Java剪刀石头布游戏：自定义回合数的for循环异常问题

原本可正常运行的Java剪刀石头布游戏，添加用户自定义回合数功能后出现异常：输入回合数后，程序连续两次输出出拳提示，且第一次无法接收用户输入。

正常代码 #

import java.util.Scanner;
import java.util.Random;
public class game {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String[] rps = {"rock","paper","scissors"};
        String computerMove = rps[new Random().nextInt(rps.length)];
        for(int i=0;i<2;i++) {
        System.out.println("Please enter your move (rock, paper or scissors)");
        String playerMove = scanner.nextLine();
        if(playerMove.equals(computerMove)) {
            System.out.println("It's a tie. Computer chose also "+computerMove);
        }
        else if (playerMove.equals("rock")) {
            if (computerMove.equals("scissors")) {
                System.out.println("You've won. Computer chose "+computerMove);
            }
            else {
                System.out.println("You loose. Computer chose "+ computerMove);
            }
        }
        else if (playerMove.equals("paper")) {
            if (computerMove.equals("rock")) {
                System.out.println("You've won. Computer chose "+computerMove);
            }
            else {
                System.out.println("You loose. Computer chose "+computerMove);
            }
        }
        else if (playerMove.equals("scissors")) {
            if (computerMove.equals("paper")) {
                System.out.println("You've won. Computer chose "+computerMove);
            }
            else {
                System.out.println("You loose. Computer chose "+computerMove);
            }
        }

        }
    }
}

添加回合数后的异常代码 #

import java.util.Scanner;
import java.util.Random;

public class game {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String[] rps = {"rock","paper","scissors"};
        String computerMove = rps[new Random().nextInt(rps.length)];
        System.out.println("How many rounds you wanna play?");
        int rounds = scanner.nextInt();
        for(int i=0;i<rounds;i++) {
        System.out.println("Please enter your move (rock, paper or scissors)");
        String playerMove = scanner.nextLine();
        if(playerMove.equals(computerMove)) {
            System.out.println("It's a tie. Computer chose also "+computerMove);
        }
        else if (playerMove.equals("rock")) {
            if (computerMove.equals("scissors")) {
                System.out.println("You've won. Computer chose "+computerMove);
            }
            else {
                System.out.println("You loose. Computer chose "+ computerMove);
            }
        }
        else if (playerMove.equals("paper")) {
            if (computerMove.equals("rock")) {
                System.out.println("You've won. Computer chose "+computerMove);
            }
            else {
                System.out.println("You loose. Computer chose "+computerMove);
            }
        }
        else if (playerMove.equals("scissors")) {
            if (computerMove.equals("paper")) {
                System.out.println("You've won. Computer chose "+computerMove);
            }
            else {
                System.out.println("You loose. Computer chose "+computerMove);
            }
        }

        }
    }
}

运行现象 #

How many rounds you wanna play?
2
Please enter your move (rock, paper or scissors)
Please enter your move (rock, paper or scissors)
rock
You've won. Computer chose scissors

问题原因及修复方案 #

原因 #

scanner.nextInt()仅读取输入中的整数部分，当输入回合数后按下回车键，换行符会残留在输入缓冲区中。第一次调用scanner.nextLine()时，会直接读取这个空的换行符，导致程序跳过用户输入，直接进入下一次循环，因此出现连续两次提示的现象。

另外原异常代码存在逻辑问题：电脑的出拳仅在初始化时生成一次，所有回合电脑都会出相同的手势，不符合游戏逻辑。

修复方案 #

1. 清空输入缓冲区：在scanner.nextInt()之后，额外调用一次scanner.nextLine()来清除残留的换行符。
2. 调整电脑出拳逻辑：将电脑出拳的代码移到for循环内部，每回合重新生成随机出拳。

修复后的完整代码：

import java.util.Scanner;
import java.util.Random;

public class game {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String[] rps = {"rock","paper","scissors"};
        
        System.out.println("How many rounds you wanna play?");
        int rounds = scanner.nextInt();
        scanner.nextLine(); // 清除输入缓冲区的换行符
        
        for(int i=0;i<rounds;i++) {
            // 每回合重新生成电脑出拳
            String computerMove = rps[new Random().nextInt(rps.length)];
            
            System.out.println("Please enter your move (rock, paper or scissors)");
            String playerMove = scanner.nextLine();
            
            if(playerMove.equals(computerMove)) {
                System.out.println("It's a tie. Computer chose also "+computerMove);
            }
            else if (playerMove.equals("rock")) {
                if (computerMove.equals("scissors")) {
                    System.out.println("You've won. Computer chose "+computerMove);
                }
                else {
                    System.out.println("You loose. Computer chose "+ computerMove);
                }
            }
            else if (playerMove.equals("paper")) {
                if (computerMove.equals("rock")) {
                    System.out.println("You've won. Computer chose "+computerMove);
                }
                else {
                    System.out.println("You loose. Computer chose "+computerMove);
                }
            }
            else if (playerMove.equals("scissors")) {
                if (computerMove.equals("paper")) {
                    System.out.println("You've won. Computer chose "+computerMove);
                }
                else {
                    System.out.println("You loose. Computer chose "+computerMove);
                }
            }
        }
    }
}

内容的提问来源于stack exchange，提问作者Cedio