我几乎完成了一个介绍类的Java项目，用户必须对汽油值执行转换。当我通过命令提示符时，该程序运行良好，直到您选择转换（从选项 1、2、3、4 或 5）的一个点。
发生的情况是程序可以很好地计算它，并显示"如果您想退出，请键入NO"部分，但是在用户甚至有机会键入任何内容之前，它会重复该程序。我不知道在这里做什么。我尝试输入keyboard.nextLine();以便它会跳到用户可以发送输入的行，但发生了同样的错误。如果我更改底部的行以在 yesOrNo = yes 时运行循环，那么它会突然停止程序。我认为这是同样的问题，尽管我不确定该怎么做，并且在其他论坛帖子上找不到此问题。感谢任何决定提供帮助的人。
这是我的代码，减去主方法/类标头：
// Ask the user for their name
System.out.println("What is your name?");
// Use Scanner to get the user's name (a string variable)
String name = keyboard.nextLine();
// Welcome the user by name & give a readable description of the program using escape sequences
System.out.println(name + ", welcome. I am Rachel and this program will help you perform gasoline analysis.n");
System.out.println("Using this program, you can extract information about any amount of gallons of gasoline, such as:");
System.out.println("its equivalent in liters, its price, and how much oil is required to produce it, for example.n");
System.out.println("Press any letter to continue.");
String yesOrNo = keyboard.next();
boolean performConversion = true;
// Perform the conversions the user wants at least once, until they want to stop.
do
{
// Get the value of gallons (floating-point value) of gasoline from the user.
System.out.println("Please enter a number that will represent your gallons of gasoline.");
double userGallons = keyboard.nextDouble();
// Declare variables and convert the gallons to the other units of measurement.
double liters = (userGallons * 3.7854);
double barrelsNeeded = (userGallons / 19.5);
double poundsCO2 = (userGallons * 20.0);
double ethanolEnergy = (userGallons * 75700);
double price = (userGallons * 4.0);
// Show the user a menu of their choices for conversion.
System.out.println("Select the conversion you would like to perform here:");
System.out.println("Press 1 for liters");
System.out.println("2 for pounds of CO2 produced");
System.out.println("3 for equivalent energy amount of ethanol gallons");
System.out.println("4 for price in USD");
System.out.println("or 5 for barrels of oil required to produce that amount of gasoline.");
int userChoice = keyboard.nextInt();
// Display the original gallons and the available conversions to the user.
switch(userChoice)
{
case 1:
System.out.println("Original gallons of gasoline: " + userGallons + "gallons");
System.out.println("Amount in liters: " + liters);
System.out.println("Type NO if you want to exit, or any other key to continue.");
break;
case 2:
performConversion = true;
System.out.println("Original gallons of gasoline: " + userGallons + "gallons");
System.out.println("Pounds of CO2 produced : " + poundsCO2);
System.out.println("Type NO if you want to exit, or any other key to continue.");
break;
case 3:
performConversion = true;
System.out.println("Original gallons of gasoline: " + userGallons + "gallons");
System.out.println("Equivalent energy amount in ethanol gallons: " + ethanolEnergy + " BTU");
System.out.println("Type NO if you want to exit, or any other key to continue.");
break;
case 4:
performConversion = true;
System.out.println("Original gallons of gasoline: " + userGallons + "gallons");
System.out.println("Price in USD: $" + price);
System.out.println("Type NO if you want to exit, or any other key to continue.");
break;
case 5:
performConversion = true;
System.out.println("Original gallons of gasoline: " + userGallons + "gallons");
System.out.println("Number of barrels of oil needed to produce that amount of gallons: " + barrelsNeeded);
System.out.println("Type NO if you want to exit, or any other key to continue.");
break;
default:
System.out.println("Invalid character. Please enter 1, 2, 3, 4, or 5.");
}
} while (!"no".equalsIgnoreCase(yesOrNo));
System.out.println("Goodbye! The program will now end.");
}
}
在不进行任何重组的情况下，您可以更改您的 while 条件：
} while (!"no".equalsIgnoreCase(keyboard.next()) );
由于您希望转换至少发生一次，因此我建议您在do { //... } while();循环中将这两个语句（如下所述）移到最后（即在switch...case语句之后）：
System.out.println("Press any letter to continue.");
String yesOrNo = keyboard.next();
原因
您要实现的是让用户执行转换，然后在他们选择是否继续之后。
当前程序不会要求用户进行此选择。相反，它只是使用 yesOrNo 变量的值进行循环，该值是在进入循环之前设置的。
将上述语句保持在循环的最末尾应该会给您带来所需的结果。
另外，我看到您已经创建了一个变量performConversion，但是它从未在某处使用过（至少没有在提供的代码片段中）。因此，除非在 main() 方法中的其他地方使用此变量，否则我建议您将其删除。