Java Scanner.nextLine() 不等待输入

Question

我以前使用过几种方法来接受用户输入并将其作为特定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，这样我就可以在菜单中使用它。每次启动应用程序时，都会出现主菜单并等待用户输入。收到这个输入后，程序不断循环，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 block ，代码循环，因为 scan.nextLine() 从不等待下一个输入。没有它，异常通常与找不到新行有关。我试过 while(scan.hasNextLine()) 似乎对其他人有用；但是，一旦输入时间到了，它就永远不会跳出循环。我也不相信我在 nextInt() 问题上绊倒了，每个人似乎都有麻烦。我将在下面发布整个类(class)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 my comment 中所述, 问题是你每次做这样的事情都会关闭System.in:

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

现在，查看 Scanner.nextLine 的规范

Throws:

• NoSuchElementException - if no line was found
• IllegalStateException - if this scanner is closed

现在，由于扫描器自身 未关闭，因此不会抛出 IllegalStateException。相反，正如您之前提到的，另一个异常“通常与找不到新行有关”——NoSuchElementException——被抛出。

假设您使用的是 JDK 7，您可以通过检查 Scanner.throwFor 了解它是如何工作的:

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

由于您的异常被抛出，getUserChar 返回值 0，然后在 run 循环中使用该值:

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

由于输入无效，您陷入了循环打印 "\nMainMenu\n"。

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