Refactoring huge if ( ... instanceof ...)

Question

I'm currently trying to refactor a part of a project that looks like this:

Many classes

 B extends A; C extends A; D extends C; E extends B; F extends A; ...

And somewhere in the code:

if (x instanceof B){
 B n = (B) x;
 ...
}else if (x instanceof C){
 C n = (C) x;
 ...
}else if (x instanceof D){
 D n = (D) x;
 ...
}else if (x instanceof E){
 E n = (E) x;
 ...
}else if (x instanceof G){
 G n = (G) x;
 ...
}...

Above if-construct currently sits in a function with a CC of 19. Now my question is: Can I split this if-construct up in mutliple functions and let Java's OO do the magic? Or are there any catches I have to look out for?

My idea:

private void oopMagic(C obj){ ... Do the stuff from the if(x instanceof C) here}
private void oopMagic(D obj){ ... Do the stuff from the if(x instanceof D) here}
private void oopMagic(E obj){ ... Do the stuff from the if(x instanceof E) here}
....

and instead of the huge if:

oopMagic(x);

Edit: I cannot change any of the classes (A,B,C,...). Inside the if statements some getters are used to read (never write) data from each object.

Answer 1

That won't fly. instanceof detects the runtime type of a variable. Polymorphism (select method by signature) depends on the compile-time type of a variable. That is, you'll always get oopMagic(A obj).

As Roger suggested, take a look at the visitor pattern, a.k.a double-dispatch.

Answer 2

It's hard to tell what would work best since you haven't given any indication what the code is doing, but another option is to add an oopMagic() method to A and override as necessary.