Java扫描器不等待输入

Question

我这里有两个代码块。一个扫描器正确地等待用户输入，另一个则直接通过它并调用NextInt()，返回NosuchelementException。下面是工作的块：

public void startGame() {
   out.println("Player1: 1 for dumb player, 2 for smart player, 3 for human player.");
   Scanner scan = new Scanner(System.in);
   p = scan.nextInt();

   if (p == 1)
        p1 = new DumbPlayer("ONE");
   if (p == 2)
        p1 = new SmartPlayer("ONE");
   else 
       p1 = new HumanPlayer("ONE");

   out.println("Player2: 1 for dumb player, 2 for smart player, 3 for human player.");
   p = scan.nextInt();

   if (p == 1)
        p2 = new DumbPlayer("TWO");
   if (p == 2)
        p2 = new SmartPlayer("TWO");
   else 
        p2 = new HumanPlayer("TWO");

   scan.close();

下面是没有的块：

public int findBestMove(Set<Integer> moves, Board b) {

    Set<Integer> set = new HashSet<Integer>();

    out.println("Player " +name+ ", select a column from 1-7: ");
    Scanner scan = new Scanner(System.in);  <--here it should wait for input, but does not!
    int move = scan.nextInt();   <-- NoSuchElementException
    scan.close();

    for (int x = 1; x <= 7; x++) {
        set.add(move);
        move += 7;
    }
    ....etc

这两个都是单独的类，并且是从另一个类中的main方法调用的。基本上，main()调用startgame()，后者又调用某个Player类的findBestMove()方法...这是非工作代码所在的地方。程序中是否有不适合接受输入的时候？我的印象是，任何时候我需要用户输入，我都可以使用这种方法。谢谢！

Answer 1

根据java.util.Scanner javadoc，scanner.close()关闭关联的流，如果该流实现了closeable接口。java.lang.System.In是一个InputStream，它实现了Closeable接口。因此，在与System.In关联的扫描器上调用Scanner.Close()后，System.In流将关闭且不再可用。

下面的SSCE对我有效。我从问题中删除了一些与实际问题无关的代码。注意，使用这种方法，当它工作时，Eclipse会给我警告“resource leak：'scan'is never close”，因此更好的解决方案是只使用一个扫描器实例。

package com.example;

import java.util.Scanner;
import java.util.Set;
import static java.lang.System.out;

public class ScannerTest {
    int p = 0;
    String name = "Test";

    public void startGame() {
        out.println("Player1: 1 for dumb player, 2 for smart player, 3 for human player.");
        Scanner scan = new Scanner(System.in);
        p = scan.nextInt();

        out.println("Result1: " + p);

        out.println("Player2: 1 for dumb player, 2 for smart player, 3 for human player.");
        p = scan.nextInt();

        out.println("Result2: " + p);

        // scan.close();   // Do not close the Scanner to leave System.in open
    }

    public int findBestMove(Set<Integer> moves, Object /*Board*/ b) {
        out.println("Player " +name+ ", select a column from 1-7: ");
        Scanner scan = new Scanner(System.in);
        int move = scan.nextInt();
        // scan.close();   // Do not close the Scanner to leave System.in open

        out.println("Move: " + move);

        return 0;
    }

    public void run() {
        startGame();
        findBestMove(null, null);
    }

    public static void main(String[] args) {
        ScannerTest st = new ScannerTest();
        st.run();
    }
}