我用兩種不同的方法從兩個不同的掃描儀對象調用Scanner.nextLine()方法.有時當我從第二種方法調用Scanner.nextLine()時,它會給我一個“行不可用”錯誤.可能是什么問題？

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();

}

}

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