collection sort

// to user defined criteria. import java.util.*; import java.lang.*; import java.io.*;   // A class to represent a student. class Student {     int rollno;     String name, address;       // Constructor     public Student(int rollno, String name,                                String address)     {         this.rollno = rollno;         this.name = name;         this.address = address;     }       // Used to print student details in main()     public String toString()     {         return this.rollno + " " + this.name +                            " " + this.address;     } }   class Sortbyroll implements Comparator<Student> {     // Used for sorting in ascending order of     // roll number     public int compare(Student a, Student b)     {         return a.rollno - b.rollno;     } }   // Driver class class Main {     public static void main (String[] args)     {         ArrayList<Student> ar = new ArrayList<Student>();         ar.add(new Student(111, "bbbb", "london"));         ar.add(new Student(131, "aaaa", "nyc"));         ar.add(new Student(121, "cccc", "jaipur"));           System.out.println("Unsorted");         for (int i=0; i<ar.size(); i++)             System.out.println(ar.get(i));           Collections.sort(ar, new Sortbyroll());           System.out.println("\nSorted by rollno");         for (int i=0; i<ar.size(); i++)             System.out.println(ar.get(i));     } }