Decimal Format Errors - String Cannot Be Converted To Double

I am working on a project that just calculates simple interest based upon a principal and a percent interest rate written as .xxxx from user.

I need the Loan amount and the final interest calculation to show up as a currency with commas in appropriate areas, the Rate to be expressed as X.xxx% instead of .xxxx

I keep getting this error when I compile:

C:JavaInterestCalculator.java:46: error: incompatible types: String cannot be converted to double
principal = formatter.format(principal);
^
C:JavaInterestCalculator.java:49: error: incompatible types: String cannot be converted to double
rate = formatter.format(rate);
^
C:JavaInterestCalculator.java:52: error: incompatible types: String cannot be converted to double
totalInterest = formatter.format(totalInterest);
^
3 errors

Tool completed with exit code 1

And here is my code

import java.util.Scanner;
import java.text.NumberFormat;
import java.text.DecimalFormat;
// class declaration
public class InterestCalculator
{
// main method declaration

[Code] .....