public class Car {
    String mark;

    public Car (String mark) {
        this.mark = mark;
    }


    public String getMark() {
        return mark;
    }
}

import java.util.Comparator;

public class SortedByMark implements Comparator <Car> {

    public int compare(Car obj1 , Car obj2) {
        String mark1 = obj1.getMark();
        String mark2 = obj2.getMark();

        return str1.compareTo(str2);
    }
}

public class Example extends SortedByMark {

    public static void main(String [] args) {

        ArrayList str = new ArrayList();
        str.add(new Car("Audi"));
        str.add(new Car("Subaru"));
        str.add(new Car("Kia"));
        str.add(new Car("BMW"));
        str.add(new Car("Alfa-Romeo"));

        Collections.sort(str, new SortedByMark());
        System.out.println("Sorting: " + str);

    }
}