将ArrayList的subList转换为ArrayList

Question

Im using an ArrayList and im trying to copy a part of it to another ArrayList therefore im using:

sibling.keys = (ArrayList<Integer>) keys.subList(mid, this.num);

Where "sibling.keys" is the new ArrayList and "keys or this.keys" is the older ArrayList. I used the casting because eclipse told me to do that but then it throws a ClassCastException :

java.util.ArrayList$SubList cannot be cast to java.util.ArrayList

Any advice?

Answer 1

subList returns a view on an existing list. It's not an ArrayList . You can create a copy of it:

sibling.keys = new ArrayList<Integer>(keys.subList(mid, this.num));

Or if you're happy with the view behaviour, try to change the type of sibling.keys to just be List<Integer> instead of ArrayList<Integer> , so that you don't need to make the copy:

sibling.keys = keys.subList(mid, this.num);

It's important that you understand the difference though - are you going to mutate sibling.keys (eg adding values to it or changing existing elements)? Are you going to mutate keys ? Do you want mutation of one list to affect the other?

Answer 2

You get the class cast exception because you are expecting an ArraList while the ArrayList.subList()does not return ArrayList. Change your sibling.keys from ArrayList to List , and should work fine. This will avoid ClassCastException as well as you will not need to and any cast.

Answer 3

Array.subList doesn't return an ArrayList , but a List .

So the following line works :

List<Integer> keys = (List<Integer>) keys.subList(mid, this.num);

Note that the cast is optional.