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

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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]]

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