JAVA-用戶從掃描儀輸入java.util.NoSuchElementException

Question

當打開多個掃描儀而不關閉時，導致異常。 但一旦關閉語句被刪除，就可以正常工作。

reader.close();

為什么關閉語句會這樣工作？

import java.util.Scanner;

public class IO {
    public static void choices() {
        boolean valid = true;
        do{
            System.out.println("What would you like to do?\n"
                    + " press '1' for event 
                                        creation.\n"
                    + " press '2' to display all 
                                        events.\n"
                    + " press '3' to exit prgram.");
            switch(inputInt()) {
                case 1: newEvent(); break;
                case 2: break;
                case 3: System.out.println("Goodbye"); valid = false; break;
                default: System.out.println("Invalid entery try again."); break;
            }
        }while(valid);
    }

    public static int inputInt() {
        Scanner reader = new Scanner(System.in);
        int input = reader.nextInt();
        reader.close();
        return input;
    }

    public static String inputString() {
        Scanner reader = new Scanner(System.in);
        String input = reader.next();
        reader.close();
        return input;
    }

    public static void newEvent() {
        System.out.println("What is the event name?");
        String name = inputString();
        Event e1 = new Event(name);
        Event.eventCreation(e1);
    }
}

使用案例1時拋出此錯誤

Exception in thread "main" java.util.NoSuchElementException
    at java.util.Scanner.throwFor(Unknown Source)
    at java.util.Scanner.next(Unknown Source)

Answer 1

正如文檔中明確指出的那樣：

 public void close() 

關閉此掃描儀。

如果尚未關閉此掃描器，則如果其基礎可讀文件也實現了Closeable接口，則將調用該可讀文件的close方法。

您也將關閉System.in 。

在此處提出問題時，重要的一點是要仔細閱讀所有適用的文檔，在這種情況下，這意味着閱讀Javadoc。

Answer 2

如果輸入已用盡，它將在方法'nextInt'上引發java.util.NoSuchElementException。 當關閉掃描器時，如果源實現了Closeable接口，它將關閉其輸入源。因此，當您調用“ reader.close（）”時，它也會在輸入流中關閉system.。

System.in是靜態屬性。因此，關閉后不可用。

public final class System {

    /* register the natives via the static initializer.
     *
     * VM will invoke the initializeSystemClass method to complete
     * the initialization for this class separated from clinit.
     * Note that to use properties set by the VM, see the constraints
     * described in the initializeSystemClass method.
     */
    private static native void registerNatives();
    static {
        registerNatives();
    }

    /** Don't let anyone instantiate this class */
    private System() {
    }

    /**
     * The "standard" input stream. This stream is already
     * open and ready to supply input data. Typically this stream
     * corresponds to keyboard input or another input source specified by
     * the host environment or user.
     */
    public final static InputStream in = null;
    ...
}