Converting a subList of an ArrayList to an ArrayList

java arraylist sublist
yoavgray picture yoavgray · May 20, 2013 · Viewed 38.1k times · Source

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

Jon Skeet picture Jon Skeet · May 20, 2013

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?

Related questions

How to take a valid sublist in Java?

I have this weird (I think) problem in Java. I have an ArrayList and I want to take a sublist. But I get the follow exception. package javatest; import java.util.ArrayList; public class JavaTest { public static void main(String[] …

Read More
Initialization of an ArrayList in one line

I wanted to create a list of options for testing purposes. At first, I did this: ArrayList<String> places = new ArrayList<String>(); places.add("Buenos Aires"); places.add("Córdoba"); places.add("La Plata"); Then, I …

Read More
Convert ArrayList<String> to String[] array

I'm working in the android environment and have tried the following code, but it doesn't seem to be working. String [] stockArr = (String[]) stock_list.toArray(); If I define as follows: String [] stockArr = {"hello", "world"}; it works. Is there something that …

Read More