Sorting list of objects based on nested values in java

Question

I have a list of Cars scheduled for delivery for multiple dates which needs to be sorted on the basis of the below points:

• If isReady>0, then it should be displayed first in the table. And then the other values come below it for that particular date.
• If isReady>0 and Object gear!=null then, it is displayed first in the table for that particular date. Followed by the other values where Object gear==null.
• If isReady>0, Object gear!=null and Object tyre!=null, then that value is displayed first in the table for that particular date. Followed by the other values where Object gear==null and tyre==null.

Here are the class codes:

public class Car {
    private int isReady;
    private Tyre tyre;
    private Gear gear;
    private Date deliveryDate;
}


public class Gear {
    private int id;
    private String type;
}


public class Tyre {
    private int id;
    private String grip;
}

public class CarComparator implements Comparator<Car> {
    @Override
    public int compare(Car entry1, Car entry2) {
        int value = 0;

        if (entry1.getIsReady() > entry2.getIsReady()) {
            value = -1;
        } else if (entry1.getIsReady() < entry2.getIsReady()) {
            value = 1;
        } else if (entry1.getIsReady() == entry2.getIsReady()) {
            value = 0;
        }
        return value;
    }
}

I have developed a Comparator which works fine for the first condition where isReady>0. Could you please help me with the other conditions mentioned above.

Thanks in advance.

Answer 1

Well, as of Java 8 you could build your comparator like this:

//order by delivery date first, ascending order
Comparator<Car> carComparator = Comparator.comparing( Car::getDeliveryDate )
  //order by isReady in ascending order
  .thenComparing( Car::getIsReady )
  //we map null to 1 and non-null to -1 and ignore the rest for now
  .thenComparing( car -> car.getGear() != null ? -1 : 1 ) 
  .thenComparing( car -> car.getTyre() != null ? -1 : 1 );

Answer 2

why not reuse Integer.compareTo to make the code shorter?

like this:

import java.util.Comparator;

public class CarComparator implements Comparator<Car> {
    @Override
    public int compare(Car entry1, Car entry2) {
        int value = 0;

        // might want to add a null check for either entry1 and entry2

        value = entry1.getDeliveryDate().compareTo(entry2.getDeliveryDate());
        if (value == 0) {
            value = ((Integer)entry1.getIsReady()).compareTo((Integer)entry2.getIsReady());
            if (value == 0) {
                value = getIntegerValueForNullCheck(entry1.getGear()).compareTo(getIntegerValueForNullCheck(entry2.getGear()));
                if (value == 0) {
                    value = getIntegerValueForNullCheck(entry1.getTyre()).compareTo(getIntegerValueForNullCheck(entry2.getTyre()));
                }
            }
        }

        return value;
    }
    private Integer getIntegerValueForNullCheck (Object o) {
        return o == null ? 0 : 1;
    }
}

including code that tests the sorting:

import java.time.Duration;
import java.time.Instant;
import java.time.LocalDateTime;
import java.util.*;

public class Sorting {

    public static void main(String[] args) {

        List<Car> cars = new LinkedList<>();


        Date today = new Date();
        Instant now = Instant.now();
        Instant after = now.plus(Duration.ofDays(1));
        Date tomorrow = Date.from(after);

        cars.add(new Car(5, new Tyre(1,"1"), new Gear(1, "1"), today ));
        cars.add(new Car(5, new Tyre(1,"1"), null, today ));
        cars.add(new Car(5, null, null, today ));
        cars.add(new Car(4, null, null, today ));
        cars.add(new Car(3, null, null, tomorrow ));


        Collections.sort(cars, new CarComparator());
        System.out.println(cars);
    }

}

the output:

[Car{isReady=4, tyre=null, gear=null, deliveryDate=Thu Oct 10 11:27:20 IDT 2019}
, Car{isReady=5, tyre=null, gear=null, deliveryDate=Thu Oct 10 11:27:20 IDT 2019}
, Car{isReady=5, tyre=Tyre{id=1, grip='1'}, gear=null, deliveryDate=Thu Oct 10 11:27:20 IDT 2019}
, Car{isReady=5, tyre=Tyre{id=1, grip='1'}, gear=Gear{id=1, type='1'}, deliveryDate=Thu Oct 10 11:27:20 IDT 2019}
, Car{isReady=3, tyre=null, gear=null, deliveryDate=Fri Oct 11 11:27:20 IDT 2019}
]

Answer 3

To be honest I don't see anything wrong with your code. That is if your intention is to return a -1 when entry1 is larger than entry 2(reverse order of the norm). Without further code to read, I think your code will work if you are trying to compare which value is larger or smaller that is built for your usage case.

However, I think there is some inefficiency in your returning method. You don't need to return value. You can just return an actual value.

For comparison, you can just compare == first then evaluate the rest after. But that might be hard to read the code so I give you two versions.

