Clean Coding

• Use methods
  • Keeps your methods short
• Refactoring (changing the structure of the code without changing its behavior)
  • Extract repetitive patterns
  • Extract highly related statements

Before:

public static void main(String[] args) {
    final byte MONTHS_IN_YEAR = 12;
    final byte PERCENT = 100;

    int principal = 0;
    float monthlyInterest = 0;
    int numberOfPayments = 0;

    Scanner scanner = new Scanner(System.in);

    while (true) {
        System.out.print("Principal: ");
        principal = scanner.nextInt();
        if (principal >= 1000 && principal <= 1_000_000)
            break;
        System.out.println("Enter a value between 1000 and 1000000");
    }

    while (true) {
        System.out.print("Annual Interest Rate: ");
        float annualInterest = scanner.nextFloat();
        if (annualInterest >= 1 && annualInterest <= 30) {
            monthlyInterest = annualInterest / PERCENT / MONTHS_IN_YEAR;
            break;
        }
        System.out.println("Enter a value between 1 and 30");
    }

    while (true) {
        System.out.print("Period (Years): ");
        byte years = scanner.nextByte();
        if (years >= 1 && years <= 30) {
            numberOfPayments = years * MONTHS_IN_YEAR;
            break;
        }
        System.out.println("Enter a value between 1 and 30.");
    }

    double mortgage = principal
        * (monthlyInterest * Math.pow(1 + monthlyInterest, numberOfPayments))
        / (Math.pow(1 + monthlyInterest, numberOfPayments) - 1);

    String mortgageFormatted = NumberFormat.getCurrencyInstance().format(mortgage);
    System.out.println("Mortgage: " + mortgageFormatted);
}

After refactoring:

final static byte MONTHS_IN_YEAR = 12;
final static byte PERCENT = 100;

public static void main(String[] args) {
    int principal = (int) readNumber("Principal: ", 1000, 1_000_000);
    float annualInterest = (float) readNumber("Annual Interest Rate: ", 1, 30);
    byte years = (byte) readNumber("Period (Years): ", 1, 30);

    double mortgage = calculateMortgage(principal, annualInterest, years);
   
    String mortgageFormatted = NumberFormat.getCurrencyInstance().format(mortgage);
    System.out.println("Mortgage: " + mortgageFormatted);
}

public static double readNumber(String prompt, double min, double max) {
    Scanner scanner = new Scanner(System.in);
    double value;
    while (true) {
        System.out.print(prompt);
        value = scanner.nextFloat();
        if (value >= min && value <= max)
            break;
        System.out.println("Enter a value between " + min + " and " + max);
    }
    return value;
}

// all the logic of calculating mortage are in one place, graceful
public static double calculateMortgage(int principal, float annualInterest, byte years) {

    float monthlyInterest = annualInterest / PERCENT / MONTHS_IN_YEAR;
    float numberOfPayments = years * MONTHS_IN_YEAR;

    double mortgage = principal
        * (monthlyInterest * Math.pow(1 + monthlyInterest, numberOfPayments))
        / (Math.pow(1 + monthlyInterest, numberOfPayments) - 1);

    return mortgage;
}