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TreeSet comparator() Method in Java with Examples

  • Last Updated : 01 Nov, 2021

TreeSet is one of the most important implementations of the SortedSet interface in Java that uses a Tree for storage. The ordering of the elements is maintained by a set using their natural ordering whether or not an explicit comparator is provided. This must be consistent with equals if it is to correctly implement the Set interface.

The comparator() method been present inside java.util.TreeSet shares an important function of setting and returning the comparator that can be used to order the elements in a TreeSet. The method returns a Null value if the set follows the natural ordering pattern of the elements.

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Syntax: 

comp_set = (TreeSet)tree_set.comparator()

Parameters: The method does not take any parameters.



Return Value: The comparator set is used to order the elements of the set in a specific order. It returns a Null value if the set follows the default or natural ordering pattern. 

Here we will be proposing two examples below one earlier we will be using the natural ordering of the elements later using a specific comparator to understand it better.

Example 1: Using the natural ordering of the elements

Java




// Java Program to illustrate the use of comparator() method
// While using the natural ordering of the elements
  
// Importing utility classes
import java.util.*;
  
// Main class
// TreeSet Demo class
public class GFG {
  
    // Main driver method
    public static void main(String[] args)
    {
  
        // Creating an empty TreeSet of integer type
        TreeSet<Integer> tree_set = new TreeSet<Integer>();
  
        // Adding elements to the set
        // using add() method
        tree_set.add(20);
        tree_set.add(24);
        tree_set.add(30);
        tree_set.add(35);
        tree_set.add(45);
        tree_set.add(50);
  
        // Printing elements inside TreeSet object
        System.out.println("Tree Set values are: "
                           + tree_set);
  
        // Creating a comparator
        Comparator comp = tree_set.comparator();
  
        // Print and display the comparator values
        System.out.println("Since the Comparator value is: "
                           + comp);
        
        // Display message only 
        System.out.println("it follows natural ordering");
    }
}
Output: 
Tree Set values are: [20, 24, 30, 35, 45, 50]
Since the Comparator value is: null
it follows natural ordering

 

Example 2: Using a specific comparator

Java




// Java code to illustrate the use of comparator()
// While using a specific comparator
  
// Importing Comparator and TreeSet classes
// from java.util package
import java.util.Comparator;
import java.util.TreeSet;
  
// Class 1
// Helper class
class Helper implements Comparator<String> {
  
    // Method
    // To compare two strings
    public int compare(String str1, String str2)
    {
  
        String first_Str;
        String second_Str;
  
        first_Str = str1;
        second_Str = str2;
  
        // using compareTo() to ensure
        return second_Str.compareTo(first_Str);
    }
}
  
// Main class
// TreeSetDemo class
public class GFG {
  
    // Main driver method
    public static void main(String[] args)
    {
  
        // Creating an empty TreeSet of string type
        TreeSet<String> tree_set = new TreeSet<String>();
  
        // Adding elements to our TreeSet object
        // using add() method
        tree_set.add("G");
        tree_set.add("E");
        tree_set.add("E");
        tree_set.add("K");
        tree_set.add("S");
        tree_set.add("4");
  
        // Printing elements in set before using comparator
        System.out.println("Set before using the comparator: " + tree_set);
  
        // Again creating an empty TreeSet of string type
        // with reference to Helper class
        TreeSet<String> tree_set1 = new TreeSet<String>(new Helper());
  
        // Adding elements to our TreeSet object
        // using add() method
        tree_set1.add("G");
        tree_set1.add("E");
        tree_set1.add("E");
        tree_set1.add("K");
        tree_set1.add("S");
        tree_set1.add("4");
  
        // Printing elements in set before using comparator
        System.out.println("The elements sorted in descending order:" + tree_set1);
    }
}
Output
Set before using the comparator: [4, E, G, K, S]
The elements sorted in descending order:[S, K, G, E, 4]



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