Sort list of Objects Java

Question

I have a little problem, i know i can use sort functions from Libraries. But this is a task from my University and i should write a sort function for my own. So i have Objects of students in my List.

Student(String Prename, String Surname, Int ID, Int gender , String Course)

Student s1 = new Student("Steven", "Schrott", 533546, 1 , "AI");

so now i have 4 Students in my list and now i have to Sort this list by Prenames.

This is my Sort function but it doesnt work.

public static List<Studen> sortStudenPrename(List<Student> data){
  List<Student> studenList = new ArrayList<StudenT>();
  for(int i = 0 ; i < data.size(); i++){
    for(int j = 0; j < data.size(); j ++){
       if(data.get(i).getPrename().compareTo(data.get(j).getPrename()) > 0) {
          studentList.add(data.get(i));
       }
    }
  }
  return studentList;
}

my output ist than like this and no every Name only is once in my list, but my Sort function kinda puts them more than once inside the new list.

Dennis
Nico
Dan
Dan
Dennis
Dan

Just write a custom Comparator and use Collections.sort. By this you will still be able to sort the way you want ... Refer : http://stackoverflow.com/questions/2784514/sort-arraylist-of-custom-objects-by-property — StackFlowed, Jan 20 '17 at 16:00
`Collections.sort((a, b) => a.getPrename().compareTo(b.getPrename()))` — Mr. Polywhirl, Jan 20 '17 at 16:04
Lets say the algorithm is at Dan now. Is `Dan Prename-comparison-with-Dennis > 0`? Yes, write Dan to the list. And the next step would be is `Dan Prename-comparison-with-Nico > 0`? Yes, add Dan to the list. That is how you get multiple names. — DuKes0mE, Jan 20 '17 at 16:04
@StackFlowed : there is no OrderedSet je JRE. Have you meant TreeSet? — rkosegi, Jan 20 '17 at 16:07
Possible duplicate of [Sort ArrayList of custom Objects by property](http://stackoverflow.com/questions/2784514/sort-arraylist-of-custom-objects-by-property) — Grzegorz Górkiewicz, Jan 20 '17 at 17:22

score 1 · Accepted Answer · edited May 23 '17 at 10:29

You can implement your own Quick-Sort algorithm.

There is a great example here:

Using comparator in custom quicksort in java

Driver

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;

public class Driver {
    public static void main(String[] args) {
        List<Student> students = new ArrayList<Student>();

        students.add(new Student(1, "Dennis", "", Gender.MALE, ""));
        students.add(new Student(2, "Nico",   "", Gender.MALE, ""));
        students.add(new Student(3, "Dan",    "", Gender.MALE, ""));
        students.add(new Student(4, "Dan",    "", Gender.MALE, ""));
        students.add(new Student(5, "Dennis", "", Gender.MALE, ""));
        students.add(new Student(6, "Dan",    "", Gender.MALE, ""));

        // This works, if you add the Comparable interface the class.
        // SortingUtils.quickSort(students); 

        // Most of the time, you will have a custom Comparator
        SortingUtils.quickSort(students, new Comparator<Student>() {
            @Override
            public int compare(Student student1, Student student2) {
                int diff = student1.getGivenName().compareTo(student2.getGivenName());
                if (diff != 0) return diff;
                return Long.compare(student1.getId(), student2.getId());
            }
        });

        for (Student student : students) {
            System.out.println(student);
        }
    }
}

Output

Student [id=3, givenName=Dan, surname=, gender=MALE, course=]
Student [id=4, givenName=Dan, surname=, gender=MALE, course=]
Student [id=6, givenName=Dan, surname=, gender=MALE, course=]
Student [id=1, givenName=Dennis, surname=, gender=MALE, course=]
Student [id=5, givenName=Dennis, surname=, gender=MALE, course=]
Student [id=2, givenName=Nico, surname=, gender=MALE, course=]

SortingUtils

import java.util.Comparator;
import java.util.List;

public class SortingUtils {
    public static <T> List<T> quickSort(List<T> list, Comparator<T> comparator) {
        return quickSort(list, comparator, 0, list.size() - 1);
    }

    public static <T extends Comparable<T>> List<T> quickSort(List<T> list) {
        return quickSort(list, 0, list.size() - 1);
    }

    private static <T> List<T> quickSort(List<T> list, Comparator<T> comparator, int left, int right) {
        int ll = left;
        int rr = right;

        if (rr > ll) {
            T pivot = list.get((ll + rr) / 2);
            while (ll <= rr) {
                while (ll < right && comparator.compare(list.get(ll),  pivot) < 0) {
                    ll += 1;
                }
                while (rr > left && comparator.compare(list.get(rr),  pivot) > 0) {
                    rr -= 1;
                }
                if (ll <= rr) {
                    swap(list, ll, rr);
                    ll += 1;
                    rr -= 1;
                }
            }
            if (left < rr)  quickSort(list, comparator, left, rr);
            if (ll < right) quickSort(list, comparator, ll, right);
        }

        return list;
    }

    private static <T extends Comparable<T>>List<T> quickSort(List<T> list, int left, int right) {
        int ll = left;
        int rr = right;

        if (rr > ll) {
            T pivot = list.get((ll + rr) / 2);
            while (ll <= rr) {
                while (ll < right && list.get(ll).compareTo(pivot) < 0) {
                    ll += 1;
                }
                while (rr > left && list.get(rr).compareTo(pivot) > 0) {
                    rr -= 1;
                }
                if (ll <= rr) {
                    swap(list, ll, rr);
                    ll += 1;
                    rr -= 1;
                }
            }
            if (left < rr)  quickSort(list, left, rr);
            if (ll < right) quickSort(list, ll, right);
        }

        return list;
    }

    private static <T> void swap(List<T> list, int left, int right) {
        T temp = list.get(left);
        list.set(left, list.get(right));
        list.set(right, temp);
    }
}

Student

public class Student implements Comparable<Student> {
    private long id;
    private String givenName;
    private String surname;
    private Gender gender;
    private String course;

    public Student(long id, String givenName, String surname, Gender gender, String course) {
        this.id = id;
        this.givenName = givenName;
        this.surname = surname;
        this.gender = gender;
        this.course = course;
    }

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }

    public String getGivenName() {
        return givenName;
    }

    public void setGivenName(String givenName) {
        this.givenName = givenName;
    }

    public String getSurname() {
        return surname;
    }

    public void setSurname(String surname) {
        this.surname = surname;
    }

    public Gender getGender() {
        return gender;
    }

    public void setGender(Gender gender) {
        this.gender = gender;
    }

    public String getCourse() {
        return course;
    }

    public void setCourse(String course) {
        this.course = course;
    }

    @Override
    public String toString() {
        return String.format("Student [id=%s, givenName=%s, surname=%s, gender=%s, course=%s]", id, givenName, surname,
                gender, course);
    }

    /** Usually logic is left out of a POJO. */
    @Override
    public int compareTo(Student student) {
        return this.getGivenName().compareTo(student.getGivenName());
    }
}

Gender

public enum Gender {
    UNKNOWN,
    MALE,
    FEMALE
}