使用Java创建GPA报告（eclipse）

Question

我目前正在尝试创建一个程序，该程序将询问用户注册用户的类数，创建一个数组来存储该数据，然后最终在print语句中显示所有这些信息。我并不担心最终的打印声明，但我似乎无法让数组工作。程序中没有错误，但一旦它要求我输入类号，它返回“线程中的异常”主“java.lang.NullPointerException     在GPAreport.main（GPAreport.java:24）“

下面是代码：

import java.util.*;
public class GPAreport {
public static void main (String [] args) { 
int classnumber[] = null; 
double gpa[];
String ClassDescription[] = null;
Scanner myScanner = new Scanner(System.in); 
String LetterGrade = "";
double GradePoints = 0;
double PercentageGrade = 0;
String currentterm, studentname = "";
int numberofclasses;

System.out.println("Please enter the current term (ex: fall 2011):");
currentterm = myScanner.nextLine();
System.out.println("Please enter Student Name:");
studentname = myScanner.nextLine();
System.out.println("Please enter number of classes:");
numberofclasses = myScanner.nextInt();

for(int i = 0; i <= numberofclasses ; i++){ 
    System.out.println("Input Class Number: " + (i+1));
    classnumber[i] = myScanner.nextInt();
    System.out.println("Input class description: " + (i+1));
    ClassDescription[i] = myScanner.nextLine();
    System.out.println("Input percentile grade for: " + classnumber[i]);
    PercentageGrade = myScanner.nextDouble();

        if ((PercentageGrade < 100) && (PercentageGrade >= 93 )){
        LetterGrade = "A";
        GradePoints = 4.0; 
            }
        else if ((PercentageGrade > 100)){ 
        LetterGrade = "A+";
        GradePoints = 4.0;
            }
        else if ((PercentageGrade < 93) && (PercentageGrade >= 90 )){ 
        LetterGrade = "A-";
        GradePoints = 4.0;
            }
        else if ((PercentageGrade < 90) && (PercentageGrade >= 83 )){ 
        LetterGrade = "B";
        GradePoints = 3.0;
            }
        else if ((PercentageGrade < 83) && (PercentageGrade >= 80 )){ 
        LetterGrade = "B-";
        GradePoints = 3.0;
            }
        else if ((PercentageGrade < 80) && (PercentageGrade >= 73 )){ 
        LetterGrade = "C";
        GradePoints = 2.0;
            }
        else if ((PercentageGrade < 73) && (PercentageGrade >= 70 )){ 
        LetterGrade = "C-";
        GradePoints = 2.0;
            }
        else if ((PercentageGrade < 70) && (PercentageGrade >= 63 )){ 
        LetterGrade = "D";
        GradePoints = 1.0;
            }
        else if ((PercentageGrade < 63) && (PercentageGrade >= 60 )){ 
        LetterGrade = "D-";
        GradePoints = 1.0;
            }
        else if ((PercentageGrade < 60) && (PercentageGrade >= 0)){ 
        LetterGrade = "F";
        GradePoints = 0;
            }
        else if (PercentageGrade < 0){ 
        System.out.println("That is not a valid score. Please try again");
            }

}

}

}