Question

Let’s say I have a super type (Shape) and two children (like Square, Circle).

I’m trying to make something like this:

Shape shape = someObject.getShape();

if (shape instanceof Square){
    shape = (Square) shape;
    // do this
} else if (shape instanceof Circle) {
    shape = (Circle) shape;
    // do that
}

Also, since getting instances doesn’t seem a correct OO principle, is there another way doing that? Cheers.

edit. I’m not trying to invoke methods on them, I want to pass them to some methods that need their specific subtype.

edit2 Basically I have a collection of type Shape so I can hold both type of subtypes in it. But when it comes to processing at one point, I need to know which of those two subclasses is, so I can pass it to a method that takes the subtype as an argument. I could easily use the supertype as argument but that wouldn’t be secure, since I deal with subtype-specific data.

Answer 1

You need to declare the variable properly, for example:

if (shape instanceof Square){
    Square square = (Square) shape;
    // do this with square
} else if (shape instanceof Circle) {
    Circle circle = (Circle) shape;
    // do that with circle
}