Question

I have an external API (I can’t modify it) with class “A” and local class “B” which overrides methods of “A” and adds an additional function.
I need to use one of them according to some parameter “is_A”.

/—— API (A.java) —–/

package A;

public class A {

    public int pingA( int value ) {

        return value;

    }

}

/—— my class (B.java) —–/

package B;

import A.*;

public class B extends A {

    @Override
    public int pingA( int value ) {
        return value;
    }

    public int pingB( int value ) {
        return value;
    }

    public static void main(String[] args) {
        final boolean is_A = false;
        A obj;
        if (is_A) {
            obj = new A();
        } else {
            obj = new B();
        }
        if (!is_A) {
            int n = obj.pingB(3);
        }
    }
}

In this case I want to use class “B”, but the command “int n = obj.pingB(3);” is not compiled because there is no method pingB in A.
The exact message is:

cannot find symbol
symbol: method pingB(int)
location: class A.A

Answer 1

You need to cast obj back to B.

if (!is_A) {
    int n = ((B) obj).pingB(3);
}

You can by the way better use the instanceof keyword instead of is_A.

if (obj instanceof B) {
    int n = ((B) obj).pingB(3);
}