Sorting Collections

Java can sort any ArrayList of Strings or numbers with Collections.sort(), just import it from java.util

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

public class Main {
    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<>();
        names.add("Tommy");
        names.add("Cortez");
        names.add("Lance");

        Collections.sort(names);

        for (String n : names) {
            System.out.println(n);
        }
    }
}

Output

Cortez
Lance
Tommy

Collections.sort() sorts in place: it modifies the original list directly. Strings are sorted alphabetically, numbers ascending

Want reverse order? Use Collections.reverse() after sorting

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

public class Main {
    public static void main(String[] args) {
        ArrayList<Integer> scores = new ArrayList<>();
        scores.add(88);
        scores.add(42);
        scores.add(95);

        Collections.sort(scores);
        Collections.reverse(scores);

        for (int s : scores) {
            System.out.println(s);
        }
    }
}

Output

95
88
42

reverse() doesn't sort, it just flips the current order. To get descending order, sort first then reverse

But how do you sort a list of objects? If you have an ArrayList<Car>, Java doesn't know what to sort by: name? speed? You have to tell it by implementing the Comparable interface

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

class Car implements Comparable<Car> {
    String name;
    int speed;

    Car(String name, int speed) {
        this.name = name;
        this.speed = speed;
    }

    public int compareTo(Car other) {
        return this.speed - other.speed;
    }
}

public class Main {
    public static void main(String[] args) {
        ArrayList<Car> garage = new ArrayList<>();
        garage.add(new Car("Infernus", 240));
        garage.add(new Car("Admiral", 150));
        garage.add(new Car("Cheetah", 230));

        Collections.sort(garage);

        for (Car c : garage) {
            System.out.println(c.name + " - " + c.speed + " km/h");
        }
    }
}

Output

Admiral - 150 km/h
Cheetah - 230 km/h
Infernus - 240 km/h

Let's trace what happens:

1. Car implements Comparable<Car>: the class promises it knows how to compare itself with other cars
2. The compareTo method returns a number:
   • negative if this comes before other
   • zero if they're equal
   • positive if this comes after other
3. this.speed - other.speed sorts ascending by speed

For descending order, flip it: other.speed - this.speed

To sort by String (e.g., by name), use String's own .compareTo()

public int compareTo(Car other) {
    return this.name.compareTo(other.name);
}

Mission: Crew Ranking

Cortez wants a crew ranking sorted by number of completed missions, from least productive to most active. Tommy completed 47 missions, Lance 12, Phil 8, and Mercedes 23

Build a CrewMember class that implements Comparable and compares by mission count. Store the four counts in variables missions1 (Tommy), missions2 (Lance), missions3 (Phil), and missions4 (Mercedes). Create an ArrayList<CrewMember> with all members, sort it with Collections.sort, and print the ranking as "name - count missions"

Example

Phil - 8 missions
Lance - 12 missions
Mercedes - 23 missions
Tommy - 47 missions