Java执行的循环在工作不正常时执行

Question

对不起，我是这个网站的新手，所以不确定这将如何显示。我想做一个简单的石头，布，剪刀游戏。在while语句之后，如果没有输入R，P，S，则程序什么也不做。我想让它回到问题的开头，这样就可以做出正确的选择。另外，如何输入打印语句，如“无效选择请重试”

package rps.gameapp;

import java.util.Scanner;

public class RPSGameApp
{

    public static void main(String[] args)
    {

    Scanner sc = new Scanner(System.in);
    String userChoice;
    String playAgain;
    int randNum = (int) (Math.random() * 3);

    do
        {
        System.out.println("Welcome to Rock, Paper, Scissors Game.");
        System.out.println("Pick R, P, or S.");
        userChoice = sc.nextLine();
        while (!userChoice.equalsIgnoreCase("P")
                && !userChoice.equalsIgnoreCase("R")
                && !userChoice.equalsIgnoreCase("S"));


            String compChoice = "";
        switch (randNum)
            {
            case 0:
                compChoice = "R";
                break;
            case 1:
                compChoice = "P";
                break;
            case 2:
                compChoice = "S";
                break;
            }

        System.out.println("The computer entered \"" + compChoice + "\".");

        if (compChoice.equalsIgnoreCase(userChoice))
            {
            System.out.println("Draw");
            } else if (userChoice.equalsIgnoreCase(userChoice)
                && compChoice.equalsIgnoreCase("S")
                || userChoice.equalsIgnoreCase("P")
                && compChoice.equalsIgnoreCase("R")
                || userChoice.equalsIgnoreCase("S")
                && compChoice.equalsIgnoreCase("P"))
            {
            System.out.println("User Wins");
            } else
            {
            System.out.println("User Loses");
            }

        System.out.print(
                "Do you want to play again? (Y/N)");
        playAgain = sc.nextLine();

        } while (playAgain.equalsIgnoreCase("Y"));

    System.out.println("Thanks for Playing!");

    }
}

Answer 1

看起来你忘记了一个do作为你的内部do{}循环

应该是：

do {
    do {
        System.out.println("Welcome to Rock, Paper, Scissors Game.");
        System.out.println("Pick R, P, or S.");
        userChoice = sc.nextLine();
    } while (!userChoice.equalsIgnoreCase("P") && !userChoice.equalsIgnoreCase("R") && !userChoice.equalsIgnoreCase("S")); 
    ...
} while (playAgain.equalsIgnoreCase("Y"));

如果没有内部do（以及环绕该循环体的花括号），内部循环将变成一个带有空体的while循环

Answer 2

正如Eran所说，您需要将do while循环包装到另一个循环中，这将不断要求用户输入正确的内容。这是完全有效的代码。一件可能更好的事情是用户输入错误的字母后的消息

编辑：还要确保每次迭代都画出随机数

编辑2：要根据用户输入更改消息，可以引入一个新变量，该变量将跟踪您请求用户正确输入的次数。如果为0，则表示第一次询问用户，我们应该打印“欢迎”消息。它不是0-您需要向用户请求正确的输入。在每一轮之后，我们再次给变量赋值零，循环重复。我已经在代码中实现了这个更改。请注意，此变量也可以是布尔值

public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String userChoice;
    String playAgain;
    int iterationNumber;

    while (true) {
        iterationNumber = 0;
        do {
            if (iterationNumber == 0) {
                System.out.println("Welcome to Rock, Paper, Scissors Game.");
                System.out.println("Pick R, P, or S.");

            } else {
                System.out.println("Please enter valid letter.");
                System.out.println("Pick R, P, or S.");
            }
            iterationNumber++;
            userChoice = sc.nextLine();
        } while (!userChoice.equalsIgnoreCase("P")
                && !userChoice.equalsIgnoreCase("R")
                && !userChoice.equalsIgnoreCase("S"));
        String compChoice = "";
        int randNum = (int) (Math.random() * 3);
        switch (randNum) {
            case 0:
                compChoice = "R";
                break;
            case 1:
                compChoice = "P";
                break;
            case 2:
                compChoice = "S";
                break;
        }

        System.out.println("The computer entered \"" + compChoice + "\".");

        if (compChoice.equalsIgnoreCase(userChoice)) {
            System.out.println("Draw");
        } else if (userChoice.equalsIgnoreCase("R")
                && compChoice.equalsIgnoreCase("S")
                || userChoice.equalsIgnoreCase("P")
                && compChoice.equalsIgnoreCase("R")
                || userChoice.equalsIgnoreCase("S")
                && compChoice.equalsIgnoreCase("P")) {
            System.out.println("User Wins");
        } else {
            System.out.println("User Loses");
        }

        System.out.print(
                "Do you want to play again? (Y/N)");
        playAgain = sc.nextLine();
        if (playAgain.equalsIgnoreCase("N")) {
            break;
        }
        iterationNumber = 0;
    }
    System.out.println("Thanks for Playing!");
}