How to Add Custom Class Objects to the TreeSet in Java? Last Updated : 24 Nov, 2022 Summarize Comments Improve Suggest changes Share Like Article Like Report TreeSet is an implementation of the SortedSet interface in java that uses a red-black tree for storage. By default, It maintains an ascending order. It contains unique elements only. It doesn't allow null elements. Access and retrieval times are quite fast. To add the user-defined object into TreeSet, we need to implement a Comparable interface. An element that we want to add in TreeSet must be a comparable type. If we don't implement the Comparable interface then it will throw ClassCastException. Example 1 Java // Importing util package import java.util.*; // Custom class Car implements Comparable interface class Car implements Comparable<Car> { // attributes int Modelno; String name, city; int stock; // Car constructor public Car(int Modelno, String name, String city, int stock) { this.Modelno = Modelno; this.name = name; this.city = city; this.stock = stock; } // Override the compareTo() method public int compareTo(Car c) { if (stock > c.stock) { return 1; } else if (stock < c.stock) { return -1; } else { return 0; } } } public class GFG { // Main driver method public static void main(String[] args) { // Define an objects of TreeSet class TreeSet<Car> set = new TreeSet<Car>(); // Creating Car objects Car c1 = new Car(132, "BMW", "Rajkot", 35); Car c2 = new Car(269, "Audi", "Surat", 20); Car c3 = new Car(560, "Kia", "Vadodara", 15); Car c4 = new Car(109, "Creta", "Ahmedabad", 26); // Adding Car objects to TreeSet set.add(c1); set.add(c2); set.add(c3); set.add(c4); // Traversing TreeSet for (Car c : set) { System.out.println(c.stock + " " + c.name + " " + c.city + " " + c.Modelno); } } } Output15 Kia Vadodara 560 20 Audi Surat 269 26 Creta Ahmedabad 109 35 BMW Rajkot 132 We need to override the compareTo() method so that it will sort our set in a particular order. Example 2: Let's take the same example but now we override the compareTo() method with respect to the name of the car. Java // Importing util package import java.util.*; // Custom class Car implements Comparable interface class Car implements Comparable<Car> { // attributes int Modelno; String name, city; int stock; // Car constructor public Car(int Modelno, String name, String city, int stock) { this.Modelno = Modelno; this.name = name; this.city = city; this.stock = stock; } // Override the compareTo() method public int compareTo(Car c) { return name.compareTo(c.name); } } public class GFG { // Main driver method public static void main(String[] args) { // Define an objects of TreeSet class TreeSet<Car> set = new TreeSet<Car>(); // Creating Car objects Car c1 = new Car(132, "BMW", "Rajkot", 35); Car c2 = new Car(269, "Audi", "Surat", 20); Car c3 = new Car(560, "Kia", "Vadodara", 15); Car c4 = new Car(109, "Creta", "Ahmedabad", 26); // Adding Car objects to TreeSet set.add(c1); set.add(c2); set.add(c3); set.add(c4); // Traversing TreeSet for (Car c : set) { System.out.println(c.name + " " + c.stock + " " + c.city + " " + c.Modelno); } } } OutputAudi 20 Surat 269 BMW 35 Rajkot 132 Creta 26 Ahmedabad 109 Kia 15 Vadodara 560 Comment More infoAdvertise with us Next Article How to Add or Append Elements to End of a TreeSet in Java? 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