If you overwrite a field in a subclass of a class, the subclass has two fields with the same name(and different type)?

I have 3 classes:

public class Alpha {
    public Number number;
}

public class Beta extends Alpha {
    public String number;
}

public class Gama extends Beta {
    public int number;
}

Why does the following code compile? And, why does the test pass without any runtime errors?

@Test
public void test() {
    final Beta a = new Gama();
    a.number = "its a string";
    ((Alpha) a).number = 13;
    ((Gama) a).number = 42;

    assertEquals("its a string", a.number);
    assertEquals(13, ((Alpha) a).number);
    assertEquals(42, ((Gama) a).number);
}

Member variables cannot be overridden like methods. The number variables in your classes Beta and Gama are hiding (not overriding) the member variable number of the superclass.

By casting you can access the hidden member in the superclass.