Converting a subList of an ArrayList to an 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?

Jon Skeet · Accepted Answer · 2013-05-20 07:45:30Z

117

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 (e.g. 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?

answered May 20, 2013 at 7:45

Jon Skeet

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3 Comments

Joshua M Over a year ago

Another easy option that worked for me was sibling.keys.addAll(keys.subList(mid, this.num)); This is assuming that sibling.keys is already initialized. addAll() simply copies each item of the sublist.

Lalchand Over a year ago

Is this issue device specific? because same code is working for me on my Realme device but not on Samsung Galaxy S9.

Jon Skeet Over a year ago

@Lalchand: I suggest you ask a new question with a complete example and the very specific error you're getting. I would expect it to be the same across all Java implementations.

Ankur Shanbhag · Accepted Answer · 2013-05-20 07:51:27Z

0

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.

answered May 20, 2013 at 7:51

Ankur Shanbhag

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