Java 超出范围

Question

除了一个问题之外，我的程序已基本完成。我遇到了 for 循环超出范围的问题。该计划的目标是根据用户输入的金额和期限计算复利。

3 年期本金 5000 美元、利息 5% 的产出示例如下:

Month:  Interest:   Principal:
1       $20.83      $5020.83 
2       $20.92      $5041.75
etc      etc         etc 

 Starting Balance        = $ 5000.00 // having problem outputting these w/ for-loop
 Final Account Balance   = $ 5807.36 // System.out.print keeps repeating multiple times
 Total Interest Paid     = $  807.36 // but i can't use variables outside of loop

我的问题是，在 for 循环期间，每次程序执行循环时，我都会不断输出起始余额、最终余额和总利息。但是，如果我尝试在循环外部使用变量，它就会超出范围，如果我尝试在循环外部声明变量，我就无法在循环内部使用它们，因为它已经在构造函数中声明了。

有人能给我一些提示或建议吗？

我的代码:

    public class Calculator
{

    public Calculator()
    {
        Scanner input = new Scanner(System.in);
        boolean error = false;

        while (!error){
        System.out.print("Please input the following: principal, interest rate, term >> ");
        double principal = input.nextDouble();
        double interest_rate = input.nextDouble(); 
        int term = input.nextInt();
        String Month = input.next();


         char dollar_sym = 36;

        if (interest_rate <= 0 || term <= 0 || principal <= 0) // input validation
           {
             System.out.println("The term, interest rate and principal must be greater 
             than zero");
             continue; 
           }
        if (!Month.equals("month")) // input validation
           {
             System.out.println("Please input month after term"); 
             continue; 
           }

        System.out.println("Month: " + "  Interest: " + "Principal:  "); 

        if (Month.equals("month"))
        {
           for (int month = 1; month <= term; month++)
            { 
              double interest = (principal * interest_rate / 100) / 12;
              principal = principal + interest; 

              System.out.printf("%4d      %c%5.2f    %c%5.2f\n", month, 
              dollar_sym, interest, dollar_sym, principal );

              double start_principal = principal - interest; // problem
              double final_principal = principal; // problem 
              double total_interest = interest * interest_rate; // problem

              System.out.println(" Starting balance = " + start_principal ); // problem
              System.out.println("Final account balance = " + final_principal ); // problem
              System.out.println("Total Interest Paid = " + total_interest); // problem
           } 
         }
       }
     }
   }

Answer 1

在循环开始之前声明它们，这样它们将存在于循环内部和循环之后:

double start_principal = 0;
double final_principal = 0;
double total_interest = 0;

Scanner input = new Scanner(System.in);
boolean error = false;

while (!error) {
    // ...
}

// ...