Сортировка объектов в arrayylist по одному из объектов

Чтобы отсортировать Fleet по Complicated, вы должны заставить Object реализовать интерфейс Comparable и переопределить метод compareTo (), как показано ниже.

Класс флота:

public class Fleet implements java.lang.Comparable<Fleet>{

    private int considerable, complicated;

    public Fleet() {
        super();
    }

    public Fleet(int considerable, int complicated) {
        super();
        this.considerable = considerable;
        this.complicated = complicated;
    }

    public int getConsiderable() {
        return considerable;
    }

    public void setConsiderable(int considerable) {
        this.considerable = considerable;
    }

    public int getComplicated() {
        return complicated;
    }

    public void setComplicated(int complicated) {
        this.complicated = complicated;
    }

    @Override
    public int compareTo(Fleet fleet) {
        int compareComplicated = ((Fleet)fleet).getComplicated();
        return this.complicated - compareComplicated;
    }

    @Override
    public String toString() {
        return "Fleet [considerable = " + considerable + ", complicated = " + complicated + "]";
    }


}

Теперь мы можем вызвать Collections.sort в ArrayList:

Основной класс:

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

public class SortFleet {

    public static void main(String[] args) {
        ArrayList<Fleet> arrayFleet = new ArrayList<Fleet>();
        Fleet fleetA = new Fleet(1, 2);
        Fleet fleetB = new Fleet(4, 12);
        Fleet fleetC = new Fleet(3, 5);
        Fleet fleetD = new Fleet(3, 50);
        Fleet fleetE = new Fleet(2, 3);
        Fleet fleetF = new Fleet(2, 50);


        arrayFleet.add(fleetA);
        arrayFleet.add(fleetB);
        arrayFleet.add(fleetC);
        arrayFleet.add(fleetD);
        arrayFleet.add(fleetE);
        arrayFleet.add(fleetF);


        System.out.println("Before sort:");

        for ( Fleet f : arrayFleet)
        {
            System.out.println(f);
        }

        System.out.println();

        sortFleet(arrayFleet);

        System.out.println("After sort:");
        for ( Fleet f : arrayFleet)
        {
            System.out.println(f);
        }
    }

    public static void sortFleet(ArrayList<Fleet> arrayFleet) {
        Collections.sort(arrayFleet);
    }

}

Консольный вывод:

Before sort:
Fleet [considerable=1, complicated=2]
Fleet [considerable=4, complicated=12]
Fleet [considerable=3, complicated=5]
Fleet [considerable=3, complicated=50]
Fleet [considerable=2, complicated=3]
Fleet [considerable=2, complicated=50]

After sort:
Fleet [considerable=1, complicated=2]
Fleet [considerable=2, complicated=3]
Fleet [considerable=3, complicated=5]
Fleet [considerable=4, complicated=12]
Fleet [considerable=3, complicated=50]
Fleet [considerable=2, complicated=50]

Обновлено: ПОДХОД JAVA 8:

Удаление интерфейса Comparable и переопределения в классе Fleet можно просто отсортировать так, возвращая тот же ожидаемый результат:

public static void sortFleet(ArrayList<Fleet> arrayFleet) {
        Collections.sort(arrayFleet, Comparator.comparingInt(Fleet::getComplicated));
}