“线程中的异常”；“主要”；java.lang.NullPointerException cann'；不要给班级写信

所以我有这个代码：

package Firstpack;

import java.io.*;
import java.util.*;

public class Main {

    public static void menu() {
        System.out.println("Welcome");
        System.out.println("1. Add a record ");
        System.out.println("2. See all records ");
        System.out.println("3. See a category");
        System.out.println("4. Total spend(Year)");
        System.out.println("5. Spend in a month");
        System.out.println("6. Chose by index");
        System.out.println("7. Exit ");

        System.out.print(">");
        Scanner in = new Scanner(System.in);
        int enteredInt = in.nextInt();
        in.close();

        switch (enteredInt) {
        case 1:
            recording();
            break;
        case 2:
            System.out.print(" You have chosen -> See all records");
            break;
        case 3:
            System.out.print(" You have chosen -> See a category");
            break;
        case 4:
            System.out.print(" You have chosen -> Total spend(Year)");
            break;
        case 5:
            System.out.print(" You have chosen -> Spend in a month");
            break;
        case 6:
            System.out.print(" You have chosen -> Chose by index");
            break;
        case 7:
            System.out.print(" Bye! ");
            break;
        default:
            menu();
        }

    }

    public static void recording() {
        System.out.println(" You have chosen -> Add a record");

        record rec = new record();
        Scanner in = new Scanner(System.in);

        System.out.print("Enter Amount > ");
        rec.amount = in.nextDouble();
        System.out.print("Enter Category > ");
        rec.category = in.next();
        System.out.print("Enter Details > ");
        rec.details = in.next();

        try {
            FileWriter fw = new FileWriter("findme.txt");
            BufferedWriter bw = new BufferedWriter(fw);

            bw.write(String.valueOf(rec.amount));
            bw.write(rec.category);
            bw.write(rec.details);

            bw.close();
            fw.close();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

    public static void main(String[] args) {
        menu();
    }
}

这让我想到：

Exception in thread "main" java.lang.NullPointerException
    at java.io.Writer.write(Unknown Source)
    at Firstpack.Main.recording(Main.java:68)
    at Firstpack.Main.menu(Main.java:25)
    at Firstpack.Main.main(Main.java:82)

---

我不知道如何解决这个问题。这里怎么了

问题是因为在菜单方法中调用了.close（）。不仅

in.close（）

关闭

扫描仪

对象，而且它还碰巧关闭

系统.in

。当您输入1作为菜单选项时，将调用

recording（）

方法，该方法创建一个新的

扫描仪

，该扫描仪尝试使用现在关闭的

系统

这是一篇针对类似问题的文章：
什么是在Maul.java中的68行？类<代码>记录>代码>什么？考虑使用<代码> java .IO。控制台< /C> >代替<代码>扫描器< /C> >。它对我来说很好。尝试将硬编码的值写入文件，如果这样做有效，请在写入值之前确保这些值是正确的。