java - 扫描仪元件异常

Question

我正在做一个简单的剪刀石头布程序，允许用户继续玩。当涉及到重复字符串扫描器的声明时，我不断收到元素异常 -

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

public class RockPaperScissors {
    String ai = null;
    String player = null;
    static int playerscore=0, aiscore=0, ties=0, rounds=0;
    static String repeatstring = null;

    public static void main(String[] args) {
        boolean repeat = false;
        RockPaperScissors rps = new RockPaperScissors();
        System.out.println("Welcome to Rock Paper Scissors!\n");
        do {
            System.out.println("Choose your output! R - Rock, P - Paper, S - Scissors");
            rps.user();
            rps.comp();
            rps.game();
            System.out.println("YOU - " + playerscore + " / COMPUTER - " + aiscore);
            System.out.println("Rounds played - " + rounds + "\n");
            System.out.println("Do you wish to continue? Y/N");

            // Error reading string input... Solution pending.
            Scanner repeatinput = new Scanner(System.in);
            repeatstring = repeatinput.nextLine();
            if(repeatstring.equals("Y")) {
                repeat = true;
            }
            else if(repeatstring.equals("N")) {
                repeat = false;
            }
            repeatinput.close();

        } while (repeat == true); 
        System.exit(0);
    }

    void user() {
        Scanner scan = new Scanner(System.in);
        String input = scan.next();
        if(input.equals("R")) {
            player = "Rock";
        }
        else if (input.equals("P")) {
            player = "Paper";
        }
        else if (input.equals("S")) {
            player = "Scissors";
        }
        scan.close();
    }

    void comp() {
        Random rand = new Random();
        int randno = rand.nextInt((3 - 1) + 1) + 1;
        switch (randno) {
            case 1: ai = "Rock";
            case 2: ai = "Paper";
            case 3: ai = "Scissors";
        }
    }

    void game() {
        // If both player and AI has the same output -
        if(ai.equals(player)) {
            ties = ties + 1;
            System.out.println("You chose - " + player);
            System.out.println("Computer chose - " + ai);
            System.out.println("We have a TIE!");
        }

        // ROCK VS SCISSORS
        if(((ai.equals("Rock")) && (player.equals("Scissors"))) || (player.equals("Rock") && (ai.equals("Scissors")))) {
            if(ai.equals("Rock")) {
                aiscore = aiscore + 1;
                System.out.println("You chose - " + player);
                System.out.println("Computer chose - " + ai);
                System.out.println("The COMPUTER WINS!");
            }
            else {
                playerscore = playerscore + 1;
                System.out.println("You chose - " + player);
                System.out.println("Computer chose - " + ai);
                System.out.println("YOU WIN!");
            }
        }

        // ROCK VS PAPER
        if(((ai.equals("Rock")) && (player.equals("Paper"))) || (player.equals("Rock") && (ai.equals("Paper")))) {
            if(ai.equals("Paper")) {
                aiscore = aiscore + 1;
                System.out.println("You chose - " + player);
                System.out.println("Computer chose - " + ai);
                System.out.println("The COMPUTER WINS!");
            }
            else {
                playerscore = playerscore + 1;
                System.out.println("You chose - " + player);
                System.out.println("Computer chose - " + ai);
                System.out.println("YOU WIN!");
            }
        }

        // PAPER VS SCISSORS
        if(((ai.equals("Scissors")) && (player.equals("Paper"))) || (player.equals("Scissors") && (ai.equals("Paper")))) {
            if(ai.equals("Scissors")) {
                aiscore = aiscore + 1;
                System.out.println("You chose - " + player);
                System.out.println("Computer chose - " + ai);
                System.out.println("The COMPUTER WINS!");
            }
            else {
                playerscore = playerscore + 1;
                System.out.println("You chose - " + player);
                System.out.println("Computer chose - " + ai);
                System.out.println("YOU WIN!");
            }
        }
        rounds = rounds + 1;
    }
}

程序在请求重复输入(Y/N)后自行终止，并显示以下错误消息:

Exception in thread "main" java.util.NoSuchElementException: No line found
at java.util.Scanner.nextLine(Unknown Source)
at RockPaperScissors.main(RockPaperScissors.java:28)

Answer 1

通常，当您遇到异常时，最好在线查找该异常(除非您知道抛出异常的原因)。在这种情况下，如果我们查找您收到的异常，我们将定向到 Javadocs，其中指出:

Thrown by the nextElement method of an Enumeration to indicate that there are no more elements in the enumeration.

这让我们查看 Scanner 的 Javadoc，看看这是否是由 nextLine() 抛出的。 。我们在Throws部分看到它说:

NoSuchElementException - if no line was found

这意味着您的程序正在尝试从尚未获得结果的扫描仪中获取数据。如果您希望它等到有结果，那么还有其他可用的方法可以帮助您的程序知道结果何时可用，特别是 hasNextLine() 。这意味着您应该在尝试获取响应之前检查用户是否已响应。

一如既往，最好尽可能检查 Javadocs。

顺便说一句，最好不要每次循环运行时都生成扫描器，而是在循环外部声明它，然后在完成后关闭它。