java - 为什么我的程序没有运行并抛出 "Exception in thread "main"java.lang.NullPointerException"

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 6 年前。

现在，按照建议编辑我的程序后，我在运行程序时没有收到任何显示消息。 AccountBalance.java 正在编译，AccountBalance.class 文件正在生成，但是当程序运行时，它什么也不显示，只有光标在闪烁。

class Balance {
    private Scanner bank;
    public int userAccount;
    public int bankAccountNumber;

    void account()
    {   bank = new Scanner(System.in);
        int openingAmount = bank.nextInt();
        System.out.print("Please deposit an amount more than Rs. 1000.00 to open a Bank account: " + openingAmount);

        if (openingAmount > 1000) {
            System.out.println("Your Bank account has opened successfully");
            userAccount = openingAmount;

            System.out.println("Enter your Bank account number : ");
            bank = new Scanner(System.in);
            bankAccountNumber = bank.nextInt();
        } else{ System.out.println("Bank account not opened.\nDo you want to open a bank account then..!!");
            this.account(); //Ask again for opening an account
        }
    }
    void withdrawal() {
        bank = new Scanner(System.in);
        int w = bank.nextInt();
        int b = userAccount - w;
        System.out.println("Withdrawal Amount is : " + w  );
        if (w < 100)
        {
            System.out.println("Unable to process your request");
        } else {
            System.out.println("Your Balance Amount is : " + b );
        }
    }

    void deposit() {
        bank = new Scanner(System.in);
        int d = bank.nextInt();
        System.out.println("Deposited amount is : ");
        userAccount += d;
        System.out.println("Your Balance Amount is : " + userAccount );
    }
}

public class AccountBalance {
    public static void main(String[] args) {
        Balance s = new Balance();
        s.account();
        s.withdrawal();
        s.deposit();
        System.out.println(" Current account balance is : "+s.userAccount);
    }
}

当我尝试运行该程序时，出现以下错误:

“线程“main”java.lang.NullPointerException 中出现异常”

我正在尝试编写一个有 2 个类的程序，其中 1 个类具有 main 方法。开设银行账户的最低金额为卢比。 1000, 开户金额、银行帐号、存取款由用户输入。需要显示当前帐户余额以及一个人是否继续该帐户，他/她可以选择一个选项来关闭它，程序将终止。我做错了什么......如何应对并解决它......？

import java.util.Scanner;
class Balance
{
    private Scanner bank;
    public int userAccount;
    public int bankAccountNumber;
    public int balance;

    public void account()
    {
        balance = bank.nextInt();
        System.out.print("Please deposit an amount of minimum Rs. 1000.00 to open a Bank account : ");

        if (balance >= 1000)
        {
            System.out.println("Account opened successfully");
            bankAccountNumber = bank.nextInt();
            System.out.println("Enter your 4 digit Bank account number : ");
        } else {
            System.out.println("Bank account not opened.\nDo you want to open a bank account then..!!");
            this.account(); //Ask again for opening an account
        }
    }
    void withdrawal() {
        int w = bank.nextInt();
        int b = userAccount - w;
        System.out.println("Withdrawal Amount is : " + w  );
        if (w < 100)
        {
            System.out.println("Unable to process your request");
        } else {
            System.out.println("Your Balance Amount is : " + b );
        }
    }

    void deposit() {
        int d = bank.nextInt();
        System.out.println("Deposited amount is : ");
        userAccount += d;
        System.out.println("Your Balance Amount is : " + userAccount );
    }
}

public class AccountBalance {
    public static void main(String[] args) {
        Balance s = new Balance();
        s.account();
        s.withdrawal();
        s.deposit();
        System.out.println("Your current balance is : " + s.userAccount);
    }
}

Answer 1

为您的 Balance 类编写一个构造函数并在其中初始化 Scanner。

    private Scanner bank;

扫描仪已定义但未初始化。将此构造函数添加到您的 Balance 类中。

public Balance()
{
    bank = new Scanner(System.in);
}