I am pretty new to Mockito and Junit 5.
I am trying to mock out the Config class’s getNumber() method so that it returns -1.

Here is a basic outline:

The class I need to mock:

class Config {
    private final int number;

    Config(int number) {
        this.number = number;
    }


    public int getNumber() {
        return number;
    }
}

The unit under test is the MyService constructor:

class MyService {
    private final Config config;


    MyService(Config config) {
        if(config.getNumber() < 0) {
            throw new IllegalStateException("Number cannot be less than 1!");
        }
        this.config = config;
    }
}

I want to test the MyService constructor when I pass it a mock Config object.
But it needs access to the getNumber() method (which I need to set the return value for).

However, since the MyService number field is private final, getNumber() returns 0 instead of -1.

My test code:

@Test
void MyService_construct() {
    Config mockConfig = Mockito.mock(Config.class);
    Mockito.when(mockConfig.getNumber()).thenReturn(-1);
    Field numberField = Config.class.getDeclaredField("number");
    numberField.setAccessible(true);
    numberField.set(mockConfig, -1);
    
    assertThrows(Exception.class, () -> MyService(mockConfig));
}

How can I mock out the behavior of Config.getNumber() such that it returns -1? Keep in mind that the number private final field is normally set in the constructor by reading a file – hence the problem with mocking the class.