Sort an array of custom objects in descending order on an int property

Question

I would like to sort my array in descending order by year of birth. My array has two other elements which are of type String. So, as an example the person who was born in the earliest year, such as 1939, would be at the top, then so on.

Here is my code:

import java.util.*;
public class StudentInformationTest
{
public static void main (String [] args){ 
    StudentInformation[] studentInfo = new StudentInformation[10];

    studentInfo[0] = new StudentInformation("Student A",1971, "BSc FDIT");
    studentInfo[1] = new StudentInformation("Student B",1964, "BSc FDIT"); 
    studentInfo[2] = new StudentInformation("Student C",1996, "BSc FDIT"); 
    studentInfo[3] = new StudentInformation("Student D",1939, "BSc FDIT"); 
    studentInfo[4] = new StudentInformation("Student E",1945, "BSc FDIT"); 
    studentInfo[5] = new StudentInformation("Student F",1991, "BSc FDIT"); 
    studentInfo[6] = new StudentInformation("Student G",1987, "BSc FDIT"); 
    studentInfo[7] = new StudentInformation("Student H",1968, "BSc FDIT"); 
    studentInfo[8] = new StudentInformation("Student I",1968, "BSc FDIT"); 
    studentInfo[9] = new StudentInformation("Student J",1973, "BSc FDIT"); 

    printInfo(studentInfo);
    printAge(studentInfo);
}
public static void printInfo(StudentInformation studentInfo[]){
    for(int i = 0; i < studentInfo.length; i++){
        System.out.println(studentInfo[i].getStudentName() + " " +   studentInfo[i].getBirthDate() + " " + studentInfo[i].getProgrammeOfStudy());
    }
    System.out.println();
}

 }

}

Once I manage to print the birth years in descending order I also need to show the student name and the university modules they are doing. I know other questions have been asked how to do this but I have not been able to see one with other objects. This is a class session so please forgive any errors in my code.

Answer 1

Use a Comparator and an ArrayList.

In Java 8

Use the new default and static methods on Comparator!

ArrayList<StudentInformation> infos = new ArrayList<StudentInformation>();
// fill array
Collections.sort(infos, 
    Comparator.comparingInt(StudentInformation::getBirthYear).reversed());

It's a brave new world! :)

Or—still better than Java 7—use lambdas!

ArrayList<StudentInformation> infos = new ArrayList<StudentInformation>();
// fill array
Collections.sort(infos, (s1, s2) ->
    Integer.compare(s2.getBirthYear(), s1.getBirthYear()));

In Java 7

Use anonymous inner classes.

class StudentDateComparator implements Comparator<StudentInformation> {
    public int compare(StudentInformation s1, StudentInformation s2) {
        return Integer.compare(s2.getBirthYear(), s1.getBirthYear());
    }
}

ArrayList<StudentInformation> infos = new ArrayList<StudentInformation>();
// fill array
Collections.sort(infos, new StudentDateComparator());

Explanation

What the Comparator does is allows anything to compare two objects of the given type (in this case, StudentInformation). You could also make StudentInformation implement Comparable<StudentInformation>, but this way is probably better because there is more than one way to compare student informations (by date, as here, but also by first name, last name, number of classes enrolled, etc.).

By swapping the order of s1 and s2 in the comparator, we induce a reverse order. Another way to do this would be to negate the compare call in the normal order, or to use a normal comparator and wrap it in Collections.reverseOrder.

---

You could also do this with a standard array.

StudentInformation[] infos = new StudentInformation[10];
// fill array
Arrays.sort(infos, new StudentDateComparator());

Answer 2

In the Java land, there is the Comparable<E> interface:

interface Comparable<E> {
  public int compareTo(E other);
}

You should make your StudentInfo implement Comparable<StudentInfo>, implement compareTo(StudentInfo other) according to your requirements, and either use methods from the standard library (but this will require a Comparator instead) or some sorting algorithm suitable for your case.