Java Scanner.nextLine（）不等待输入

Question

我以前有几种方法用于接受用户输入，并将其作为某种数据类型返回给调用它的方法。我在以前的项目中使用过这些方法或它们的变体。

我采用了用于字符串的方法来选择给定的第一个字符并返回它，以便可以在菜单中使用它。每次我启动该应用程序时，都会出现主菜单，并等待用户输入。收到此输入后，程序将连续循环直到停止。这是我捕获字符的方法：

private char getUserChar(String prompt) {
    try {
        Scanner scan = new Scanner(System.in);
        System.out.print(prompt);
        String tempString = "";
        tempString = scan.nextLine();
        scan.close();
        char userChar = tempString.charAt(0);
        return userChar;
    } catch(Exception ex) {
        System.out.println(ex.getMessage());
    }
    return 0;
}

由于try /
catch块，代码循环，因为scan.nextLine（）永远不会等待下一个输入。没有它，异常通常与找不到新行有关。我尝试了while（scan.hasNextLine（））似乎对其他人有用的方法；但是，一旦输入时间到了，它就永远不会中断。我也不相信我会遇到每个人似乎都遇到麻烦的nextInt（）问题。我将在下面发布整个课程的代码：

import java.util.Scanner;
import controller.Controller;
public class TUI {
    private Controller controller;
    public TUI() {
        Controller controller = new Controller();
        this.controller = controller;
    }

    public void run() {
        boolean wantToQuit = false;
        char userInput = 0;
        System.out.println("Welcome to the Mart.");
        do{
            userInput = mainMenu();
            if(isUserInputValid(userInput))
                switch(userInput){
                    case 'a': addItem();
                    break;
                    case 'r': controller.removeItem();
                    break;
                    case 'i': controller.printInventory();
                    break;
                    case 'p': controller.customerPurchase();
                    break;
                    case 'w': controller.weeklyStock();
                    break;
                    case 'c': wantToQuit = true;
                    break;
                }
            else System.out.println("\nMainMenu");



        } while(!(wantToQuit));
        System.out.println("WolfMart is now closed.  Thank you and good-bye.");
    }




    private boolean isUserInputValid(char userInput) {
        char[] testSet = {'a', 'r', 'i', 'p', 'c', 'w'};
        for(char currentChar : testSet) {
            if(currentChar == userInput)
                return true;
        }
        return false;
    }

    private char mainMenu() {
        System.out.println();
        controller.printInventory();
        String mainMenuSelection = "What would you like to do: (a)dd item, (r)emove item, print (i)nventory, " +
            "(p)urchase by customer, (c)lose store?\r\n";

        char mainMenuInput = getUserChar(mainMenuSelection);
        return mainMenuInput;
    }

    private char getUserChar(String prompt) {
        try {
            Scanner scan = new Scanner(System.in);
            System.out.print(prompt);
            String tempString = "";
            tempString = scan.nextLine();
            scan.close();
            char userChar = tempString.charAt(0);
            return userChar;
        } catch(Exception ex) {
            System.out.println(ex.getMessage());
        }
        return 0;
    }


    private int getUserInt(String prompt) {
        Scanner scan = new Scanner(System.in);
        int userInt = -1;
        try {
            System.out.print(prompt);
            String input = scan.nextLine();
            userInt = Integer.parseInt(input);
        }
        catch(NumberFormatException nfe) {
            System.out.println("I did not recognize your command, please try again.");
        }
        scan.close();
        return userInt;
    }

    private String getUserString(String prompt) {
        Scanner scan = new Scanner(System.in);
        String userString = null;
        try{
            System.out.print(prompt);
            userString = scan.nextLine();
        } catch(Exception ex)
        {
            System.out.println("I did not recognize your command, please try again.");
        }
        scan.close();
        return userString;
    }

    private double getUserDouble(String prompt) {
        Scanner scan = new Scanner(System.in);
        double userDouble = -1.0;
        try {
            System.out.print(prompt);
            String input = scan.nextLine();
            userDouble = Double.parseDouble(input);
        }
        catch(NumberFormatException nfe) {
            System.out.println("I did not recognize your command, please try again.");
        }
        scan.close();
        return userDouble;
    }

    private void addItem() {
        String itemName = "";
        double price;
        int quantity;
        String namePrompt = "Enter the name of the item being added to the inventory: ";
        String pricePrompt = "Enter the cost of " + itemName + ": ";
        String quantityPrompt = "Enter the quantity of " + itemName + ": ";
        itemName = getUserString(namePrompt);
        price = getUserDouble(pricePrompt);
        quantity = getUserInt(quantityPrompt);
        controller.addItem(itemName, quantity, price);
    }




}

Answer 1

正如我在评论中所述，问题是
您System.in每次执行以下操作时都将关闭：

Scanner scan = new Scanner(System.in);
...
scan.close();

现在，看一下
Scanner.nextLine

抛出 ：

• NoSuchElementException -如果找不到行
• IllegalStateException -如果关闭此扫描仪
  

现在，由于扫描仪 本身 未关闭，IllegalStateException因此不会抛出。相反，如前所述，将引发另一个异常，“
通常与找不到新行有关 ”- NoSuchElementException。

假设您正在使用JDK
7，可以通过检查Scanner.throwFor以下内容来了解其工作原理：

if ((sourceClosed) && (position == buf.limit()))
    throw new NoSuchElementException();

由于引发了异常，因此将0返回的值，getUserChar然后将其用于run循环中：

    do {
        userInput = mainMenu();
        if (isUserInputValid(userInput)) {
            ...
        } else {
            System.out.println("\nMainMenu");
        }
    } while (!wantToQuit);

由于输入无效，您将陷入循环打印之中"\nMainMenu\n"。

若要更正此问题，请尝试使用一个Scanner并且不要关闭System.in;-)