在 Java ATM 项目中使用 If 语句进行切换

Question

现在我正在处理一个ATM 项目。 用户将输入他们的姓名、余额、交易类型和交易金额。交易类型为Withdraw (W)、Deposit (D)和R (Report)，我用的是switch，有3个casesW、D、R。如果您出现以下情况，程序将输出错误： 输入错误代码，您没有足够的资金来进行您选择的交易类型，并且您输入的金额为负数。现在我很难设置switch。我的代码如下，任何帮助将不胜感激！谢谢

import java.io.*;
import java.util.*;
public class atmValidator {

   public static void main(String[] args) {

      String fname = " ", lname = " ";

      double balbefore = 0.0, balafter = 0.0, transamount = 0.0;

      char code = ' ';

      Scanner screen = null;
      FileInputStream fis = null;
      FileOutputStream fos = null;
      PrintWriter pw = null;

      //Declaring the variables/scanner/etc.

      try {
        fis = new FileInputStream ("transactions.txt");
        fos = new FileOutputStream ("statement.txt");

        screen = new Scanner (fis);
        pw = new PrintWriter (fos);

      } catch (FileNotFoundException e) {
          System.out.println("File not found");
          System.exit(0);
      }

      //The try/catch for reading from the file

      while (screen.hasNext()) {
        fname = screen.next();
        lname = screen.next();
        balbefore = screen.nextDouble();
        code = screen.next().charAt(0);
        transamount = screen.nextInt();

      //A while loop to gather the info from the file, and the variables getting their values from the files.

        switch (code) {

            case 'D':
                balafter = balbefore + transamount;
                if (transamount <0) {
                    pw.println("ERROR: Enter positive amount.");
                }
                else if (balafter < 300) {
                    pw.println("Warning, balance below $300");
                }

                {

                   // Where I am stuck at :(   
                }
          }
       }        
       screen.close();
       pw.close();

    }   
}
//Sorry for the weird braces and spaces

Answer 1

您可以在开关体内编写如下案例。您可以进一步延长条件，但在这里我只是给您一个如何做的例子。如果字符不是 D、W 或 R，则执行默认大小写。

    switch (code) {
        case 'D':
            balafter = balbefore + transamount;
            System.out.println("");
            if (balafter < 0) {
                pw.println("Insufficent funds");

            } else if (balafter < 300) {
                pw.println("Warning, balance below $300");
            }else{
                pw.println("Well. You have now " + balafter + "$");
            }
            break;
        case 'W':
            balafter = balbefore - transamount;
            if (transamount > balbefore) {
                pw.println("Sorry. You don't enough funds.");

            } else if (transamount <= balbefore && balafter < 300) {
                pw.println("Warning, balance below $300");
            } else {
                pw.println("The withdrawl was successful.");
            }
            break;
        case 'R':
            balafter = balbefore + transamount;
            if (balafter < 0) {
                pw.println("Insufficent funds");

            } else if (balafter < 300) {
                pw.println("Warning, balance below $300");
            } else {
                pw.println("Good. Your balance is " + balAfter);
            }
            break;
        default:
            System.out.println("Nothing to do.");
            break;
    }

Answer 2

你快到了！在每个案例之后，您需要跟进break; 并转到下一个case。最后，您需要有一个default: break，它的意思是“如果变量不适合任何情况，则退出开关”。

所以 switch 的工作方式如下：

switch(code) {
    case 'D': 
        // do deposit code
        break;
    case 'W':
        // do withdrawl code
        break;
    ...
    default:
        break;
}

Switch 具有不同的语法，因为它没有花括号来定义代码块。它宁愿寻找 break; 和下面的 case: 或 default: 继续前进。

https://docs.oracle.com/javase/tutorial/java/nutsandbolts/switch.html 有很好的例子供你学习。

评论后编辑：

针对您的评论，您需要执行以下操作：

case 'W':
    tempBal = balbefore - transamount;
    if(tempBal < 0) {
        System.out.println("Insufficient Funds");
    }
    else {
        balafter = balbefore - transamount;
        if(balafter < 300) {
            System.out.println("Warning, balance below $300");
        }
    }
    break;
...
default:
    System.out.println("Please select "D", "W", or "R");
    break;