How to Implement a Custom Order or Sorting for Key-Value Pairs in a TreeMap in Java?

The Java Collections Framework includes the TreeMap class, in which Java offers a sorted collection of key-value pairs. By default, TreeMap uses a custom Comparator or arranges components according to their natural ordering. In this article, we will learn how to apply a custom order for key-value pairs in a TreeMap.

Sorting for Key-Value Pairs in a TreeMap

We must provide a custom Comparator during TreeMap instantiation to create a custom order for key-value pairs in a TreeMap. The items’ storage order will be specified by this comparator.

Program to Implement a Custom Order for Key-Value Pairs in a TreeMap

Assume for the moment that we have a TreeMap with people’s names as values and their ages as keys. The entries should be arranged in decreasing order according to age.

Below is the Program to Implement a Custom Order for Key-Value Pairs in a TreeMap:

Java

// Java program to apply a custom order
// For key-value pairs in a TreeMap
import java.util.*;

// Driver Class
public class CustomOrderTreeMap
{
      // Main Method
    public static void main(String[] args) {
        // Creating a TreeMap with a custom comparator
          // For descending order based on keys (ages)
        TreeMap<Integer, String> customOrderedMap = new TreeMap<>(Collections.reverseOrder());

        // Adding key-value pairs
        customOrderedMap.put(25, "Rahul");
        customOrderedMap.put(30, "Abhi");
        customOrderedMap.put(22, "Charan");
        customOrderedMap.put(35, "Gopi");

        // Displaying the TreeMap
        System.out.println("Custom Ordered TreeMap: " + customOrderedMap);
    }
}

Output

Custom Ordered TreeMap: {35=Gopi, 30=Abhi, 25=Rahul, 22=Charan}

Explanation of the Program:

• In the above program, we have instantiated a customOrderedMap TreeMap, we provide Collections.reverseOrder() as the Comparator. This guarantees that the items are arranged according to keys (ages) in decreasing order.
• We expand the TreeMap with key-value pairs.
• The TreeMap with its items Sorted according to the unique order specified by the Comparator is Shown in the Output.