Compareto Method

The compareTo method is used for sorting collection of class instances.

The method is also an implementation of the Comparable interface. It can be applied to both List and ArrayList.

Syntax

Interface implementation:

compareTo method:

Importing necessary package:

Sorting class collection via the compareTo method:

Examples

Ascending Sort

import java.util.ArrayList;
import java.util.Collections;

class Smartphone implements Comparable<Smartphone>
{
    private String model;
    private double price;

Smartphone(String model, double price)
    {
        this.model = model;
        this.price = price;
    }

public int compareTo(Smartphone s) {
        if(s.getPrice() > this.getPrice()) {
            return -1;
        }
        else if(s.getPrice() == this.getPrice()) {
            return 0;
        }

return 1;
    }

public double getPrice() {
        return this.price;
    }

public String getModel() {
        return this.model;
    }
}

class Shop
{
    public ArrayList<Smartphone> smartphones;

Shop()
    {
        this.smartphones = new ArrayList<Smartphone>();
    }
}

public class Main {

public static void main(String[] args) {
        Shop shop = new Shop();

Smartphone phone_1 = new Smartphone("Samsung Galaxy S 20", 1300);
        Smartphone phone_2 = new Smartphone("Iphone 11", 2600);
        Smartphone phone_3 = new Smartphone("Samsung Galaxy Fold", 2400);

shop.smartphones.add(phone_1);
        shop.smartphones.add(phone_2);
        shop.smartphones.add(phone_3);

// Sorting collection via the compareTo method:
        Collections.sort(shop.smartphones);

for (Smartphone smartphone: shop.smartphones) {
            System.out.println("--------");
            System.out.println(smartphone.getModel());
            System.out.println(smartphone.getPrice());
        }
    }
}

Output:

Explanation:

At lines 15 - 24 the compareTo method is defined. It's used to sort the phones by price ascending.
If at line 16 the sign > is changed to <, the phones will be sorted by descending.
At line 59 the collection of phones is sorted. The sorting uses the compareTo method.

Descending Sort

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;