Sorting ArrayList in Kotlin

Question

I have an ArrayList of ArrayList what I want to achieve is, to sort them in descending order (based on the Highest to lowest List size ) to get the largest first and onwards.

I did the same thing in java using Collections. It worked perfectly fine there but now I'm unable to achieve this in Koltin I'm getting an ERROR.

Consider I've a class Foo()

 private var allHistoryList: ArrayList<ArrayList<Foo>> =   arrayListOf()


     /**
                 * Sorting array of array list to get Biggest to smallest array List based of size
                 */

      Collections.sort(allHistoryList, object: Comparator<ArrayList>{
            override fun compare(var a1, var a2) :Int {
                return a2.size() - a1.size()
            }
                    
        }
 

 Collections.sort(allHistoryList, object: Comparator<ArrayList>{
                    override fun compare(var a1 : ArrayList<*>, var a2 :ArrayList<*>) :Int {
                        return a2.size() - a1.size()
                    }
    
                }
    
                )
                //end of sorting

                 /**
                 * Sorting array of array list to get Biggest to smallest array List based of size
                 */
    
 Collections.sort(allHistoryList, new Comparator<ArrayList>() {
                    public int compare(ArrayList a1, ArrayList a2) {
                        return a2.size() - a1.size(); // working fine in Java.
                    }
                });//end of sorting

Answer 1

In Kotlin, it's just one line:

allHistoryList.sortByDescending { list -> list.size }

The method sortByDescending mutates the original list sorting it descending using the size of the list as selector to make the comparison between elements.