扫描仪行不可用错误

Question

我用Scanner.nextLine()两种不同的方法从两个不同的扫描仪对象中调用该方法.有时当我Scanner.nextLine()从第二种方法调用它时,它会给我一个" 行不可用 "的错误.可能是什么问题？

import java.util.Scanner;

public class TicTacToe {

    private final String COMPUTER = "computer";
    private final String PLAYER = "player";

    /**
     * Asks the user to choose a letter ('X' or 'O')
     * and checks if the input is valid.
     */
    public void askLetter() {
        System.out.println("Please enter 'X' or 'O': ");
        Scanner input = new Scanner(System.in);
        String letter = input.nextLine().toUpperCase().trim();
        while (!letter.equals("X") && !letter.equals("O")) {
            System.out.println("Please enter 'X' or 'O': ");
            letter = input.nextLine().toUpperCase().trim();
        }
        input.close();
    }


    /**
     * Asks the user to choose who goes first. 
     * @return 'computer' or 'player'
     */
    public String decide() {
        //System.out.println("Choose who goes first('player' or 'computer'): ");
        Scanner input = new Scanner(System.in);
        String first = input.nextLine().toUpperCase().trim();
        while (!first.equals(COMPUTER) && !first.equals(PLAYER)) {
            System.out.println("Choose who goes first('player' or 'computer'): ");
            first = input.nextLine().toUpperCase().trim();
        }
        input.close();
        return first;
    }

    public static void main(String[] args) {
        TicTacToe tictac = new TicTacToe();
        tictac.askLetter();
        tictac.decide();
    }
}

Answer 1

您的askLetter方法会关闭扫描仪......它将关闭System.in.然后你试图在System.in内部重用decide.

将构造函数更改为接受Scanner您在其中创建的a main.然后将该扫描仪保留在字段中并在两种方法中使用它:

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    TicTacToe tictac = new TicTacToe(scanner);
    tictac.askLetter();
    tictac.decide();
    // You might want to close the scanner here, but you don't really have to.
}

这也有助于使您的代码更易于测试 - 您现在可以使用Scanner由伪数据支持的代码(例如a StringReader).