搞清java中数据基本类型的转换

import javax.swing.JOptionPane;

public class ComputeLoan {

  /** Main method */

  public static void main(String[] args) {

    // Enter yearly interest rate
    String annualInterestRateString = JOptionPane.showInputDialog(

      "Enter yearly interest rate, for example 8.25:");



    // Convert string to double
    double annualInterestRate =

      Double.parseDouble(annualInterestRateString);



    // Obtain monthly interest rate
    double monthlyInterestRate = annualInterestRate / 1200;



    // Enter number of years
    String numberOfYearsString = JOptionPane.showInputDialog(

      "Enter number of years as an integer, \nfor example 5:");



    // Convert string to int
    int numberOfYears = Integer.parseInt(numberOfYearsString);



    // Enter loan amount
    String loanString = JOptionPane.showInputDialog(

      "Enter loan amount, for example 120000.95:");



    // Convert string to double
    double loanAmount = Double.parseDouble(loanString);



    // Calculate payment
    double monthlyPayment = loanAmount * monthlyInterestRate / (1

      - 1 / Math.pow(1 + monthlyInterestRate, numberOfYears * 12));

    double totalPayment = monthlyPayment * numberOfYears * 12;



    // Format to keep two digits after the decimal point
    monthlyPayment = (int)(monthlyPayment * 100) / 100.0;

    totalPayment = (int)(totalPayment * 100) / 100.0;



    // Display results
    String output = "The monthly payment is " + monthlyPayment +

      "\nThe total payment is " + totalPayment;

    JOptionPane.showMessageDialog(null, output);

  }