Complete Program

In completing the program, be sure that rate is correct. The user is supposed to enter percent interest, for example 6 for six percent. The program needs to convert that to a rate such as 0.06 to work correctly.

import java.util.Scanner;

class DollarsAfterForty
{

  public static void main( String[] args )  
  {
    double dollars = 1000.00 ;
    double rate;
    int    year =  1 ;     

    // get the interest rate from the user
    Scanner scan = new Scanner( System.in );
    System.out.print("Enter the interest rate in percent: "); 
    rate = scan.nextDouble()/100.0 ;
 
    while (  year <= 40 )
    {
      // add another year's interest
      dollars =  dollars + dollars*rate ; 

      // add in this year's contribution
      dollars = dollars + 1000 ;

      year    =  year + 1 ;
    }

    System.out.println("After 40 years at " + rate*100 
      + " percent interest you will have " + dollars + " dollars" );
  }

}

Here is an example run:

C:\users\default\JavaLessons\chap19>java DollarsAfterForty
Enter the interest rate in percent: 11
After 40 years at 11.0 percent interest you will have 646826.9337201559 dollars

C:\users\default\JavaLessons\chap19>java DollarsAfterForty
Enter the interest rate in percent: 12
After 40 years at 12.0 percent interest you will have 860142.3907860613 dollars

C:\users\default\JavaLessons\chap19>java DollarsAfterForty
Enter the interest rate in percent: 13
After 40 years at 13.0 percent interest you will have 1146485.7949682677 dollars

It looks like the interest rate should be somewhere between 12 and 13 percent.