空指针异常是什么意思

Question

public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
Students1[] stus = new Students1[3];
Students1Biz biz = new Students1Biz();

    Students1 stu1 = new Students1();
    stu1.score = 96;

    Students1 stu2 = new Students1();
    stu2.score = 56;

    Students1 stu3 = new Students1();
    stu3.score = 78;

    stus[0] = stu1;
    stus[1] = stu2;
    stus[2] = stu3;
/*  
    for (int i = 0; i < stus.length; i++) {
        System.out.println("请输入" + (i + 1) + "个学员的成绩：");
        stu[i].score = scanner.nextInt();
    }
    */
    biz.seaScore(stus);

}
为什么stu[i].score = scanner.nextInt();
不能接受输入数据 报空指针异常

Answer 1

空指针异常就是对象为空
java一切都是对象，比如，字符串有个length()方法，
但是，null是没length方法的，你String.length()是没问题的，但是，null.length()那就肯定有问题，这就是空指针异常