java - 使用扫描器 nextLine() 会抛出 NoSuchElementException

Question

在同一控制台上运行 nextLine() 两次，同时在两次运行之间使用 println 时，我得到 NoSuchElementException。

首先我运行这个:

  public void ask() {
    System.out.print(title);
    answer = getAnswer();
  }
  private String getAnswer() {
    System.out.println("Before first nextLine()");
    final Scanner in = new Scanner(System.in);
    final String input = in.nextLine();
    in.close();
    System.out.println("After first nextLine()");
    return input;
  }

然后我运行这个:

  @Override
  public void ask() {
    System.out.println(title);
    int index = 0;
    for (Option o : options)
      System.out.println(index++ + " : " + o.getTitle());
    System.out.println("Before second nextLine()");
    final Scanner in = new Scanner(System.in);
    String answer = in.nextLine();
    in.close();
    System.out.println("After second nextLine()");

    this.answer = options.get(Integer.parseInt(answer)).getTitle();
  }

这是输出:

 How old are you?Before first nextLine()
32
Exception in thread "main" java.util.NoSuchElementException: No line found
After first nextLine()
    at java.util.Scanner.nextLine(Scanner.java:1540)
Sex
    at SingleChoise.ask(SingleChoise.java:25)
0 : Male
    at Main.main(Main.java:17)
1 : Female
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
Before second nextLine()
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)

为什么会抛出异常，我该如何解决？

Answer 1

关于 Scanner 类的一件事是调用 Scanner.close()在扫描器上也会关闭它正在读取的输入流。在这种情况下，这就是关闭 System.in，一旦关闭就无法重新打开。

如果可能，您应该在类级别声明您的Scanner，并保持打开状态。从 System.in 读取的类通常不需要关闭。