我的try-catch塊有問題嗎？

Question

所以我們必須做一個抵押貸款計算項目，我們必須要求用戶再次計算，現在我們必須這樣做，因此每次用戶輸入任何輸入的字符串值時都會輸出錯誤消息。我以爲我做得對，但每次運行它時都會發生奇怪的事情，我無法弄清楚爲什麼，並且我知道Try-Catch塊有什麼問題。

這裏是我的輸出：http://imgur.com/cbvwM5v

正如你可以看到我第三次運行程序我進入一個「二」作爲第二個輸入，它仍然做了計算。然後，第三次嘗試時，我輸入了一個負數，然後是「兩」，一切都按照我想要的方式工作。然後，我上次運行它時，我爲第一個輸入設置了一個正數，它仍然進行了計算，任何你們看到的可能是這樣做的？另外，我想我可能使用了錯誤的例外，我不確定它的含義，我只是猜測。我應該使用NumberFormatException，並且在nfe下還有一行表示該值沒有被使用。

這裏是我的代碼：

package MortgageCalculation2c; 

import java.text.NumberFormat; 
import java.util.Scanner; 
/** 
* 
* @author Akira 
*/ 
public class MortgageCalculation2c { 

/** 
* @param args the command line arguments 
*/ 
public static void main(String[] args) { 
    Scanner in = new Scanner(System.in); 


    double loanAmount = 0; 
    double interestRate = 0; 
    double numberYears = 0; 
    double months; 
    double monthlyPayment; 
    double numerator; 
    double denominator; 
    double formula; 
    boolean userInput = true; 
    String answer = ("y"); 


    while (userInput) { 
     try { 
      loanAmount = 0; 
      interestRate = 0; 
      numberYears = 0; 

    //prompt user for the loan amount  
    System.out.print("Enter the loan amount: "); 
    loanAmount = Double.parseDouble(in.nextLine()); 

    //prompt the user for the interest rate 
    System.out.print("Enter the rate: "); 
    interestRate = Double.parseDouble(in.nextLine()); 

    //prompt the user for thenumber of years 
    System.out.print("Enter the number of years: "); 
    numberYears = Double.parseDouble(in.nextLine()); 

     } catch (NumberFormatException nfe) { 
      System.out.println("You must enter positive numerical data!"); 
     } 


    //if the user enters a negative number print out a error message, if not, continue calculations 
    if ((loanAmount <= 0) || (interestRate <= 0) || (numberYears <= 0)) { 
     System.out.println("ALL NUMERICAL VALUES MUST BE POSITIVE!"); 

    } else { 
     //convert the interest rate 
     interestRate = interestRate/100/12; 

     //the number of years must be converted to months 
     months = numberYears * 12; 

     //numerator of the monthly payment formula 
     numerator = (Math.pow(1 + interestRate, months)); 

     //denominator of the monthly payment formula 
     denominator = ((numerator)-1); 

     //the formula equals the numerator divided by the denominator 
     formula = (numerator/denominator); 

     //monthly payment calculation 
     monthlyPayment = (interestRate * loanAmount * formula); 

     //sytem output 
     NumberFormat defaultFormat = NumberFormat.getCurrencyInstance(); 
     System.out.println("The monthly payment is: " + defaultFormat.format(monthlyPayment)); 

    } 

     //prompt the user if they would like to calculate the program again. 
     System.out.println("Would you like to calculate again (y/n) : "); 

     //if the user enters "y" the program will run again and if the user enters anything else it ends 
      answer = in.nextLine(); 
     answer = answer.toLowerCase(); 
      if (answer.equals("y")){ 
       userInput = true; //tests the program if it needs to run again 
      }else{ 
       break; //ends the program 
      } 

} 

} 
} 

有什麼事情，你們可以看到，可能是什麼問題？

Answer 1

看起來你需要continue在你的catch塊，所以程序流程回到循環。

... 
    } catch (NumberFormatException nfe) { 
     System.out.println("You must enter positive numerical data!"); 
     continue; // <---------- here 
    } 
    ... 

如果這不是在try-catch構造特殊的運動，它是將是更好的使用Scanner的方法hasNextDouble()爲validatiing和nextDouble讀取和轉換的數字。

它會是這個樣子：

//prompt user for the loan amount  
loanAmount = enterPositiveDouble("Enter the loan amount: ") 

//prompt the user for the interest rate 
interestRate = enterPositiveDouble("Enter the rate: "); 

//prompt the user for the number of years 
numberYears = enterPositiveDouble("Enter the number of years: "); 

，你將有一個特殊的static方法enterPositiveDouble類似以下內容：

static void enterPositiveDouble(String prompt) { 
    Scanner in = new Scanner(System.in); 
    boolean ok = true; 
    double result = -1; 
    do { 
     System.out.print(prompt); 
     ok = (in.HasNextDouble() && (result = in.NextDouble()) > 0) 
     if ! ok 
     System.out.println("You must enter positive numerical data!"); 
    } while (!ok); 
} 

以上是不是最佳的代碼，但公正的插圖可能的方案。