Let's assume, there is a method that returns an Object, but we know it is some sort of typed List (e.g. LinkedList<Long> or ArrayList<String>):
public abstract Object open(); Directly using its return value
List<Long> result = (List<Long>)dialog.open(); will cause a warning such as:
Type safety: Unchecked cast from Object to List
To overcome this problem, we can cast to List<?> and then check each list item individually:
List<?> result = (List<?>)dialog.open(); List<Long> safeResult = new LinkedList<Long>(); for (Object obj: result) { if (obj instanceof Long) { safeResult.add((Long)obj); } } Now I thought, wouldn't it be nice to have a generic method that can cast an Object (that is known to contain a List) to any typed instance of List.
This is the closest I could get:
private <T> List<T> doSafeCast(Object listObject, Class<T> type) { List<T> result = new LinkedList<T>(); if (listObject instanceof List) { List<?> list = (List<?>)listObject; for (Object obj: list) { if (type.isInstance(obj)) { result.add(type.cast(obj)); } } } return result; } Now I can use it like this:
List<Long> safeResult = this.doSafeCast(dialog.open(), Long.class); However, now I am bound to have LinkedLists as result only.
Is it possible to specify the list type by passing an additional parameter, too?
I would want to to something like this:
List<Long> safeResult = this.doSafeCast(dialog.open(), Long.class, ArrayList.class); List<String> anotherSafeResult = this.doSafeCast(anotherDialog.open(), String.class, LinkedList.class); I know that all this seems overly complicated and I could just add @SuppressWarnings("unchecked") to my initial unsafe cast, if I were sure that the dialog will always return the desired type. But I came up with this idea of having a general method to do such safe casts and now I want to know, if this is possible at all. So please don't suggest other solutions, I just want to learn about using generics a little more, here. :-) Thank you.
