How to avoid downcasting when using a list of subclass items in Java

Question

I have an abstract class A and classes B and C. Classes B and C extend A. I need to hold B and C in the list together, say List. However, B and C have unique methods, so not all items in List can be used to invoke certain methods unless I am downcasting which is considered to be a design smell.

I need to keep B and C on the same list because I want to sort them based on their shared attributes which I can. Is keeping them on the same list of their parent type and then downcasting a bad design in this situation?

Answer 1

I need to keep B and C on the same list because I want to sort them based on their shared attributes which I can.

As long as the requirement is to keep the list sorted with mixed types B, C both extending A an to call their non-inherited methods, you have no choice than using the List<A> containing all types.

Although I also try to avoid downcasting when possible, it doesn't mean there are situations when it is neither useful nor necessary.

for (A item: sortedList) {
    if (item instanceof B) {
        Whatever fieldB = ((B) item).getFieldB();  // using non-inherited method of B
    } else if (item instanceof C) {
         Whatever fieldC = ((C) item).getFieldC();  // using non-inherited method of C
    } else {
         // either only A or anything different that extends A
    }
}

As of Java 14 thanks to JEP 305: Pattern Matching for instanceof, such thing gets less verbose:

for (A item: sortedList) {
    if (item instanceof B b) {
        Whatever fieldB = b.getFieldB();  // using non-inherited method of B
    } else if (item instanceof C c) {
        Whatever fieldC = c.getFieldC();  // using non-inherited method of C
    } else {
        // either only A or anything different that extends A
    }
}