I need to sort an array according to another array, eg:
list1: [221, 54, 50, 138, 125, 145]
list2: [50, 125]
then the sorted list1 should be: [50, 125 ,221, 54, 138, 145]
I tried the following code:
Collections.sort(list1, Comparator.comparing(listItem -> list2.indexOf(listItem)));
But this results in: [221, 54, 138, 145, 50, 125]
How can I get the 50 and 125 at the top and the rest after?
When list1 element does not contain in list2 then indexOf return -1 that's why those values come first. So for those value use list1.size() then those value will come later.
Collections.sort(list1, Comparator.comparing(listItem -> list2.contains(listItem) ? list2.indexOf(listItem): list1.size()));
The reason why you are getting the items of list1 first is that the index of its elements which are not there in list2 is returned as -1 and therefore they are getting placed first. All you need to do is to reverse the order as shown below:
list2.sort(Comparator.reverseOrder());
Collections.sort(list1, Comparator.comparing(listItem -> list2.indexOf(listItem)).reversed());
Note that you need to sort list2 in reverse order before applying the reversed. If you do not want to change ordering or list2, you can clone list2 and use it with Comparator.
Demo:
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
public class Main {
public static void main(String[] args) {
List<Integer> list1 = new ArrayList<>(List.of(221, 54, 50, 138, 125, 145));
List<Integer> list2 = new ArrayList<>(List.of(50, 125));
// Clone list2
List<Integer> list2Clone = new ArrayList<>(list2);
// Sort the cloned list in reverse order
list2Clone.sort(Comparator.reverseOrder());
// Sort list1
Collections.sort(list1, Comparator.comparing(listItem -> list2Clone.indexOf(listItem)).reversed());
// Display the result
System.out.println(list1);
}
}
Output:
[50, 125, 221, 54, 138, 145]
Collections.sort(roomSets, Comparator.comparing(room -> list.indexOf(room.getId()))); worked(resulting in the reverse order). But on adding the reversed() function there Collections.sort(roomSets, Comparator.comparing(room -> list.indexOf(room.getId())).reversed());, it suddenly cannot resolve the method at all.(The Id is String). Please let me know if you have an idea why the object behaves like that on adding the reversed function