Create TreeSet with Custom Comparator in Java

In this article, we will learn to create a TreeSet with a custom Comparator in Java. TreeSet is a set that stores elements in sorted order. By default, elements are sorted in natural ordering (numbers in ascending order).

Understanding TreeSet and Comparators

Following are some key points for TressSet and Comparators ?

• A TreeSet is a part of the Java Collections Framework and implements the NavigableSet interface.
  
• It automatically sorts elements as they are added.
  
• By default, it sorts elements using their natural ordering (determined by the Comparable interface).
  
• To define a custom sorting order, we use the Comparator interface.

Creating a TreeSet with a Custom Comparator

To create a TreeSet with a custom comparator, let us first create an Integer array and set it to TreeSet ?

Integer arr[] = { 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 };

Using the Comparator with reverseOrder(), which returns a comparator that imposes the reverse of the natural ordering ?

Set<Integer> set = new TreeSet<Integer>(Collections.reverseOrder());

Adding elements from the array to TreeSet using for loop and add() method ?

for (int i = 0, n = arr.length; i < n; i++) {
         set.add(arr[i]);
      }

Example

Below is an example of creating a TreeSet with a custom Comparator in Java ?

import java.util.Collections;
import java.util.Set;
import java.util.TreeSet;
public class Demo {
   public static void main(String args[]) throws Exception {
      Integer arr[] = { 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 };
      Set<Integer> set = new TreeSet<Integer>(Collections.reverseOrder());
      for (int i = 0, n = arr.length; i < n; i++) {
         set.add(arr[i]);
      }
      System.out.println("TreeSet = "+set);
      System.out.println(((TreeSet<Integer>) set).comparator());
   }
}

Output

TreeSet = [100, 90, 80, 70, 60, 50, 40, 30, 20, 10]
java.util.Collections$ReverseComparator@7ff95560

Time Complexity: O(n log n ) for inserting n elements (due to balanced tree operations).
Space Complexity: O(n) as TreeSet stores n elements.

Conclusion

Using a TreeSet with a custom comparator lets us change the default ordering behavior. With Collections.reverseOrder(), it is quite simple to order elements in reverse order rather than the default order. This technique comes in handy while dealing with ordered data where elements have to be ordered differently as per particular needs.