String#compareTo()

sortに渡すComparatorのbaseになる考え方
returnが1以上になるとsortはひっくり返す

public class Outer {
    public static void main(String[] args) {
        String s1 = new String("A");
        String s2 = new String("B");
        System.out.println(s1.compareTo(s2));
        System.out.println(s1.compareTo(s1));
        System.out.println(s2.compareTo(s1));
    }
}

-1
0
1