Student[] feld = {
                new Student("Meier","Peter",122),
                new Student("Meier","Andrea",142),
                new Student("Brandt","Uwe",12),
                new Student("Meyer","Jens",121),
                new Student("Schulz","Jasmine",11),
                new Student("Schulz","Jürgen",14),
                new Student("Beyer","Ute",17)
        };

        ComparatorStudent comp = new ComparatorStudent();

        MergeSortGeneric mergeSort = new MergeSortGeneric();
        Student[] feldNeu= (Student[]) mergeSort.sort(feld, comp);
        System.out.println("Ausgabe");
        for (Student value: feldNeu) {
            System.out.println(" "+value);
        }



import java.util.Comparator;

public class ComparatorStudent implements Comparator<Student> {


    @Override
    public int compare(Student a, Student b) {
        return a.compareTo(b);
    }

}



public class Student implements  Comparable<Student>{

    public String lastname="";
    public String firstname="";
    public int mnr=0;

    public Student(String lastname,String firstname, int mnr) {
        this.lastname=lastname;
        this.firstname=firstname;
        this.mnr=mnr;
    }

    @Override
    public  String toString(){
        return lastname+", "+firstname+", "+mnr;
    }

    @Override
    public int compareTo(Student std) {
        int result =lastname.compareTo(std.lastname);
        if (result==0) {
            result =firstname.compareTo(std.firstname);
        }
        return result;
    }
}