Remove value version:

public class CarComparator implements Comparator<Car> {
    @Override
    public int compare(Car entry1, Car entry2) {
        if (entry1.getisReady() > entry2.getisReady()) {
            return -1;
        } else if (entry1.getisReady() < entry2.getisReady()) {
            return 1;
        } else if (entry1.getisReady() == entry2.getisReady()) {
            return 0;
        }
    }
}

Remove value and different style of comparison:

public class CarComparator implements Comparator<Car> {

    @Override
    public int compare(Car entry1, Car entry2) {
        if (entry1.getisReady() == entry2.getisReady()) return 0;            
        return entry1.getisReady() > entry2.getisReady()? -1 : 1;
    }
}

I am not sure if I got you correctly but if I don't, I hope this code can help you. I separated them into 3 methods and I hope you got a getTyre() and getGear() method. You can combine them as needed and for the last method, its values were separated into variable order for easy reading of code.

class CarComparator implements Comparator<Car> {

    public int compare(Car entry1, Car entry2) {
        if (entry1.getisReady() == entry2.getisReady()) return 0;
        return entry1.getisReady() > entry2.getisReady()? -1 : 1;
    }

    public int compareGear(Car entry1, Car entry2){
        if ( (entry1.getGear() != null && entry2.getGear() != null) 
           ||(entry1.getGear() == null && entry2.getGear() == null)
           ){
            return compare(entry1, entry2);
        }  
        return entry1.getGear() != null && entry2.getGear() == null? -1 : 1;

    }

    public int compareTye(Car entry1, Car entry2){
        int order1 = entry1.getGear() != null && entry1.getTyre() != null? 1 : 0;
        int order2 = entry2.getGear() != null && entry2.getTyre() != null? 1 : 0;

        if ( order1 == order2 ) return compare(entry1, entry2);
        return order1 > order2? -1 : 1;
    } 
}

Answer 4

Check this comparator so you can sort with multiple attributes

public class CarComparator implements Comparator<Car> {

    @Override
    public int compare(Car entry1, Car entry2) {
        int value;
        if (entry1.getDeliveryDate().before(entry2.getDeliveryDate())){
            value = -1;
        }else if (entry1.getDeliveryDate().equals(entry2.getDeliveryDate())){
            value = 0;
        }else{
            value =1;
        }
        //For same day
        if (value==0){
            if (entry1.getIsReady() > entry2.getIsReady()) {
                value = -1;
            } else if (entry1.getIsReady() < entry2.getIsReady()) {
                value = 1;
            } else if (entry1.getIsReady() == entry2.getIsReady()) {
                value = 0;
            }
        }
        //if same isReady
        if (value==0){
            if (entry1.getGear()!=null && entry2.getGear()==null) {
                value = -1;
            } else  if (entry1.getGear()==null && entry2.getGear()==null) {
                value = 0;
            } else{
                value = 1;
            }
        }
        //if still equals
        if (value==0){
            if (entry1.getTyre()!=null && entry2.getTyre()==null) {
                value = -1;
            } else  if (entry1.getTyre()==null && entry2.getTyre()==null) {
                value = 0;
            } else{
                value = 1;
            }
        }


        return value;
    }
}

I am not sure if this is what you try to do. What the above comparator does is: First to sort with Dates, if it finds equal dates (value=0), it compares the isReady, then getGear() and finally the getTyre().

That way you can add as many attributes as you need in your comparator.

Including the main method with 3 cars

public class Main {
    public static void main (String[]args) throws UnsupportedEncodingException, ParseException {

        List<Car> carL = new ArrayList<Car>();

        Car car1 = new Car();
        car1.setDeliveryDate(new Date());
        Gear gear1 = new Gear();
        car1.setGear(gear1);
        Tyre tyre1 = new Tyre();
        car1.setTyre(null);
        car1.setId(1);
        car1.setDeliveryDate((new SimpleDateFormat("dd-MM-yyyy")).parse("01-01-2000"));
        car1.setIsReady(0);

        Car car2 = new Car();
        car2.setDeliveryDate(new Date());
        Gear gear2 = new Gear();
        car2.setGear(gear2);
        Tyre tyre2 = new Tyre();
        car2.setTyre(tyre2);
        car2.setId(2);
        car2.setDeliveryDate((new SimpleDateFormat("dd-MM-yyyy")).parse("02-01-2000"));

        car2.setIsReady(1);

        Car car3 = new Car();
        car3.setDeliveryDate(new Date());
        Gear gear3 = new Gear();
        car3.setGear(gear3);
        Tyre tyre3 = new Tyre();
        car3.setTyre(tyre3);
        car3.setId(3);
        car3.setDeliveryDate((new SimpleDateFormat("dd-MM-yyyy")).parse("01-01-2000"));

        car3.setIsReady(1);

        carL.add(car1);
        carL.add(car2);
        carL.add(car3);
        Collections.sort(carL, new CarComparator());
        for (Car car : carL) {
            System.out.println("car: " + car.toString());
        }
    }
}

outputs:

car: Car{id=3, isReady=1, tyre=false, gear=false, deliveryDate=Sat Jan 01 00:00:00 EET 2000}
car: Car{id=1, isReady=0, tyre=true, gear=false, deliveryDate=Sat Jan 01 00:00:00 EET 2000}
car: Car{id=2, isReady=1, tyre=false, gear=false, deliveryDate=Sun Jan 02 00:00:00 EET 2000}