Sort user defined objects using TreeSet in java (Comparator interface)

What is TreeSet?

• TreeSet is a NavigableSet implementation based on a TreeMap.
• Elements in TreeSet are ordered using natural order or Comparator/Comparable interface.
• In case of user defined objects or POJO, we need to supply the Compartor (to specify on which fields, we would like to sort objects)
• Comparator is specified during construction of TreeSet.
• We have already discussed to sort user defined objects using Comparable interface.

Example – Sort POJO or user defined object using comparator interface.

Given a TreeSet containing Vehicle objects. Vehicle class has following data members.

• public String model
• public String owner
• public String registerCity
• public LocalDate serviceDate

We would like to Sort Vehicle objects by model, owner and serviceDate. We will have a specialized Comparator for each sorting need. We will have following Comparators:

1. Comparator to sort Vehicle objects by model
2. Comparator to sort Vehicle objects by owner
3. Comparator to sort Vehicle objects by serviceDate

Program – sort user defined objects contained in a TreeSet using java

package org.learn.collection.set.tset;

import java.time.LocalDate;
import java.time.Month;
import java.util.Comparator;
import java.util.TreeSet;

class Vehicle {
 public String model; 
 public String owner;
 public String registerCity;
 public LocalDate serviceDate;

 public Vehicle(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("[Model:%s,Owner:%s,registeredCity:%s,ServiceDate:%s]", 
     model,owner,registerCity,serviceDate.toString());
  }
}

class SortByModel implements Comparator<Vehicle> {

 @Override
 public int compare(Vehicle vehicle1, Vehicle vehicle2) {
  return vehicle1.model.compareTo(vehicle2.model);
 }
}

class SortByOwner implements Comparator<Vehicle> {
 @Override
 public int compare(Vehicle vehicle1, Vehicle vehicle2) {
  return vehicle1.owner.compareTo(vehicle2.owner);
 }
}

class SortByServiceDate implements Comparator<Vehicle> {

 @Override
 public int compare(Vehicle vehicle1, Vehicle vehicle2) {
  return vehicle1.serviceDate.compareTo(vehicle2.serviceDate);
 }
}

public class DemoTreeSetComparator {

 public static void main(String[] args) {
  TreeSet<Vehicle> vehicleList = new TreeSet<Vehicle>(new SortByModel());
  vehicleList.add(new Vehicle("S-Class", "Guzan", "NewYork", LocalDate.of(2014, Month.AUGUST, 01)));
  vehicleList.add(new Vehicle("C-Class", "Bedoya", "California", LocalDate.of(2009, Month.DECEMBER, 05)));
  vehicleList.add(new Vehicle("A-Class", "Cameron", "Illinois", LocalDate.of(2014, Month.JULY, 25)));
  vehicleList.add(new Vehicle("SL-Class", "Johnson", "Kansas" ,LocalDate.of(2011, Month.DECEMBER, 10)));
  System.out.println("1. Sorted Vehicle list by Model:\n"+ vehicleList);
  
  vehicleList = new TreeSet<Vehicle>(new SortByOwner());
  vehicleList.add(new Vehicle("S-Class", "Guzan", "NewYork", LocalDate.of(2014, Month.AUGUST, 01)));
  vehicleList.add(new Vehicle("C-Class", "Bedoya", "California", LocalDate.of(2009, Month.DECEMBER, 05)));
  vehicleList.add(new Vehicle("A-Class", "Cameron", "Illinois", LocalDate.of(2014, Month.JULY, 25)));
  vehicleList.add(new Vehicle("SL-Class", "Johnson", "Kansas" ,LocalDate.of(2011, Month.DECEMBER, 10)));
  System.out.println("\n2. Sorted Vehicle list by Owner:\n"+ vehicleList);
  
  vehicleList = new TreeSet<Vehicle>(new SortByServiceDate());
  vehicleList.add(new Vehicle("S-Class", "Guzan", "NewYork", LocalDate.of(2014, Month.AUGUST, 01)));
  vehicleList.add(new Vehicle("C-Class", "Bedoya", "California", LocalDate.of(2009, Month.DECEMBER, 05)));
  vehicleList.add(new Vehicle("A-Class", "Cameron", "Illinois", LocalDate.of(2014, Month.JULY, 25)));
  vehicleList.add(new Vehicle("SL-Class", "Johnson", "Kansas" ,LocalDate.of(2011, Month.DECEMBER, 10)));
  System.out.println("\n3. Sorted Vehicle list by service date:\n"+ vehicleList);
  
 }
}

Output – sort user defined objects contained in a TreeSet using java

1. Sorted Vehicle list 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]]

2. Sorted Vehicle list by Owner:
[
[Model:C-Class,Owner:Bedoya,registeredCity:California,ServiceDate:2009-12-05], 
[Model:A-Class,Owner:Cameron,registeredCity:Illinois,ServiceDate:2014-07-25], 
[Model:S-Class,Owner:Guzan,registeredCity:NewYork,ServiceDate:2014-08-01], 
[Model:SL-Class,Owner:Johnson,registeredCity:Kansas,ServiceDate:2011-12-10]]

3. Sorted Vehicle list by service date:
[
[Model:C-Class,Owner:Bedoya,registeredCity:California,ServiceDate:2009-12-05], 
[Model:SL-Class,Owner:Johnson,registeredCity:Kansas,ServiceDate:2011-12-10], 
[Model:A-Class,Owner:Cameron,registeredCity:Illinois,ServiceDate:2014-07-25], 
[Model:S-Class,Owner:Guzan,registeredCity:NewYork,ServiceDate:2014-08-01]]