Your final code block is correct and is the standard practice, you don't need a constructor in this test class for the Calculator instance.
For the test class you should define the fields and initialize them in a @BeforeAll or @BeforeEach method.
Using a constructor for a test class in JUnit is reserved for parameterized unit tests, which is why you are getting an error stating that you're missing a ParameterResolver. This would be an example where you would use a constructor for a parameterized test:
public class Calculator { public int subtract(int a, int b) { return a - b; }}@RequiredArgsConstructor@RunWith(Parameterized.class)class CalculatorTest { private final int x; private final int y; private final int z; private Calculator calculator; // Constructor is generated by lombok with @RequiredArgsConstructor // and accepts three parameters, x, y, and z @BeforeEach void setUp() { calculator = new Calculator(); } @Test void testSubtract() { assertEquals(z, calculator.subtract(x, y)); } /** * This method will pass each parameter into the constructor * of this test class. In this case, the testSubtract method * will be ran 4 times with each set of parameters. */ @Parameterized.Parameters static Collection parameters() { return Arrays.asList( new Object[][] { {5, 3, 2}, {10, 1, 9}, {120, 40, 80}, {1, 1, 0} } ); }}You can find additional details about Parameterized Unit Testing here for JUnit 4 and here for JUnit 5.