Question

How to declare mixedList with generics for such snapshot without modifying the rest of the code?

List mixedList = new ArrayList();
if(flagA) {
    ClassA a = new ClassA(); //comes from elsewhere
    mixedList.add(a)
} else {
    List<ClassB> bList = new ArrayList<ClassB>();  //comes from elsewhere
    mixedList = bList; //error
}

I can do:

List<Object> mixedList = new ArrayList<Object>();
if(flagA) {
    ...
} else {
    ...
    mixedList.addAll(bList);
}

but is there a way to avoid changing the code?

Answer 1

It’s not safe to assign bList (List<ClassB>) to mixedList (List<Object>).

The service from which you obtained bList might retain a reference to it; this service will assume its list contains only ClassB instances. If you were allowed to assign that list to a List<Object> reference, you could then add any type of object to the list without a warning. But when the service, thinking that every element in its list was a ClassB, attempted to access the elements, a ClassCastException would be raised.

Creating a new List<Object>, and adding elements to it with add() or addAll(), prevents this “type pollution”. You can safely modify this copy of the list, and let the source of the list keep its own copy “pure.”