Whats the wrong in the code

package myproject;
import java.util.*;

class Student{
	
	int id;
	String name;
	int sub;
	int marks[] = new int[100];
	double avg;
	char grade;
	int i;
	
	public int getId() {
		return id;
	}
	public String getName() {
		return name;
	}
	public int getSub() {
		return sub;
	}
	
	public void setId(int id) {
		this.id=id;
	}
	public void setName(String name) {
		this.name=name;
	}
	public void setSub(int sub) {
		this.sub=sub;
	}
	
	public double getAvg() {
		return avg;
	}
	public void setAvg(double avg) {
		this.avg=avg;
	}
	
	public int[] getMarks() {
	return marks;
	}
	public void setMarks(int a[]) {
		this.marks=a;
	}
	
	public char getGrade() {
		return grade;
	}
	public void setGrade(char grade) {
		this.grade=grade;
	}
	public void calcAvg() {
		int total=0;
		for(int i=0; i<marks.length; i++) {
			total= total + marks[i];
		}
		avg=(total/marks.length);
	}
	
	public void findGrade() {
		
		if(marks[i]<50) {
			grade = 'F';
		}
		else {
			if(avg>=80 && avg<=100)
				grade='O';
			if(avg>=50 && avg<=80)
				grade='A';
			if(avg<50)
				grade='F';
		}
	}
}

public class Methods8 {

	public static void main(String[] args) {
		
		Scanner sc = new Scanner(System.in);
		
		System.out.println("Enter the id ");
		int id = sc.nextInt();
		System.out.println("Enter the name ");
		String name = sc.next();
		System.out.println("Enter no of subjects ");
		int sub = sc.nextInt();
		
		int a[] = new int[sub];
		
		for(int i=0; i<sub; i++) {
			System.out.println("Enter the marks ");
			a[i]=sc.nextInt();
		}
		
		Student s = new Student();
		s.setId(id);
		s.setName(name);
		s.setSub(sub);
		
		System.out.println("Id :" +s.getId());
		System.out.println("Name :" +s.getName());
		s.calcAvg();
		s.findGrade();
		System.out.println("average :" +s.getAvg());
		System.out.println("grade :" +s.getGrade());
		
	}

}