如何提示用户只输入他们可以选择的三个选项之一，并显示错误输入的错误消息

Question

该程序将提示用户输入介质（空气，水或钢）和距离。 然后计算声波通过介质的距离。

我写了整个程序，但我没有读到我的教授在家庭作业中添加的最后一点，这是以下段落。 现在我被卡住了，因为我不太确定如何将它添加到我的程序中。 我正在使用if语句，但也许我可以将其添加到一个？

程序提示介质：“输入以下其中一项：空气，水或钢：”并读取介质。 如果介质不是空气，水或钢，则程序会打印消息：“抱歉，您必须输入空气，水或钢”，而不是其他任何内容。 否则，程序会提示输入以下距离。

我尝试了一个while循环并添加了另一个if语句，但实际上我的问题是语法。 因为我从来没有命令用户输入特定的字符串。

public class SpeedOfSound {
    public static void main(String[] args) {

        double distance;
        double time;

        Scanner keyboard = new Scanner(System.in);
        //prompt the user to enter the medium through which sound will 
        System.out.print("Enter one of the following: air, water, or steel:");
        String input;
        input = keyboard.nextLine();

        // prompt the user to enter a distance

        System.out.print("Enter distance in feet: ");

        distance = keyboard.nextDouble();

        // determine if medium is air, water, steele and calculate

        if (input.equals("air")) {
            time = (distance / 1100);
            System.out.println("The total time traveled is " + time + " feet per second.");
        }
        else if (input.equals("water"))

        {
            time = (distance / 4900);
            System.out.println("The total time traveled is " + time + " feet per second.");
        }

        else if (input.equals("steel"))
        {
            time = (distance / 16400);
            System.out.println("The total time traveled is " + time + " feet per second.");
        }
    }
}

我的预期结果是让用户只输入空气，水或钢。

Answer 1

你的代码有几个问题，我冒昧地纠正它们。 阅读注释以更好地理解代码的每个部分。

public class SpeedOfSound
{
    /* Best to declare it here so other methods have access to it. */
    private static final Scanner keyboard = new Scanner(System.in);
    /*
     * Declared as a class field so you can use it if you
     * have a need for it in addition to time calculated in main.
     */
    private static double distance;

    /**
     * Blocks program execution until a number has been detected as user input.
     * @return numeric representation of user input.
     */
    public static double getDistance()
    {
        System.out.println("Enter distance in feet: ");
        // CAREFUL: This will throw an exception if the user enters a String
        // return keyboard.nextDouble();
        while (keyboard.hasNext())
        {
            /*
             * Check if the user input is actually a number
             * and if it isn't print an error and get next token
             */
            String input = keyboard.nextLine();
            try {
                return Double.valueOf(input);
            }
            catch (NumberFormatException e) {
                System.out.println("Incorrect input, try again.");
            }
        }
        throw new IllegalStateException("Scanner doesn't have any more tokens.");
    }

    /**
     * Calculate the speed of sound for user input which is limited to:
     * <ul>
     *     <li>Air</li>
     *     <li>Water</li>
     *     <li>Steel</li>
     * </ul>
     * @return total time traveled in feet per second.
     */
    public static Double calculate()
    {
        Double time = null;

        //prompt the user to enter the medium through which sound will travel through
        System.out.println("Enter one of the following: air, water, or  steel:");

        // The loop will break the moment time is calculated
        while (time == null && keyboard.hasNext())
        {
            double distance;
            String input = keyboard.nextLine();

            //determine if medium is air, water, steele and calculate

            if (input.equals("air"))
            {
                distance = getDistance();
                time = (distance / 1100);
            }
            else if (input.equals("water"))
            {
                distance = getDistance();
                time = (distance / 4900);
            }
            else if (input.equals("steel"))
            {
                distance = getDistance();
                time = (distance / 16400);
            }
            else System.out.println("Incorrect input, try again.");
        }
        return time;
    }

    public static void main(String[ ] args)
    {
        Double time = calculate();
        System.out.println("The total time traveled is " + time + " feet per second.");
    }
}

但是我解决这个任务的方法是在排序enum中实现元素并在那里移动大部分calculate()方法。 这将允许您快速创建更多元素，如air ， water和steel而无需创建额外的if块来处理它们。

元素枚举器

public enum Element {

    AIR("air", 1100),
    WATER("water", 4900),
    STEEL("steel", 16400);

    private final String name;
    private final int factor;

    Element(String name, int factor) {
        this.name = name;
        this.factor = factor;
    }

    /**
     * @param element name of the element to calculate time for
     * @return total time traveled in feet per second for given element or
     *         {@code null} if no element matched the given name.
     */
    public static Double getTimeTraveledFor(String element)
    {
        /* Find an element that matches the given name */
        for (Element e : Element.values()) {
            /*
             * Validate the parameter without case consideration.
             * This might be a better way of validating input unless
             * for some reason you really want a case-sensitive input
             */
            if (e.name.equalsIgnoreCase(element)) {
                return SpeedOfSound.getDistance() / e.factor;
            }
        }
        return null;
    }
}

修改方法

public static Double calculate()
{
    Double time = null;

    //prompt the user to enter the medium through which sound will travel through
    System.out.println("Enter one of the following: air, water, or  steel:");

    // The loop will break the moment time is calculated
    while (time == null && keyboard.hasNext())
    {
        String input = keyboard.nextLine();
        time = Element.getTimeTraveledFor(input);
        if (time == null) {
            System.out.printf("%s is not a recognized element, try again.", input);
        }
    }
    return time;
}

Answer 2

    while(true){  
     System.out.print("Enter distance in feet: ");

    String input;
                input = keyboard.nextLine();


     //prompt the user to enter a distance


     System.out.print("Enter distance in feet: ");

     distance = keyboard.nextDouble();

     //determine if medium is air, water, steele and calculate

        if (input.equals("air"))
        {


        time = (distance / 1100);

        System.out.println("The total time traveled is " + time + " 
     feet per second.");
    break;
        }

        else if (input.equals("water"))

        {
        time = (distance / 4900);

        System.out.println("The total time traveled is " + time + " 
     feet per second.");
    break;

         }



         else if (input.equals("steel"))

         {

          time = (distance / 16400);

          System.out.println("The total time traveled is " + time + " 
     feet per second.");
    break;


          }
else
 System.out.println("wrong choice");

}