Java, use correct constructor based on instance

Question

I need my Class C to choose correct constructor based on instance variable. I have basic code shown below. Whenever I create instance of an class B and store it as reference to A. 'Wrong' constructor is used on class C. What are my options to change this if I don't want to use (b instanceOf B) because it is in other package.

Class A {
}

Class B extends A {
}


Class C {
    C(A a){...}
    C(B b){...}
}

Class Main{
    private createClass(String s){
        if (...){
            return new B();
        }
    }

    public static void main(String[] args){
        A b = createClass("s");
        new C(b); //--> constructor C(A a) is used but i need C(B b)
    }
}

Answer 1

new C(A a) is called, because the b variable is of type A at compile-time. The compiler doesn't know that at Runtime it will hold reference to an instance of B and that's why it binds to the new C(A a) constructor.

In general, I think your should reconsider your design, but if you want to keep it like this, you could at least make the createClass() method Generic and pass the Class<T> of the resuling type:

private <T extends A> T createClass(String s, Class<T> clazz){
    if (...) {
        return clazz.newInstance();
    }
    //
}

This will allow you to point out (and easily switch) the type of the result you need:

B b = createClass("s", B.class);
new C(b);