Is using .getClass() regarded as a bad design?

Question

I am currently implementing a function, using the superclass as a parameter.

For example:

private void foo(Parent parent) {
    if(parent.getClass() == Child1.class) {
        Child1 c1 = (Child1) parent;
        System.out.println(c1.getChild1Attribute());
    }
    else if(parent.getClass() == Child2.class) {
        Child2 c2 = (Child2) parent;
        System.out.println(c1.getChild2Attribute());
    }
    else if(parent.getClass() == Parent.class) {
        System.out.println(parent.getParentAttribute());
    }
}

Is this a bad idea?

I've read some threads here saying that using getClass() or instanceof is bad design:

• why super.getClass() in a subclass returns subclass name
• How to determine an object's class (in Java)?

Answer 1

It is not necessarily a bad design, but it is an indication that something wrong may be going on.

Your specific case looks bad, because it appears that a single method is aware of multiple classes. This could be an indication that you are missing an overload possibility, or an opportunity to use one of the multiple dispatch patterns:

// Three overloads - one per target class
private void foo(Parent obj) {
}
private void foo(Child1 obj) {
}
private void foo(Child2 obj) {
}

One of the common multiple dispatch patterns is visitor pattern, see if it is applicable to the problem you're solving.

Answer 2

Yes, it is bad design.

Instead, you should make a single abstract method in the superclass and override it in each subclass to perform the desired action.