【JAVA】学生管理系统，调用方法时候出现not null提示，运行了以后方法无效

Question

package 复习试炼场;

import java.util.ArrayList;
import java.util.Scanner;

public class Student {
    public static void main(String[] args) {
        ArrayList<StudentDemo> array=new ArrayList<StudentDemo>();
        while (true){
        System.out.println("--------欢迎来到学生管理系统--------");
        System.out.println("1.添加学生");
        System.out.println("2.删除学生");
        System.out.println("3.查看学生");
        System.out.println("4.修改学生");
        System.out.println("5.退出系统");
        System.out.println("请输入你的选择:");
        Scanner sc=new Scanner(System.in);
        String line = sc.nextLine();
        switch (line){
            case "1":
//                System.out.println("添加成功");
                addStudent(array);
                break;
            case "2":
                System.out.println("删除成功");
                break;
            case "3":
                findStudent(array);
                System.out.println("查看成功");
                break;
            case "4":
                System.out.println("修改成功");
                break;
            case "5":
                System.out.println("退出学生管理系统");
                System.exit(0);
        }
    }
}
    public static void addStudent(ArrayList<StudentDemo> array){
        Scanner sc=new Scanner(System.in);
        System.out.println("请输入你的学号:");
        String sid = sc.nextLine();
        System.out.println("请输入你的姓名:");
        String name = sc.nextLine();
        System.out.println("请输入你的年龄:");
        String age = sc.nextLine();
        System.out.println("请输入你的居住地:");
        String adress = sc.nextLine();
        StudentDemo s=new StudentDemo();
        s.setSid(sid);
        s.setName(name);
        s.setAge(age);
        s.setAdress(adress);
    }
    public static void findStudent(ArrayList<StudentDemo> array){
        for(int i=0;i<array.size();i++){
            System.out.println("学号\t姓名\t年龄\t居住地");
            StudentDemo s=array.get(i);
            System.out.println(s.getSid()+"\t"+s.getName()+"\t"+s.getAge()+"岁"+s.getAdress());
        }
    }
}

package 复习试炼场;

public class StudentDemo {
        private String sid;
        private String name;
        private String age;
        private String adress;
        public StudentDemo() {
        }
        public StudentDemo(String sid, String name, String age, String adress) {
                this.sid = sid;
                this.name = name;
                this.age = age;
                this.adress = adress;
        }
        public String getSid() {
                return sid;
        }

        public void setSid(String sid) {
                this.sid = sid;
        }

        public String getName() {
                return name;
        }

        public void setName(String name) {
                this.name = name;
        }

        public String getAge() {
                return age;
        }

        public void setAge(String age) {
                this.age = age;
        }

        public String getAdress() {
                return adress;
        }

        public void setAdress(String adress) {
                this.adress = adress;
        }
}

Answer 1

package 复习试炼场;
import java.util.ArrayList;
import java.util.Scanner;
public class Student {
    public static void main(String[] args) {
        ArrayList<StudentDemo> array=new ArrayList<StudentDemo>();
        while (true){
        System.out.println("--------欢迎来到学生管理系统--------");
        System.out.println("1.添加学生");
        System.out.println("2.删除学生");
        System.out.println("3.查看学生");
        System.out.println("4.修改学生");
        System.out.println("5.退出系统");
        System.out.println("请输入你的选择:");
        Scanner sc=new Scanner(System.in);
        String line = sc.nextLine();
        switch (line){
            case "1":
//                System.out.println("添加成功");
                addStudent(array);
                break;
            case "2":
                System.out.println("删除成功");
                break;
            case "3":
                findStudent(array);
                System.out.println("查看成功");
                break;
            case "4":
                System.out.println("修改成功");
                break;
            case "5":
                System.out.println("退出学生管理系统");
                System.exit(0);
        }
    }
}
    public static void addStudent(ArrayList<StudentDemo> array){
        Scanner sc=new Scanner(System.in);
        System.out.println("请输入你的学号:");
        String sid = sc.nextLine();
        System.out.println("请输入你的姓名:");
        String name = sc.nextLine();
        System.out.println("请输入你的年龄:");
        String age = sc.nextLine();
        System.out.println("请输入你的居住地:");
        String adress = sc.nextLine();
        StudentDemo s=new StudentDemo();
        s.setSid(sid);
        s.setName(name);
        s.setAge(age);
        s.setAdress(adress);
        array.add(s);
    }
    public static void findStudent(ArrayList<StudentDemo> array){
        for(int i=0;i<array.size();i++){
            System.out.println("学号\t姓名\t年龄\t居住地");
            StudentDemo s=array.get(i);
            System.out.println(s.getSid()+"\t"+s.getName()+"\t"+s.getAge()+"岁"+s.getAdress());
        }
    }
}

你的问题在于添加学生的时候，没有往集合中存学生对象，在添加学生的方法最后加array.add(s);就行了