How to convert ArrayList<subClass> to ArrayList<super> in java

Question

I hava a method setData(ArrayList<super> list), so anyone who want set data can have their own entity, just need to extends from super class. but when they call setData, how they convert ArrayList to ArrayList?

Now i use setData(new ArrayList(list)), but it's not a good solution.

Anyone can tell me how to convert or another solution can avoid the problem. Very appreciated!

Answer 1

It depends on what you want to do with the list afterwards. If you're just going to read from it, you can declare your method like this:

setData(ArrayList<? extends SuperType> list)

You can't add anything to that list, though, because you won't know what ? is. Another option is to create a copy when calling your method:

List<SubType> subtypeList = ...;
setData(new ArrayList<SuperType>(subtypeList));

Answer 2

Instead of coding

setData(new ArrayList(list));

you could simply use

setData((ArrayList<super>)((Object)list));

Casting list to Object first and then to ArrayList will force the compiler to accept the method invocation. As an added bonus, you don't suffer the performance expense of creating a new array list.