Constructor Injection

1_UserService.java

public class UserService {

	private UserDAO userDAO;

	public UserService(UserDAO userDAO) {
		this.userDAO = userDAO;
	}

	public User signIn(Email email, Password password) {
		User user;

		user = userDAO.findUserBy(email);
		if (user.isEmpty()) {
			throw new RuntimeException("user doesn't have permissions");
		}
		// do something
		return user;
	}
}

2_ProductionCode.java

UserService userService = new UserService(new UserDAOOracle());
userService.signIn(email, password);

3_TestCode.java

@Test(expected = RuntimeException.class)
public void should_throw_exception_when_user_wasnt_found() throws Exception {
	FakeUserDAO fakeUserDAO = new FakeUserDAO();

	UserService userService = new UserService(fakeUserDAO);
	userService.signIn(email, password);
}