Java运行报NullPointerException空指针错误，该如何排查修复？

• 空指针异常触发有两个核心原因：
  1. 你执行Student[] icsClass = new Student[MAX];时，仅创建了一个可存储Student类型对象的数组，数组内每个位置默认值都是null，没有实例化Student对象就直接调用setXxx方法，直接触发空指针。
  2. 你先执行了counter = counter + 1再用counter作为数组下标取值，假设你的MAX是10，数组合法下标范围是0~9，当counter累加到10时访问icsClass[counter]已经超出数组边界，该位置本身也是null。
• 额外逻辑问题：你当前代码仅在counter == MAX的时候才会把当前输入的学生信息存入数组，前面循环过程中输入的前9个学生数据全部没有存入数组，不符合存10个学生信息的初始需求。

1. 调整counter的累加逻辑，确认信息无误后再累加，直接用当前counter作为数组下标
2. 每输入完一个学生的信息确认无误后，直接将已经实例化完成的person对象赋值给数组对应位置即可，不需要逐个调用set方法复制属性
3. 去掉counter == MAX的判断，每个学生输入完都可以确认是否正确，输入错误可以重新录入

public static void main(String[] args) {
    // 需提前定义常量：MAX=10、MIN_YEAR=1895、MAX_YEAR=2021、MIN_MONTH=1、MAX_MONTH=12、MIN_DAY=1、MAX_DAY=31
    Student[] icsClass = new Student[MAX];
    Scanner s = new Scanner(System.in);

    int counter = 0;
    int year;
    int month;
    int day;

    System.out.println("This program will store 10 student's information.");
    System.out.println("Do you want to continue? (Yes/No)");
    String a = s.next();

    if (a.equalsIgnoreCase("Yes")) {
        while (counter < MAX) {
            Student person = new Student();
            System.out.println("===== 输入第" + (counter+1) + "个学生信息 =====");
            System.out.println("Please enter your first name:");
            person.setfName(s.next());
            System.out.println("Please enter your last name:");
            person.setlName(s.next());

            System.out.println("Please enter your year of birth:");
            year = s.nextInt();
            while ((year < MIN_YEAR) || (year > MAX_YEAR)) {
                System.out.println("Error: Year of birth must be between 1895 and 2021");
                System.out.println("Please re-enter your year of birth:");
                year = s.nextInt();
            }
            person.setYear(year);

            System.out.println("Please enter your month of birth:");
            month = s.nextInt();
            while ((month < MIN_MONTH) || (month > MAX_MONTH)) {
                System.out.println("Error: Month of birth must be between 1 and 12");
                System.out.println("Please re-enter your month of birth:");
                month = s.nextInt();
            }
            person.setMonth(month);

            System.out.println("Please enter your day of birth:");
            day = s.nextInt();
            while ((day < MIN_DAY) || (day > MAX_DAY)) {
                System.out.println("Error: Day of birth must be between 1 and 31");
                System.out.println("Please re-enter your day of birth:");
                day = s.nextInt();
            }
            person.setDay(day);

            person.display();
            System.out.println("Is the information above correct? (Yes/No)");
            String b = s.next();
            if (b.equalsIgnoreCase("Yes")) {
                // 直接把实例化好的person对象存入数组对应位置
                icsClass[counter] = person;
                // 确认无误再累加计数器
                counter++;
            } else {
                System.out.println("请重新输入该学生信息");
            }
        }
        // 遍历输出所有学生信息
        System.out.println("===== 所有学生信息如下 =====");
        for (int i = 0; i < MAX; i++) {
            icsClass[i].display();
        }
    }
    s.close();
}

内容的提问来源于stack exchange，提问作者Faruk