错误：无法从静态上下文引用非静态变量inout

Question

我正在为我的CS类开发一个涉及方法和变量的程序。我知道这是范围的问题，或者至少我认为，但不知道如何解决。我不能使用对象。代码尚未完成，因为我目前只专注于这一个问题。

import java.util.Scanner;
public class MethodBankingSystem {
Scanner input = new Scanner(System.in);

public static double Deposit(double userDeposit, double userBalance) {
         System.out.print("Enter the amount you would like to deposit: ");
            userDeposit = input.nextDouble(); //Create variable userDeposit for amount entered
            userBalance = userBalance + userDeposit; //Replace value for userBalance
            System.out.printf("New balance after deposit is: $" + "%.2f \n\n",userBalance );
            return userBalance;
        }
public static double Withdraw(double userWithdraw, double userBalance) {
        System.out.print("Enter the amount you would like to withdraw: ");
            userWithdraw = input.nextDouble(); //Create variable userWithdraw for amount entered
            if (userWithdraw > userBalance) { //Create if/else statement to make sure enough funds are available
                System.out.printf("Funds are not available for this withdraw \n\n");
            }
            else {
                userBalance = userBalance - userWithdraw; //Replace value for userBalance
                System.out.printf("New balance after withdraw is: $" + "%.2f \n\n",userBalance );
            }
            return userBalance;
        }
public static void CurrentBalance(double userBalance) {
        System.out.printf("Current balance is $" + "%.2f \n\n",userBalance ); 
    }







public static void main(String[] args) {
 Scanner input = new Scanner(System.in);
 double userBalance = 100.00;
 double userDeposit;
 double userWithdraw;
 double userInput;
 String cardNumber;

    String menu = "Welcome to Mustang Bank\nPlease choose from the options below:\n1 - Make a Deposit\n" +
            "2 - Make a Withdraw\n3 - Check your Current Balance\n9 - Exit the Program\nEnter Option:\n";





    do {
        System.out.print(menu);
        userInput = input.nextDouble();

        switch (userInput) {
        case '1':
            Deposit(userDeposit, userBalance);
            break;
        case '2':
            Withdraw(userWithdraw, userBalance);
            break;
        case '3':
            CurrentBalance(userBalance);
            break;
        case '4':
            CurrentBalance(userBalance);
            break;
        default:
            System.out.println("This option is not acceptable.");
        }

        } while (userInput !=9);
    }





    }

Answer 1

基本上问题是：

static表示每个类只有一个该字段的实例（不是实例）。该字段基本上在所有实例之间共享;在一个实例中修改它将在所有实例中影响它。您可以在实例中引用静态字段，但不能静态引用实例成员。

要么input静态（不是真的推荐）：

static Scanner input = new Scanner(System.in);

或者创建一个MethodBankingSystem的实例并从你的方法中删除静态修饰符（这是我认为你试图做的）：

import java.util.Scanner;
public class MethodBankingSystem {
    Scanner input = new Scanner(System.in);

    public double Deposit(double userDeposit, double userBalance) {
             System.out.print("Enter the amount you would like to deposit: ");
                userDeposit = input.nextDouble(); //Create variable userDeposit for amount entered
                userBalance = userBalance + userDeposit; //Replace value for userBalance
                System.out.printf("New balance after deposit is: $" + "%.2f \n\n",userBalance );
                return userBalance;
    }

    public double Withdraw(double userWithdraw, double userBalance) {
            System.out.print("Enter the amount you would like to withdraw: ");
                userWithdraw = input.nextDouble(); //Create variable userWithdraw for amount entered
                if (userWithdraw > userBalance) { //Create if/else statement to make sure enough funds are available
                    System.out.printf("Funds are not available for this withdraw \n\n");
                }
                else {
                    userBalance = userBalance - userWithdraw; //Replace value for userBalance
                    System.out.printf("New balance after withdraw is: $" + "%.2f \n\n",userBalance );
                }
                return userBalance;
    }
    public void CurrentBalance(double userBalance) {
        System.out.printf("Current balance is $" + "%.2f \n\n",userBalance ); 
    }







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

    MethodBankingSystem () {

     Scanner input = new Scanner(System.in);
     double userBalance = 100.00;
     double userDeposit;
     double userWithdraw;
     double userInput;
     String cardNumber;

    String menu = "Welcome to Mustang Bank\nPlease choose from the options below:\n1 - Make a Deposit\n" +
            "2 - Make a Withdraw\n3 - Check your Current Balance\n9 - Exit the Program\nEnter Option:\n";





    do {
        System.out.print(menu);
        userInput = input.nextDouble();

        switch (userInput) {
        case '1':
            Deposit(userDeposit, userBalance);
            break;
        case '2':
            Withdraw(userWithdraw, userBalance);
            break;
        case '3':
            CurrentBalance(userBalance);
            break;
        case '4':
            CurrentBalance(userBalance);
            break;
        default:
            System.out.println("This option is not acceptable.");
        }

        } while (userInput !=9);
    }



    }

我在修改问题之前修改了你的代码

因为我是一个好人，我修改了你的代码。有各种各样的问题，请阅读它们。你创建一个输入实例两次，你正在打开一个你正在调用存款的双重（userDeposit，userBalance）;如果没有initialisinguserDeposit，你声明cardNumber但是从不实例化甚至尝试在任何地方使用它，你就会遇到静态作用域的问题。

import java.util.Scanner;

public class MethodBankingSystem {

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

    Scanner input;
    double userBalance = 100.00;

    MethodBankingSystem() {

        input = new Scanner(System.in);
        int userInput;

        String menu = "Welcome to Mustang Bank\nPlease choose from the options below:\n" + "1 - Make a Deposit\n"
                + "2 - Make a Withdraw\n" + "3 - Check your Current Balance\n"
                + "9 - Exit the Program\nEnter Option:\n";

        do {
            System.out.print(menu);
            userInput = input.nextInt();
            switch (userInput) {
                case 1:
                    Deposit();
                    break;
                case 2:
                    Withdraw();
                    break;
                case 3:
                    CurrentBalance();
                    break;
                case 9:
                    break;
                default:
                    System.out.println("This option is not acceptable.");
            }

        } while (userInput != 9);

        input.close();
    }

    public double Deposit() {
        System.out.print("Enter the amount you would like to deposit: ");
        double userDeposit = input.nextDouble(); // Create variable userDeposit for amount entered
        userBalance = userBalance + userDeposit; // Replace value for userBalance
        System.out.printf("New balance after deposit is: $" + "%.2f \n\n", userBalance);
        return userBalance;
    }

    public double Withdraw() {
        System.out.print("Enter the amount you would like to withdraw: ");
        double userWithdraw = input.nextDouble(); // Create variable userWithdraw for amount entered
        if (userWithdraw > userBalance) { // Create if/else statement to make sure enough funds are available
            System.out.printf("Funds are not available for this withdraw \n\n");
        } else {
            userBalance = userBalance - userWithdraw; // Replace value for userBalance
            System.out.printf("New balance after withdraw is: $" + "%.2f \n\n", userBalance);
        }
        return userBalance;
    }

    public void CurrentBalance() {
        System.out.printf("Current balance is $" + "%.2f \n\n", userBalance);
    }

}