Java inheritance and Scope of variable

Question

The Program is:

class A
{
   int i = 10;
}
class B extends A
{
   int j = 20;
}
class C extends B
{
   int k = 30;
}
class D extends C
{
   int m = 40;
}

public class asg2
{
   public static void main(String[] args)
   {
       A[] a = {new A(),new B(), new C(), new D()};

    System.out.println(a[3].i); //No error!!!
    System.out.println(a[2].j); //throws error 
    System.out.println(a[1].k); //throws error (Understood why error)
    System.out.println(a[0].m); //throws error (Understood why error)
}

}

I understand why the last two throw error. But I dont understand why the 2nd print statement throws error. And first one runs smoothly.

asg2.java:29: error: cannot find symbol         
System.out.println(a[2].j);                                    
                       ^
symbol:   variable j                    
location: class A

M A · Accepted Answer · 2015-06-22 20:59:55Z

5

The compiler does not see the element a[2] of type C. It sees it of type A because that's the type of the declared array. Hence it cannot accept accessing a field that belongs to a subclass of A. If you cast the element to C, the compile will accept it:

System.out.println(((C) a[2]).j); // compiles OK

answered Jun 22, 2015 at 20:59

M A

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Comments

khelwood · Accepted Answer · 2015-06-22 21:00:05Z

2

Every entry in your array a is of type A. It might actually be holding an instance of B or C or D, but the variable is of type A, because that is how your array was declared. So you can't access fields that A doesn't have (unless you cast to another type, telling the compiler explicitly what type you think the object is).

answered Jun 22, 2015 at 21:00

khelwood

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