调用Scanner.close（）抛出异常 [英] Calling Scanner.close() throws nosuchelement exception

问题描述

我有一个简单的方法，将命令打印到屏幕上，扫描用户的输入，并将其作为字符串返回。如果用户的输入无效，则通知用户并再次询问。这个方法是完美的，但是我的教练提到我们应该总是关闭资源，所以我回去并添加了close方法，现在我每次调用该方法时都会得到一个NoSuchElementException，无论用户输入什么。这是代码...

  private String getUserString（String userCommand）{
 System.out.println（userCommand ）; 
扫描仪扫描=新扫描仪（System.in）; 
 String userinput = scan.nextLine（）。trim（）; 
 
 if（userinput.isEmpty（））{
 System.out.println（Invalid choice）; 
 return getUserString（userCommand）; 
} 
 
 else {
 return userinput; 
} 
} 
  

异常总是回到userinput的行启动为scan.nextLine（）。trim（）
注意*我在每个返回语句之前的每一行添加了scan.close（），但是我没有在上面添加它。

解决方案

当您关闭扫描时，您正在关闭 System.in 当你尝试从它重新读取时，它会抛出异常。

您的教练是正确的一方面，您应该清理资源，只是不当涉及到系统。在。

---

而不是每次需要输入重新打开流，你可以创建您的扫描仪一次，只需在需要输入的地方重新使用：

  public static void main（String [] args）throws IOException {
扫描仪扫描=新的扫描仪（System.in）; 
 System.out.println（getUserString（Write something：，scan））; 
 System.out.println（getUserString（Write other else：，scan））; 
} 
 
 private static String getUserString（String userCommand，Scanner scan）{
 
 System.out.println（userCommand）; 
 
 String userinput = scan.nextLine（）。trim（）; 
 
 if（userinput.isEmpty（））{
 System.out.println（Invalid choice）; 
 return getUserString（userCommand，scan）; 
} else {
 return userinput; 
} 
} 
  

I have a simple method that prints a command to the screen, scan's a user's input, and returns it as a String. If the user's input is invalid, it notifies the user and asks again. This method worked perfectly but my instructor mentioned that we should always close resources, so I went back and added in the close method and now I'm getting a NoSuchElementException every time the method is called, regardless of user input. Here is the code...

private String getUserString(String userCommand) {
    System.out.println(userCommand);
    Scanner scan = new Scanner(System.in);
    String userinput = scan.nextLine().trim();

    if (userinput.isEmpty()){
        System.out.println("Invalid choice");
        return getUserString(userCommand);
    }

    else {
        return userinput;
    }
}

The exceptions always point back to the line where userinput is initiated as scan.nextLine().trim() NOTE* I added scan.close() on each line before each return statement, however I didn't include it above.

解决方案

When you close scan you are closing System.in and when you try to re-read from it, it will throw the exception.

Your instructor is right in one aspect, you should clean up resources, just not when it comes to System.in.

---

Instead of re-opening the stream every time you need input you could create your Scanner once and just re-using it where you need input:

public static void main(String[] args) throws IOException {
    Scanner scan = new Scanner(System.in);
    System.out.println(getUserString("Write something:", scan));
    System.out.println(getUserString("Write something else:", scan));
}

private static String getUserString(String userCommand, Scanner scan) {

    System.out.println(userCommand);

    String userinput = scan.nextLine().trim();

    if (userinput.isEmpty()) {
        System.out.println("Invalid choice");
        return getUserString(userCommand, scan);
    } else {
        return userinput;
    }
}

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