java - 当我第二次按下键盘输入时，我的 Do-While 循环中断

Question

我才刚开始编码几周，所以我真的很新。这是一个简单的程序，可以帮助用户定制新车。该程序对每个菜单使用不同的方法。每个非主要方法都会询问用户他们想要 2 个定制汽车选项中的哪一个，然后该方法将成本发送回主要方法。每个非 main 方法都使用与 engineCost 方法相同的格式。我不明白为什么我的键盘输入仅在 do while 循环的第一次迭代期间起作用。这是我在 eclipse 上提示的错误:

Exception in thread "main" java.lang.IllegalStateException

我对堆栈溢出进行了一些查找，但我认为我的知识不足以正确搜索答案。任何帮助将非常感激。

import java.util.Scanner;

public class Ch5HW {
    public static void main(String[] args){
        double baseCar = 14000;
        double addCost = 0;
        double ttlCost = 0;
        int userOpt;
        Scanner keyboard = new Scanner(System.in);
        System.out.println("Basic Car includes: Silver color, 4 cilinder engine, Cooper Tires; cost: $14,000");
        System.out.println("");
        //this do-while loops until the user enters 4 to exit 
        do{
            System.out.println("Build Car Quote");
            System.out.println("1. Engine");
            System.out.println("2. Color");
            System.out.println("3. Tires");
            System.out.println("4. Exit");

            System.out.print("Enter Option: ");
            userOpt = 0;
            // this keyboard input only works on the first iteration of the loop
            userOpt = keyboard.nextInt();
            System.out.println(userOpt);
            //nested switch to determine userOpt
            switch (userOpt)
            {
            case 1:
                // calls the engineCost method
                addCost = engineCost();
                //adds the value received to a total value
                ttlCost += addCost;
            break;

            case 2:
                //calls the colorOpt method
                addCost = colorOpt();
                //adds the value received to a total value
                ttlCost += addCost;
            break;

            case 3:
                //calls the tireOpt method
                addCost = tireOpt();
                //adds the value received to a total value
                ttlCost += addCost;
            break;
            }
            //System.out.print(addCost + baseCar);
            keyboard.close();
        } 
        while (userOpt !=4);
        //once loop has finished the total of additional purchases is added to the base car price 
        System.out.print("Total cost: " + (ttlCost + baseCar));
    }
    // this method has options for the engine
    static double engineCost()
    {
        double engineCost = 0;
        double addCost =0;
        int engineOpt;
        Scanner keyboard = new Scanner(System.in);

        System.out.println("Engine Options: ");
        System.out.println("1. 8 cylinder: $5000.00");
        System.out.println("2. 6 cylinder: $3500.00");
        System.out.println("3. Exit");
        System.out.println("Enter Option: ");
        engineOpt = keyboard.nextInt();

        switch (engineOpt)
        {
        case 1:
            engineCost = 5000.00;
            addCost += engineCost;
        break;
        case 2:
            engineCost = 3500.00;
            addCost += engineCost;
        break;  
        case 3:
            engineCost = 0;
            addCost += engineCost;
        }
        keyboard.close();
        return addCost; 
    }
}

Answer 1

这是因为循环中的行:keyboard.close();。这将在第一次迭代后关闭Scanner。之后，您尝试调用 nextInt() (来自 docs )将抛出:

IllegalStateException - if this scanner is closed

您应该仅在完全使用完资源后才关闭该资源。

也就是说，不要关闭System.in。一般规则是，如果您没有打开它，则不应关闭它。