public void outputBarChart() {
        
        System.out.println("Overall grade distribution :");
        
        int[] frequency =  new int[11];
        
        for(int[] studentGrades : grades) {
            for(int grade : studentGrades) {
                ++frequency[grade/10];
            } 
        }
        
        for(int count = 0; count < frequency.length; count++) {
            
            if(count == 10)
                System.out.printf("%5d: ", 100);
            
            else
                System.out.printf("%2d-%2d: ", 
                        count * 10 , count * 10 + 9);
            
            for (int stars = 0; stars < frequency[count]; stars++)
                System.out.print("*");
            System.out.println();           
        }
    }