Java:j' ai une erreur dans mon prg

import java.io.*;  

public class StudentMarks {  
 int r1 = 0, r2 = 0, r3 = 0, r4 = 0, rn = 1;  
 static BufferedReader br;  
 public StudentMarks(int num) {  
  int arr[] = new int[num];  
  try {  
   for (int i = 0; i < arr.length; i++) {  
    System.out.println("enter the marks for roll nb" + rn);  
    arr[i] = Integer.parseInt(br.readLine());  
    rn++;  
   }  
   for (int j = 0; j < arr.length; j++) {  
    if (arr[j] >= 0 && arr[j] <= 40) {  
     r1++;  
    } else if (arr[j] > 40 && arr[j] <= 60) {  
     r2++;  
    } else if (arr[j] > 60 && arr[j] <= 80) {  
     r3++;  
    } else if (arr[j] > 80 && arr[j] <= 100) {  
     r4++;  
    }  
   }  
   System.out.println("Students securing marks in the range of 0-40 are \t"+ r1);  
   System.out.println("Students securing marks in the range of 40-60 are \t"+ r2);  
   System.out.println("Students securing marks in the range of 60-80 are \t"+ r3);  
   System.out.println("Students securing marks in the range of 80-100 are \t"+ r4);  
  }  
  // end of try  
  catch (Exception exp) {  
   System.out.println(exp);  
  }  
  // end catch  
 }  

 // end constructor  
 public static void main(String args[]) {  
    
  try {  
   br = new BufferedReader(new InputStreamReader(System.in));  
   System.out.println("Enter the total nb of students in the class");  
   int total_students = Integer.parseInt(br.readLine());  
   StudentMarks sm = new StudentMarks(total_students);  
  }  
  // end of try  
  catch (Exception exp) {  
   System.out.println(exp.getMessage());  
  }// end of catch  
 }// end main  
}// end of class