为什么这个do while循环是无限运行的？

Question

尝试编写一个程序，其中用户输入他们一周的费用。麻烦在于我希望程序重新询问用户是否输入了不可接受的输入。我想出了如何检测负值的方法，但是当尝试捕获输入不匹配异常（如果像输入字符或字符串一样）时，循环只是无限运行，要求“星期一费用：”我如何使它，以便用户有另一个回答的机会？我试着Rest一下;但是这也打破了做而循环，我不想要。

这是我目前为止的代码:

  import java.util.Scanner;

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

Scanner myScanner;
myScanner = new Scanner(System.in);

double mon, tues, wed, thurs, fri, sat, sun;

do{
    try{
        System.out.print("Expenses for Monday: ");
         mon = myScanner.nextDouble();
    }catch( Exception e){
        mon = -1;
    }
}while(mon<0);

System.out.println(mon);

}
}  

谢谢你的帮助

Answer 1

这是因为当扫描器无法解析双精度值时，它总是转到catch块。这符合你的条件

mon<0

它总是得到-1值，它会导致无限循环，你可能把你的条件误认为

while(mon>0)

Answer 2

可以使用my scanner . nextline()；在开始下一个循环之前，转储缓存中所有不必要的字符

import java.util.Scanner;

public class BarGraph {

    public static void main(String[] args) {

        Scanner myScanner;
        myScanner = new Scanner(System.in);

        double mon, tues, wed, thurs, fri, sat, sun;

        do {
            try {
                System.out.print("Expenses for Monday: ");
                mon = myScanner.nextDouble();
            } catch (Exception e) {
                mon = -1;
                myScanner.nextLine();
            }
        } while (mon < 0);

        System.out.println(mon);
    }
}

Answer 3

对 nextDouble（） 的调用不会占用扫描仪的非双精度值。你需要这样的东西，

if (myScanner.hasNextDouble()) {
    mon = myScanner.nextDouble();
} else {
    System.out.printf("%s is not a double.%n", myScanner.next());
}

此外，我建议您将该逻辑提取到一个方法中。喜欢

static double getExpensesForDay(Scanner scanner, String day) {
    while (true) {
        System.out.printf("Expenses for %s: ", day);
        System.out.flush();
        if (scanner.hasNextDouble()) {
            return scanner.nextDouble();
        }
        System.out.printf("%s is not a double.%n", scanner.nextLine());
    }
}

然后你可以这样称呼它

public static void main(String args[]) {
    Scanner myScanner = new Scanner(System.in);
    double mon = getExpensesForDay(myScanner, "Monday");
    double tues = getExpensesForDay(myScanner, "Tuesday");
    double wed = getExpensesForDay(myScanner, "Wednesday");
    double thurs = getExpensesForDay(myScanner, "Thursday");
    double fri = getExpensesForDay(myScanner, "Friday");
    double sat = getExpensesForDay(myScanner, "Saturday");
    double sun = getExpensesForDay(myScanner, "Sunday");
    // ...
}