【java】为什么代码全部标红？之前能运行的

package com.company;

import java.util.ArrayList;
import java.util.Arrays;
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.修改学生");
        System.out.println("3.删除学生");
        System.out.println("4.查看学生");
        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 s = sc.nextLine();
        switch (s){
            case "1":
                addStudent(array);
                break;
            case "2":
                System.out.println("修改成功");
                break;
            case "3":
                System.out.println("删除成功");
                break;
            case "4":
                findStudent(array);
                break;
            case "5":
                System.out.println("退出成功");
                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 xuehao = 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.setXuehao(xuehao);
        s.setName(name);
        s.setAge(age);
        s.setAdress(adress);
        array.add(s);
        System.out.println("添加完成");
    }
    public static void findStudent(ArrayList<StudentDemo> array){
        if(array.size()==0){
            System.out.println("无信息，请添加后再查询");
            return;
        }
        for(int i=0;i<array.size();i++){
            System.out.println("学号"+"姓名"+"年龄"+"居住地");
            StudentDemo s = array.get(i);
            System.out.println(s.getXuehao()+s.getName()+s.getAge()+s.getAdress());
            System.out.println("查看成功");
        }
    }
}

package com.company;

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

    public void setXuehao(String xuehao) {
        this.xuehao = xuehao;
    }

    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;
    }
}

检查这三个配置： 

看一下你这个项目的jdk配置

jdk没配好。File --- Project Structure --- Project 和 SDK 配置jdk

题主：jdk和maven配置检查一下，应该是这两个地方的问题