Basic Java inheritance exercise

Question

I'm learning Java and I'm doing this exercise and I have to say what the main method prints. Its goal is to better understand Java inheritance.

interface X extends Remote{
    A m(B bb) throws RemoteException;
}

class A implements Serializable{
     int a;
     A m(A aa){ 
         aa.a = 3; 
         System.out.println("A.m"); 
         return aa;
     }
}

class B extends A{
     int b;
     A m(B bb){
        bb.b = 7; 
        System.out.println("B.m");
        return bb;
     }
}

class C extends A implements X{
    public B m(B bb){
        if(bb.b == 7) bb.b = 9; else bb.b = 1;
        System.out.println("C.m");
        return bb;
    }
}

Now I have a main method in which I call:

X x = new C();
B b = new B();
(x.m(b)).m(b);   // prints C.m() A.m()

The third line of my main method, prints "C.m() A.m()" but I don't understand why it prints A.m(). The x.m(b) returns an object that has both static and dynamic type == B; on this object it is invoked m(b) method; so why it is not called the m() method of B class?

I've seen that the m() mehtod in B class is not an overriding of the m() method in A class because they have different explicit parameter.

thanks in advance

I would start from avoiding naming things as A,a,aa, m(A aa), a.m, aa.a. It just makes code harder to read. — Marcin Szymczak
– Marcin Szymczak, Commented Feb 6, 2015 at 15:24
You are right and I totally agree with you. It's an exercise taken from a book, it's only to explore inheritance. Any ideas about why it prints C.m() A.m() instead of C.m() B.m() as I supposed? Thanks m8 — Roberto Milani
– Roberto Milani, Commented Feb 6, 2015 at 15:33

njzk2 · Accepted Answer · 2015-02-06 15:32:49Z

5

b in an instance of B, which extends A. As such, B has 2 m methods.

To know which is called, what is important is the type of the object on which the method is called.

(x.m(b))

is the result of the call to the m method from the X interface, because the declaring type of x is X.

Therefore (x.m(b)) is an object of type A (even though the actual implementation is a B).

This being a type A, the m method from the A class is called.

If you change the names of the methods, it will become clearer that the m methods from A and B are really different objects.

Your mistake is assuming that

The x.m(b) returns an object that has both static and dynamic type == B;

Which is wrong, because x is declared of type X, making the result of x.m a type A.

answered Feb 6, 2015 at 15:32

njzk2

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1 Comment

njzk2 Over a year ago

another important point is that the method in B A m(B bb){ does not override the method in A A m(A aa){ because the argument is more specific, meaning it cannot answer all calls of the method in A. (But I think you already understood that part).

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