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

Question

现在，按照建议编辑我的程序后，我在运行程序时没有收到显示消息。帐户余额.java正在编译，帐户余额.class文件正在生成，但是当程序运行时，它什么都不显示，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：

"Exception in thread 'main' java.lang.NullPointerException">

我正在尝试编写一个程序，该程序有 2 个类，其中 1 个具有 main 方法。开立银行账户的最低金额为1000卢比，开户金额，银行帐号存款和取款由用户输入。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..如何应对和修复它...？

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

类 Balance 编写一个构造函数并初始化其中的扫描仪。

    private Scanner bank;

扫描程序已定义，但未初始化。将此构造函数添加到 Balance 类中。

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

Answer 2

尝试在account方法开头调用balance = bank.nextInt();之前添加bank = new Scanner(System.in);，因为它没有初始化，所以它正在创建NullPointerException。