) {
        this.apiService = apiService;
    }

    public void insertNewUser(User user){
        apiService.createNewUser(user);
    }
}

ApiServiceNotifierTest.java

// test package
@DisplayName("Api Service Test")
public class ApiServiceNotifierTest {

    private MockApiServiceDouble apiServiceDouble;

    @BeforeEach
    void init(){
        apiServiceDouble = new MockApiServiceDouble();
    }

    @Test
    @DisplayName("Insert a new user using ApiServiceManager")
    void testInsertNewUser(){
        ApiServiceManager notifier = new ApiServiceManager(apiServiceDouble);

        User user = new User("Mitch", "Mitch@tabian.ca", 29);
        notifier.insertNewUser(user);

        assertEquals(1, apiServiceDouble.createdUsers.size());
        final User expectedUser = apiServiceDouble.createdUsers.get(0);

        assertAll(
                new Executable() {
                    @Override
                    public void execute() throws Throwable {
                        assertEquals("Mitch", expectedUser.getName());
                    }
                },
                new Executable() {
                    @Override
                    public void execute() throws Throwable {
                        assertEquals("Mitch@tabian.ca", expectedUser.getEmail());
                    }
                },
                new Executable() {
                    @Override
                    public void execute() throws Throwable {
                        assertEquals(29, expectedUser.getAge());
                    }
                }
        );

    }
}

MockApiServiceDouble.java

// test package
public class MockApiServiceDouble implements ApiService {

    ArrayList<User> createdUsers = new ArrayList<>();

    @Override
    public void createNewUser(User user) {
        createdUsers.add(user);
    }

}

User.java

// main package
public class User {

    private String name;
    private String email;
    private int age;

    public User(String name, String email, int age) {
        this.name = name;
        this.email = email;
        this.age = age;
    }

    public User() {
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}