- Elements in TreeSet are ordered using natural order or Comparator/Comparable interface.
- In case of user defined objects or POJO, POJO needs to implement Comparable interface to sort objects.
- TreeSet will invoke the compareTo method of POJO, to sort POJOs (contained in TreeSet)
- We have already discussed to sort user defined objects using Comparator interface.
Example – Sort Car POJO contained in TreeSet
TreeSet is containing Car objects. Car class has following attributes.
- public String model
- public String owner
- public String registerCity
- public LocalDate serviceDate
- Car class will implements Comparable interface.
- Car class will override compareTo method (to achieve sorting).
- Car objects will be sorted by model.
Program – Sort user defined objects contained in TreeSet
package org.learn.collection.set.tset;import java.time.LocalDate;import java.time.Month;import java.util.TreeSet;class Car implements Comparable<Car>{ public String model; public String owner; public String registerCity; public LocalDate serviceDate; public Car(String model,String owner, String registerCity, LocalDate serviceDate) { this.model = model; this.serviceDate = serviceDate; this.owner = owner; this.registerCity = registerCity; } public String toString() { return String.format("\n[Model:%s,Owner:%s,registeredCity:%s,ServiceDate:%s]", model,owner,registerCity,serviceDate.toString()); } @Override public int compareTo(Car car) { return this.model.compareTo(car.model); }}public class DemoTreeSetComparable { public static void main(String[] args) { TreeSet<Car> carList = new TreeSet<Car>(); carList.add(new Car("S-Class", "Guzan", "NewYork", LocalDate.of(2014, Month.AUGUST, 01))); carList.add(new Car("C-Class", "Bedoya", "California", LocalDate.of(2009, Month.DECEMBER, 05))); carList.add(new Car("A-Class", "Cameron", "Illinois", LocalDate.of(2014, Month.JULY, 25))); carList.add(new Car("SL-Class", "Johnson", "Kansas" ,LocalDate.of(2011, Month.DECEMBER, 10))); System.out.println("Sorted car objects by Model:\n"+ carList); }} |
Output – Sort user defined objects contained in TreeSet
Sorted car objects by Model:[[Model:A-Class,Owner:Cameron,registeredCity:Illinois,ServiceDate:2014-07-25], [Model:C-Class,Owner:Bedoya,registeredCity:California,ServiceDate:2009-12-05], [Model:S-Class,Owner:Guzan,registeredCity:NewYork,ServiceDate:2014-08-01], [Model:SL-Class,Owner:Johnson,registeredCity:Kansas,ServiceDate:2011-12-10]] |