同时循环：负输入检查和添加字符串的try catch

Question

我必须做一个计算器

>

使用正数输入运行程序，显示输出并询问用户是否要循环。

使用字符串输入（两个而不是2个）运行程序，使用try和cat显示一条消息，告诉用户错误，并询问用户是否想要循环。

使用负数输入运行程序，在不显示输出的情况下显示错误消息，并询问用户是否要循环。

当我尝试catch时，对于try catch之后的变量，我得到的变量尚未初始化。

try {
    System.out.print("Enter loan amount: ");
    loanNum = Double.parseDouble(in.nextLine());

    System.out.print("Enter rate: ");
    rateNum = Double.parseDouble(in.nextLine());

    System.out.print("Enter number years: ");
    yearNum = Double.parseDouble(in.nextLine());
}
catch(NumberFormatException e) { 
    System.out.println ("You must enter positive numeric data!");
}

如果没有尝试捕获，我不知道如何在不使用System.exit（0）；不显示输出的情况下执行数字2。这结束了用户能够输入到循环。

我不知道如何修复我的while循环。当我运行程序时，它只运行一次，然后循环一次。之后，它将自动循环，无需提示，并在一行上吐出行，如：

Would you like to calculate again (y/n):
Enter loan amount:

代码：

package whileloopyn;

import java.util.Scanner;

public class Whileloopyn {

public static void main(String[] args) {

    Scanner in = new Scanner(System.in);

    //Declare Variables
    double loanNum;
    double rateNum;
    double trateNum;
    double yearNum;
    double monthNum;
    double equNum;
    double outputNum;

    //Prompt for input and try catch for exception error (rate inputed as two)

    System.out.print("Enter loan amount: ");
    loanNum = Double.parseDouble(in.nextLine());

    System.out.print("Enter rate: ");
    rateNum = Double.parseDouble(in.nextLine());

    System.out.print("Enter number years: ");
    yearNum = Double.parseDouble(in.nextLine());


    //Postive input data check (negative then ask again)
    if(loanNum <= 0 || rateNum <=0 || yearNum <=0)
    {
        System.out.println("You must enter positive numeric data! ");
        System.exit(0);
    }


    //Convert years to months 
    monthNum = yearNum * 12;

    //Convert rate to percent and monthly
    trateNum= (rateNum / 100) /12; 

    //Complete numerator 
    equNum = Math.pow((1+ trateNum), monthNum );

    //Plug equNum into formula
    outputNum = trateNum * loanNum * (equNum / (equNum - 1));

    //output answer
    System.out.printf("The monthly payment is: $ %.2f%n", outputNum);

    //prompt user y n to run program again
    System.out.println("Would you like to calculate again (y/n): ");
    String loop = in.nextLine();

    //while loop with (input

    boolean isContinuing = true;

    while (isContinuing) {
        while(loop.equals("y")){
        System.out.print("Enter loan amount: ");
        loanNum = in.nextDouble();

        System.out.print("Enter rate: ");
        rateNum = in.nextDouble();

        System.out.print("Enter number years: ");
        yearNum = in.nextDouble();

        //Postive input data check (negative then loop again)
        if(loanNum <= 0 || rateNum <=0 || yearNum <=0)
        {
           System.out.println("You must enter positive numeric data! ");
           System.exit(0);
        }


        //Convert years to months 
        monthNum = yearNum * 12;

        //Convert rate to percent and monthly
        trateNum= (rateNum / 100) /12; 

        //Complete numerator 
        equNum = Math.pow((1+ trateNum), monthNum );

        //Plug equNum into formula
        outputNum = trateNum * loanNum * (equNum / (equNum - 1));

        //output answer
        System.out.printf("The monthly payment is: $ %.2f%n", outputNum);
        //ask user if you want to use program again
        System.out.printf("Would you like to calculate again (y/n): ");

        if(loop.equals("n"))
        {
            System.exit(0);
        }
    }
    }

}

}

有人建议（input.equals（String））是我应该如何比较同时循环的字符串，并使用in.nextLine（）；。




Answer 1

如果用户按下Y，则必须继续该循环：

        System.out.printf("Would you like to calculate again (y/n): ");
        Scanner s = new Scanner(System.in);
        if(s.nextLine().equalsIgnoreCase("y")) {
            continue;
        } else {
            break;
        }

而不是而（loop.equals（y））{，你应该写而（true）{，因为你已经在循环中检查过了