How can I slice an ArrayList out of an ArrayList in Java

Question

How do I get an array slice of an ArrayList in Java  Specifically I want to do something like this    ArrayList lt Integer gt  inputA   input subList 0  input size   2      where  input  is a prepouplated ArrayList lt Integer gt    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

User · Accepted Answer

In Java  it is good practice to use interface types rather than concrete classes in APIs   Your problem is that you 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       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   It is not a slice in the normal sense   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    It would be a lot of work to design  implement and test  You have now added significant new class to your code base  possibly with dependencies on undocumented aspects  and therefore  subject to change   aspects of the ArrayList class  You would need to change relevant places in your codebase where you are creating 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

User · Answer

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 lt Integer gt  intList   Lists newArrayList 1  2  3  4  5  6  7  8       List lt List lt Integer gt  gt  subSets   Lists partition intList  3

User · Answer

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

User · Answer

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 lt Integer gt  inputA   new ArrayList lt Integer gt  input subList 0  input size   2

User · Answer

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

[java] How can I slice an ArrayList out of an ArrayList in Java?

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