为什么这样做会无限循环运行？

Question

尝试编写一个程序，让用户输入他们本周的开支。问题在于我希望程序在输入不可接受的输入时重新询问用户。我想出了如何检测负值，但是当试图捕获输入不匹配异常时（如果输入了字符或字符串）循环只是无限地运行询问 "Monday Expenses:" 我该怎么做才能让用户有另一次回答的机会？我试着休息一下；但这也脱离了 do while 循环，这是我不想要的。

到目前为止，这是我的代码：

  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);

}
}

感谢您的帮助

Answer 1

替换

mon = myScanner.nextDouble();

与以下

mon = Double.parseDouble(myScanner.next());

Answer 2

这是因为它总是在扫描器无法解析双精度值时转到 catch 块。符合你的条件

mon<0

它总是得到-1值，它导致无限循环你可能把你的条件误认为是

while(mon>0)

Answer 3

对 nextDouble() 的调用不会消耗来自 Scanner 的非 double。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");
    // ...
}

Answer 4

你可以使用myScanner.nextLine();在开始下一个循环之前转储缓存中所有不需要的字符

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 5

您可以试试下面的代码。它将遍历整个数组。您可以根据需要添加更多验证。

import java.util.Scanner;
public class BarGraph 
{

public static void main(String[] args)
{
    System.out.println("Welcome to Expenses Program: " + "\n");

    week(); 
}

public static void week()
{
    Scanner myScanner;
    myScanner = new Scanner(System.in);
    String [] day = new String[] {"mon", "tues", "wed", "thurs", "fri", "sat", "sun"};

        for (String days : day)
        {
            System.out.println("Expenses for " + days + ":");
            float exp = myScanner.nextFloat();

        }   
}
}

为什么这样做会无限循环运行？

Why is this do while loop running infinitely?

java

loops

infinite-loop

do-while

inputmismatchexception