java - 餐厅菜单 : how to efficiently implement a nested loop to collect user input and conduct error checking

Question

我有这两种方法，但我无法完全找出执行其算法的最佳方法。 我正在编写一个类似于餐厅菜单并收集用户订单的程序。

The implementation is,
welcome user and present him with the menu
while the user has not entered 'q', send the user input to a method called getGoodOrderLine(String str)
this method will then check the input for the following,
- First, it must check that a number is present before trying to read it; if there is no number, the entry is an error unless it starts with ‘q’ or ‘Q’, which tells the program to quit.
- then, determine which item the user is asking for by looking at just the first letter. So an input “2 Hello” means 2 hamburgers. the assumption is that if there is a digit in the string, the digit appears before the word, for simplicity
- finally, if the first letter is not (H,C,F or D), print an error message and ask that it be re-entered.

我的问题是，我创建了一个 while 循环，它应该循环直到用户输入有效，但它似乎不起作用，这是我的代码:

 import java.util.*;
    public class MenuApp
    {
      //global variables
      public static double HAM  = 3.75;
      public static double CHEESE  = 4.10;
      public static double FRIES  = 2.50;
      public static double DRINKS  = 1.75;

      public static void main(String [] args)
      {
        //variables
        String order;
        double total = 0.0;
        boolean stopLoop;
        //print welcome message && collect order
        welcomeCustomer();
        order = collectItem();
        order = getGoodOrderLine(order);
        stopLoop = order.equalsIgnoreCase("q");


        while(!stopLoop)//while user hasnt typed q
        {
          if(order.equalsIgnoreCase("q"))
          {
            break;
          }
          order = getGoodOrderLine(order);
          //will add the value of user order to total here if order is valid
          //leave loop if useer inputs q
        }


        //ending program
        Date today = new Date();
        System.out.println("Date: " + today);
        System.out.println("Please pay " + total + "\n");
        System.out.println("End of processing");
      }
      public static void welcomeCustomer()
      {
        System.out.println("Welcome to QuickieBurger!");
        System.out.println("Hamburgers \t\t $" + HAM);
        System.out.println("cheeseBurgers\t\t $" + CHEESE);
        System.out.println("Fries\t\t\t $" + FRIES);
        System.out.println("Drinks\t\t\t $" + DRINKS+"\n");
      }
      public static String collectItem()
      {
        String userInput = null;
        Scanner kbd = new Scanner(System.in);

        System.out.println("Please place your order (e.g., 3 ham). Enter Q to quit.");
        userInput = kbd.nextLine();
        System.out.println(userInput);
        return userInput;
      }
      public static String getGoodOrderLine(String userInput)
      {
        String result = "";
        boolean pass = false;
        if(userInput.equalsIgnoreCase("q"))
        {
          return userInput;//early exit, return q
        }

        //check if it has at least a digit first
        for(char c: userInput.toCharArray())
        {
          if(Character.isDigit(c))
          {pass = true;}
        }

        //if it doesn't have a digit || string doesnt begin with a digit
        if(!Character.isDigit(userInput.charAt(0)))
        {
          if(!pass)
            System.out.println("Your entry "+ userInput + " should specify a quantity");
          else
            System.out.println("Your entry "+ userInput + " does not begin with a number");
        }
        else
        {
          //do the remaining tests here
        }
        return result;
      }

}

在测试 Character.isDigit(userInput.charAt(0)); 时，我不断收到空指针和索引越界异常

Answer 1

问题是您返回的是 空 字符串，因此 charAt(0) 给出错误，因为 char 数组没有元素。如果您想收集需要使用的项目像 list 这样的集合类型，你不能使用数组，因为数组具有固定长度。要获取用户输入产品的价格，你需要将价格与产品名称映射。所以你可以使用 map。但我使用了 2 个数组作为 map。检查它和它的输出。

import java.util.ArrayList;
import java.util.List;
import java.util.Date;
import java.util.Scanner;

public class myMenu {

    public static String names[] = {"HAM", "CHEESE", "FRIES", "DRINKS"};
    public static double prices[] = {3.75, 4.10, 2.50, 1.75};
    public static ArrayList<List<String>> allitems = new ArrayList<>();   
    static double total = 0.0;

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

    public static void welcomeCustomer() {
        System.out.println("Welcome to QuickieBurger!");
        for (int i = 0; i < names.length; i++) {
            System.out.println(names[i] + "\t\t\t" + prices[i]);
        }
    }

    public static void collectItem() {
        String userInput = "";
        Scanner kbd = new Scanner(System.in);
        System.out.println("Please place your order (e.g., 3 ham). Enter Q to quit.");
        userInput = kbd.nextLine();
        while (!getGoodOrderLine(userInput)) {
           userInput = kbd.nextLine();
        }

    }
    private static boolean getGoodOrderLine(String userInput) {

        if (userInput.equalsIgnoreCase("q")) {
            transaction();
        } else if (!Character.isDigit(userInput.charAt(0))) {
            System.out.println("quesntity should be specified. try again");
            return false;
        } else {
            for (int i = 0; i < names.length; i++) {
                String items = names[i];
                //get the first charactor from userinput
                char c = 0;
                for(int z=0;z<userInput.length();z++){
                    c=userInput.charAt(z);
                    if(Character.isAlphabetic(c)){
                        break;
                    }
                }
                if (Character.toLowerCase(items.charAt(0)) ==Character.toLowerCase(c)) {

                    String s="";
                    int x=0;
                    while(Character.isDigit(userInput.charAt(x))){
                        s+=userInput.charAt(x);
                        x++;
                    }
                    int quentity=Integer.parseInt(s);
                    double pri = prices[i];
                    double sub = quentity * pri;
                    total += sub;
                    ArrayList<String> subitem = new ArrayList<>();
                    subitem.add(items);
                    subitem.add(String.valueOf(quentity));
                    subitem.add(String.valueOf(sub));
                    allitems.add(subitem);
                    return false;
                }
            }
            System.out.println("this not a valid food item.try again");
        }

        return false;

    }

    private static void transaction() {
        //ending program
        Date today = new Date();
        System.out.println("-------------------------------------");
        System.out.println("Date: " + today);
        for (List<String> menu : allitems) {
            System.out.println(menu.get(0)+"   "+menu.get(1)+"  = "+menu.get(2));
        }
        System.out.println("Please pay " + total + "\n");
        System.out.println("------------------------------------");
        System.out.println("End of processing");
    }
}

输出>>

Welcome to QuickieBurger!
HAM         3.75
CHEESE          4.1
FRIES           2.5
DRINKS          1.75
Please place your order (e.g., 3 ham). Enter Q to quit.
2 Hello
4 CHEESE
q
-------------------------------------
Date: Sun Nov 02 02:59:56 PST 2014
HAM   2  = 7.5
CHEESE   4  = 16.4
Please pay 23.9

------------------------------------
End of processing