我有一个ArrayList的ArrayList，我想实现的是，按降序排序（基于最高到最低的列表大小），首先获得最大的列表。
我在java中使用集合做了同样的事情。它在那里工作得很好，但现在我无法在Koltin中实现这一点，我得到了Error.

假设我有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