您的代码有几个问题，我冒昧地纠正了它们。通读 cmets 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.");
}
}
然而，我处理这个任务的方法是在一个enum 中实现元素，并将大部分calculate() 方法移动到那里。这将允许您快速创建更多元素，例如 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;
}