Question

I’m creating some random classes to understand Polymorphism better.
Coding is as follows:

Poly1:

public abstract class Poly1 {
    int comPoly;
}

SubPoly1:

public class SubPoly1 extends Poly1 {
    String testPoly;
}

SubPoly2:

public class SubPoly2 extends Poly1 {
    int x;
}

testPoly:

public class testPoly {
public static void main(String[] args) {
    Poly1[] testObj = new Poly1[2];
    testObj[0] = new SubPoly1();
    testObj[1] = new SubPoly2();
    testObj[1].x = 1;
    testObj[1].comPoly = 2;
    System.out.println("Test Output : " + testObj[1].x+ " and " + testObj[1].comPoly);
    testObj[0].testPoly = "Hello";
    testObj[0].comPoly = 8;
    System.out.println("Test Output : " + testObj[0].testPoly+ " and " + testObj[1].comPoly);
   }
}

But the program is not getting past the compilation stage as I get the symbol not found error whenever I try to access the variables from SubPoly1 or SubPoly2(for example testObj[1].x would return an error).

Any help would be appreciated.

Answer 1

That’s because you’ve declared testObj to be Poly1[] while x is not definied in Poly1, but in SubPoly2. On a Poly1 reference you can only access comPoly.

To fix this, you need to move x to Poly1 or to use SubPoly2[] instead or to cast a Poly1 reference to SubPoly2 (which is only possible when the instance is actually a SubPoly2). Which one the best solution is depends on the functional requirements.

See also:

• Inheritance tutorial