Java超出范围

Question

除了一个问题，我的程序几乎完成了。 我在for循环中遇到范围外的问题。 该计划的目标是使用户输入的金额和期限的每月利息增加。

以3年期，利率为5％的本金$ 5000计算的输出示例为：

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

我的问题是在我的for循环中，每次程序执行循环时，我都会一直输出“ 起始余额”，“最终余额”和“总利息 ”。 但是，如果我尝试在循环外使用变量，则超出范围；如果我尝试在循环外声明变量，则无法在循环内使用变量，因为它已在构造函数中声明。

谁能给我一些提示或建议？

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，以便它们将在循环内部及其之后存在：

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

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

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

// ...

Answer 2

在我的回答中，我假设当您说goes out of scope ，您的意思是您遇到了编译时错误。 （请注意，如果您提供了错误消息和引起错误消息的行，它将使答案更容易解决）。

变量的范围是指在哪里可以访问该变量。 例如，如果你声明的内部的一个变量if声明，范围是， if声明。 一些示例代码：

public void updateStatus(){
    Boolean shouldCheckStatus = true;//this variable is declared inside of the updateStatus method, so that is the scope of the variable
    if(shouldCheckStatus == true){
        Int hitCounter = 0;//this variable is declared inside of the if statement, so it is only accessible inside of the if statement
        //do some work
        if(hitCounter > 100){
            self.registerHits(hitCounter);//hitCounter is still accessible here, because it is still inside of the if statement
        }
        else{
            shouldCheckStatus = false;
        }
    }//close the if statement and so close the scope...
    //the hitCounter variable is no longer in scope, because we are no longer in the if statement
    //but shouldCheckStatus is still in scope, because we are still in the method
    if(shouldCheckStatus == true){
       self.callAnotherMethod();
    }
}

因此，在您遇到的问题中，您需要在要使用它的范围内，在要使用它的位置上方声明变量。 然后不再声明。 因此在循环之前声明。