How to reverse the order of SortedSet

Question

I want to print an ordered list in Map using the following:

Map<Float, String> mylist = new HashMap<>();

mylist.put(10.5, a);
mylist.put(12.3, b);
mylist.put(5.1, c);

SortedSet<Float> orderlist = new TreeSet<Float>(mylist.keySet());

for (Float i : orderlist) {
    System.out.println(i+" "+mylist.get(i));
}

The above code prints:

5.1 c
10.5 a
12.3 b    

But how do I the print the orderlist in reverse order like below:

12.3 b
10.5 a
5.1 c

Answer 1

If you're willing to store the elements in the SortedSet in reverse order, the only change you need to make is to construct the TreeSet with an appropriate constructor which takes a custom Comparator:

Map<Float, String> mylist = new HashMap<>();

mylist.put(10.5, a);
mylist.put(12.3, b);
mylist.put(5.1, c);

SortedSet<Float> orderlist = new TreeSet<Float>(Collections.reverseOrder());
orderList.addAll(mylist.keySet());

for (Float i : orderlist) {
    System.out.println(i+" "+mylist.get(i));
}

Note the neat method here is Collections.reverseOrder() which returns a Comparator which compares in the opposite of the natural ordering of elements.