Question

I ran into this example about inheritances on the web, and I am not easy regarding its results. I’m missing something critical.

public class A {

int i = 5;

public A() {
    foo();
}

private void foo() {
    System.out.println(i);

}}

public class B extends A {
int i = 6;}

public class C extends B {
int i = 7;

public void foo() {
    System.out.println("C's foo");
    System.out.println(super.i);
}}

I’m trying to figure out what’s happening by the following command: C c = new C();
System.out.println(C.i);

I know that when we create a new instance of C we approach A’s and B’s constructures, So We reach to A() – (question 1) Does i (of A) being initialize on the way?
Now we need to call to foo()– (question 2)- Does C’s foo() consider as an override of A’s foo()? and what if B had a foo() of itself? then it was consider as a override and C’s foo() was operated?

As far as I know, there is no override when it relates to local variables. How come that System.out.println(c.i) is 7 and not 5? shouldn’t it b the i of the highest-father?

EDIT: My question is not about which foo and i will be used when I use c, is about what happens during these two specific commands, cause obviously A’s foo was used and not C’s.

Thank you very much.

Answer 1

The three i variables here are completely independent. Which one is used by any statement is determined at compile-time – there’s no polymorphism involved. So A.foo() will always print out the value of the variable declared in A.

Note that these aren’t local variables by the way – they’re instance variables.

When you print out c.i that uses the variable declared in C because the compile-time type of c is C. You can see this if you write:

C c = new C();
A a = c;
B b = c;
System.out.println(a.i); // 5 - variable declared in A
System.out.println(b.i); // 6 - variable declared in B
System.out.println(c.i); // 7 - variable declared in C

Note that in a well-written program this sort of thing almost never causes a problem, as variables should be private.