TreeMap comparator() method in Java with Examples

The comparator() method of java.util.TreeMap class is used to return the comparator used to order the keys in this map, or null if this map uses the natural ordering of its keys.

--> java.util Package
    --> TreeMap Class
        --> comparator() Method 

Syntax:

public Comparator comparator()

Return Type: This method returns the comparator used to order the keys in this map, or null if this map uses the natural ordering of its keys.

Note: descendingIterator() method is natural ordering by default.

Example 1: For Natural ordering 

Java

// Java program to Illustrate comparator() Method
// for Natural Ordering (Descending Order)

// Importing required classes
import java.util.*;

// Main class
public class GFG {

    // Main driver method
    public static void main(String[] argv) throws Exception
    {

        // Try block to check for exceptions
        try {

            // Creating an empty TreeMap by
            // creating object of NavigableMap
            NavigableMap<Integer, String> treemap
                = new TreeMap<Integer, String>();

            // Populating TreeMap
            // using put() method
            treemap.put(1, "one");
            treemap.put(2, "two");
            treemap.put(3, "three");
            treemap.put(4, "four");
            treemap.put(5, "five");

            // Printing the TreeMap
            System.out.println("TreeMap: " + treemap);

            // Getting used Comparator in the map
            // using comparator() method
            Comparator comp = treemap.comparator();

            // Printing the comparator value
            System.out.println("Comparator value: " + comp);
        }

        // Catch block to handle the exception
        catch (NullPointerException e) {

            // Display message if exception occurs
            System.out.println("Exception thrown : " + e);
        }
    }
}

Output:

TreeMap: {1=one, 2=two, 3=three, 4=four, 5=five}
Comparator value: null

Example 2: For Reverse ordering

Java

// Java program to demonstrate comparator() Method
// for Reverse Ordering

// Importing required classes
import java.util.*;

// Main class
public class GFG {

    // Main driver method
    public static void main(String[] argv) throws Exception
    {

        // Try block to check for exceptions
        try {

            // Creating an empty TreeMap
            NavigableMap<Integer, String> treemap
                = new TreeMap<Integer, String>(
                    Collections.reverseOrder());

            // Populating TreeMap
            // using put() method
            treemap.put(1, "one");
            treemap.put(2, "two");
            treemap.put(3, "three");
            treemap.put(4, "four");
            treemap.put(5, "five");

            // Printing the TreeMap
            System.out.println("TreeMap: " + treemap);

            // Getting used Comparator in the map
            // using comparator() method
            Comparator comp = treemap.comparator();

            // Printing the comparator value
            System.out.println("Comparator value: " + comp);
        }

        // Catch block to handle the exceptions
        catch (NullPointerException e) {

            // Display message if exception occurs
            System.out.println("Exception thrown : " + e);
        }
    }
}

Output:

TreeMap: {5=five, 4=four, 3=three, 2=two, 1=one}
Comparator value: java.util.Collections$ReverseComparator@232204a1