ArrayList sorting objects using Comparator

Question

I'm currently getting an error when attempting to sort these Pet objects by weight. I'm sure it's something simple, but I can't see why this compare isn't working.

Error: The return type is incompatible with java.util.Comparator.compare(Pet, Pet)

ArrayListNoDups class

import java.util.*;
import java.util.Collections;
import java.util.Comparator;
import java.lang.Object;

public class ArrayListNoDups {
    public static void main(String[] args) {
        ArrayList<Pet> list = new ArrayList<Pet>();
        String name;
        Integer age;
        Double weight;
        Scanner keyboard = new Scanner(System.in);
        System.out.println("If you wish to stop adding Pets to the list, please put a 0 for all 3 fields.");

        do {
            System.out.println("Enter a String for Pet name: ");
            name = keyboard.next();
            System.out.println("Enter an int for Pet age: ");
            age = keyboard.nextInt();
            System.out.println("Enter a double for Pet weight: ");
            weight = keyboard.nextDouble();

            if (name.length() > 0 && age > 0 && weight > 0)
                list.add(new Pet(name, age, weight));
        } while (name.length() > 0 && age > 0 && weight > 0);

        System.out.println("Your list sorted by WEIGHT ========================= ");
        Collections.sort(list, Pet.SortByWeight);
        for (Pet p2 : list)
            p2.writeOutput();

    }
}

Pet class

import java.util.*;

public class Pet {
    private String name;
    private Integer age; // in years
    private double weight; // in pounds

    public void writeOutput() {
        System.out.println("Name: " + name);
        System.out.println("Age: " + age + " years");
        System.out.println("Weight: " + weight + " pounds");
    }

    public void set(String newName) {
        name = newName;
        // age and weight are unchanged.
    }

    public void set(int newAge) {
        if (newAge <= 0) {
            System.out.println("Error: illegal age.");
            System.exit(0);
        } else
            age = newAge;
        // name and weight are unchanged.
    }

    public void set(double newWeight) {
        if (newWeight <= 0) {
            System.out.println("Error: illegal weight.");
            System.exit(0);
        } else
            weight = newWeight;
        // name and age are unchanged.
    }

    public Pet(String name, int age, double weight) {
        this.name = name;
        this.age = age;
        this.weight = weight;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    public double getWeight() {
        return weight;
    }

    public static Comparator<Pet> SortByWeight = new Comparator<Pet>() {
        public double compare(Pet pet1, Pet pet2) {
            return pet1.getWeight() - pet2.getWeight();
        }
    };
}

Answer 1

compare method of Comparator interface returns an int , not double .

Change:

public double compare(Pet pet1, Pet pet2)

to:

public int compare(Pet pet1, Pet pet2)

The Comparator can look like this:

public static Comparator<Pet> SortByWeight = new Comparator<Pet>() {
    public int compare(Pet pet1, Pet pet2) {
        return (int)(pet1.getWeight() - pet2.getWeight());
    }
};

Answer 2

The problem is compare needs to return an integer to comply with that Comparator interface.

You could cast down to an int in your comparison, or perhaps better, just use Double.compare() .

Side note: using the @Override annotation in these situations (implementing common interfaces) will make the compiler help you see these problems earlier.

Answer 3

Try this :

public int compare(Pet pet1, Pet pet2) {
            return new Double(pet1.getWeight()).compareTo(new Double(pet2.getWeight()));
        }