code: public static Integer getGrade(Integer year, Integer grade, Integer onYear) {
  		if (year == null || grade == null || onYear == null)
  			return null;
  
  		grade += onYear - year;
  		if (grade < 0)
  			grade = 0;
  		else if (grade > 13)
  			grade = 13;
  
  		return grade;
  	}
docs: 学生在year年的9月份是grade年级，计算他在onYear的9月份是几年级