Functions

Note

Any fool can write code that computers can understand.
Good programmers write code that humans can understand.

Functions are code blocks with parameters, Sample code of function:

public class Main {
    
    public static void main(String[] args) {
        greetUser("Mosh", "Hamadani");
        greetUser("New", "Facade");
    }
    
    public static void greetUser(String firstName, String lastName) {
        System.out.println("Hello " + firstName + " " + lastName);
    }
}

Refactoring Mortgage Calculator using functions

Refactoring means changing the structure of the code without changing its bahavior. Whenever you want to refactor your method, you should look for two things:

1. The concepts or lines of code that always go together.

2. Look for repetitive patterns in your code.

We extract the repetitive read number pattern to function readNumber and the meaningful concepts of calculate mortgage to fucntion calculateMortgage.

import java.text.NumberFormat;
import java.util.Scanner;

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

        double mortgage = calculateMortgage(principal, annualInterestRate, years);
        System.out.println("Mortgage: " + NumberFormat.getCurrencyInstance().format(mortgage).trim());

    }
    
        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.nextDouble();
            if (value >= min && value <= max) {
                break;
            }
            System.out.println("Enter a value between " + min + " and " + max);
        }
        return value;
    }

    public static double calculateMortgage(
            int principal, float annualInterest, byte years) {
        final byte MONTHS_IN_YEAR = 12;
        final byte PERCENT = 100;

        float monthlyInterestRate = (annualInterest / PERCENT) / MONTHS_IN_YEAR;
        short numberOfPayments = (short)(years * MONTHS_IN_YEAR);
        return principal * (monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments) /
                (Math.pow(1 + monthlyInterestRate, numberOfPayments) - 1));
    }
}