如何获取用户输入并在不同类中的数组列表中搜索用户输入？

Question

更新：应用@ user27158建议的改进（谢谢）后，当我运行程序时遇到了另一个问题。

弹出错误，从本质上使程序停止运行。 在调查了这一点之后，我无法弄清问题所在。 再一次，我是编程新手，很可能我只是想念一些完全简单的东西。

错误信息：

线程“主”中的异常java.lang.UnsupportedOperationException：不支持。 在country.game.Main.main（Main.java:36）的country.game.EuropeanCountries.EuropeanCountriesList（EuropeanCountries.java:17）处返回Java：1 BUILD FAILED（总时间：12秒）

错误发生在以下行：

List<String> euCountries = EuropeanCountries.EuropeanCountriesList();

主类：

package country.game;

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


public class Main {

    public static List<String> EuropeanCountriesList() {
        List<String> EuropeanCountries = new ArrayList<>();
        return EuropeanCountries;
    }



    public static void main(String[] args) {

        boolean Running = true;

        List<String> UsedCountries = new ArrayList<>();

        Scanner Reader = new Scanner(System.in);

        while(Running == true){



            System.out.println("Pick a Country: ");
            String UserChoice = Reader.next();




            List<String> euCountries = EuropeanCountries.EuropeanCountriesList();

            if(!euCountries.contains(UserChoice)){
                System.out.println("That is not a valid country");
            } else {
                if(UsedCountries.contains(UserChoice)) {
                    System.out.println("Sorry you have already used that country");
                } else {
                    System.out.println("Correct! That Country is in Europe!");
                    UsedCountries.add(UserChoice);
                }
            }
        }        
    }           
}

“欧洲”类

package country.game;

import java.util.Arrays;
import java.util.List;

public class Europe {

    private static final List<String> EuropeanCountries = Arrays.asList(
            new String[]{
                "Albania",
                "Andorra",
                "Austria",

                "Belarus",
                "Belgium",
                "Bosnia and Herzegovina",
                "Bulgaria",

                "Croatia",
                "Czechia",

                "Denmark",

                "England",
                "Estonia",

                "Finland",
                "France",

                "Germany",
                "Greece",

                "Hungary",

                "Iceland",
                "Ireland",               
                "Italy",

                "Kosovo",

                "Latvia",
                "Liechtenstein",
                "Lithuania",
                "Luxembourg",

                "Malta",
                "Moldova",
                "Monaco",
                "Montenegro",

                "Netherlands",
                "Northern Ireland",
                "North Macedonia",
                "Norway",

                "Poland",
                "Portugal",

                "Romania",

                "San Marino",
                "Scotland",
                "Serbia",
                "Slovakia",
                "Slovenia",
                "Spain",
                "Sweden",
                "Switzerland",

                "Turkey",

                "Ukraine",

                "Vatican City",

                "West Russia",
            }
    );


    public static List<String> EuropeanCountriesList(){

    return EuropeanCountries;
    }    

}

任何帮助将不胜感激！

Answer 1

首先，您应该对代码进行一些改进。

1. 运行永远不会增加，因此while循环永远不会结束
2. 您只想初始化扫描仪一次，因此可以在循环外进行
3. 您正在尝试将String与int进行比较
4. 您的列表“ UsedCountries”是通用的。 这绝不是一个好主意，因为它不限制可以输入的类型，这意味着您可能会出现仅在运行时显示的错误。 应该声明为List<String> UsedCountries = new ArrayList<>();
5. 变量和方法名称应以小写字母https://www.oracle.com/technetwork/java/codeconventions-135099.html开头

解决方法：如何从其他类访问arraylist

• 从你的方法返回
• 在返回的列表上工作

    public static List<String> EuropeanCountriesList() {
        // declaring as List as it is best to not tie yourself to a specific implementation
        List<String> EuropeanCountries = new ArrayList<>();
        ...
        return EuropeanCountries;
    }

然后

    String UserChoice = Reader.next()
    List<String> euCountries = EuropeanCountries.EuropeanCountriesList();
    ...

上面的代码允许您在当前代码中使用arraylist。 我会建议一些进一步的改进

1. 获得列表后，只需使用contains方法而不是for循环

    List<String> euCountries = EuropeanCountries.EuropeanCountriesList();
    // step 1 - check if it is one of the acceptable countries
    if (!euCountries.contains(UserChoice)) {
        System.out.println("That is not a valid country");
    } else {
        // step 2 - check if it has already been selected
        if (UsedCountries.contains(UserChoice)) {
            System.out.println("Sorry You Have Already Used That Country");
        } else {
            System.out.println("Correct! That Country is in Europe!");
            UsedCountries.add(UserChoice);
        }
    }

2. 不必在每次调用该方法时都创建国家列表，只需声明一次并返回存储的列表即可

public class Europe {
    private static final List<String> EuropeanCountries = Arrays.asList(
        new String[]{
            "Albania", 
            "Andorra",
            ...
        }
    );

    public static List<String> EuropeanCountriesList() {
        return EuropeanCountries;
    }
}

希望这对你有帮助