How do I get an array slice of an ArrayList in Java? Specifically I want to do something like this:
ArrayList<Integer> inputA = input.subList(0, input.size()/2);
// where 'input' is a prepouplated ArrayList<Integer>
So I expected this to work, but Java returns a List - so it's incompatible. And when I try to cast it, Java won't let me. I need an ArrayList - what can I do?
In Java, it is good practice to use interface types rather than concrete classes in APIs.
Your problem is that you1 are using ArrayList (probably in lots of places) where you should really be using List. As a result you created problems for yourself with an unnecessary constraint that the list is an ArrayList.
This is what your code should look like:
List input = new ArrayList(...);
public void doSomething(List input) {
List inputA = input.subList(0, input.size()/2);
...
}
this.doSomething(input);
1 - Based on your comments, "you" was actually someone else ... who set this problem in an interview question. It is possible that this was actually a trick question, designed to see how you would cope with creating a (real) slice of an ArrayList that was a assignment compatible with ArrayList.
Your proposed solution to the problem was/is this:
new ArrayList(input.subList(0, input.size()/2))
That works by making a copy of the sublist (slice) returned by the sublist call. The resulting ArrayList is not a slice in the normal sense. It is a distinct list. Mutating this list does not change the original list or vice-versa. Furthermore, if the sublist is big, then making the copy will be expensive.
If you are constrained by APIs that you cannot change, such that you have to declare inputA as an ArrayList, you might be able to implement a custom subclass of ArrayList in which the subList method returns a subclass of ArrayList. However:
ArrayList class.ArrayList instances to create instances of your subclass instead.The "copy the array" solution is more practical ... bearing in mind that these are not true slices.
new ArrayList(input.subList(0, input.size()/2))
I have found a way if you know startIndex and endIndex of the elements one need to remove from ArrayList
Let al be the original ArrayList and startIndex,endIndex be start and end index to be removed from the array respectively:
al.subList(startIndex, endIndex + 1).clear();
If there is no existing method then I guess you can iterate from 0 to input.size()/2, taking each consecutive element and appending it to a new ArrayList.
EDIT: Actually, I think you can take that List and use it to instantiate a new ArrayList using one of the ArrayList constructors.
Although this post is very old. In case if somebody is looking for this..
Guava facilitates partitioning the List into sublists of a specified size
List<Integer> intList = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8);
List<List<Integer>> subSets = Lists.partition(intList, 3);
This is how I solved it. I forgot that sublist was a direct reference to the elements in the original list, so it makes sense why it wouldn't work.
ArrayList<Integer> inputA = new ArrayList<Integer>(input.subList(0, input.size()/2));
ArrayList? I think you may lack a little bit of understanding how interfaces work becauseListandArrayListare not “incompatible”—ArrayListimplementsList, andListprobably contains all necessary methods you need.ArrayListbecause he then needs to call a method with it which accepts anArrayList. Arguably such a method is poorly designed and should acceptListinstead, but such situations can arise not only in interview questions but in code written by others that one can't just go and change. Co-workers and libraries aren't always perfect.