Question

A class C has a void method m with no parameters. Another class D extends C and overrides m. Each class has a constructor with no parameters. In each of the following, say whether it is legal, and if so, which definition of m will be used.

i) C x = new D();
x.m();

ii) D x = new C();
x.m();

I think i is legal, and ii is not illegal. Not sure how I get my head around this question, any hints are welcome.

Answer 1

The best way to answer the question is to write some code and see what happens. Use System.out.println("method called from C"); in your implementation of m to tell which implementation is called. Having said that, the whole point of overriding a method is so that the new implementation will get used. If you object is of type C then Cs method will get called. If you object is of type D then Ds method will get called regardless of what type the reference is.

The first answer:

C x = new D();

is legal because and object of type D is a C as well (because D extends C).

The second answer:

D x = new C();

is not legal because a reference to D cannot hold an object of its supertype C.