Creating Methods

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public class Main {

    public static void main(String[] args) {
        String message = greetUser("Lucas", "YE");
        System.out.println(message);
    }

    public static String greetUser(String firstName, String lastName){
        return "Hello " + firstName + " " + lastName;
    }
}

Extracting Methords

改变代码的结构却不改变它的行为

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

        double mortgage = getMortgage(principal, annualInterest, years);

        String mortgageFormatted = NumberFormat.getCurrencyInstance().format(mortgage);
        System.out.print("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.nextDouble();
            if (value >= min && value <= max)
                break;
            System.out.println("Enter a number between " + min + " and " + max + ".");
        }
        return value;
    }

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

        float monthlyInterest = annualInterest / MONTHS_IN_YEAR / PERCENT;
        short numberOfPayments = (short) (years * MONTHS_IN_YEAR);

        double mortgage = principal
                * (monthlyInterest * Math.pow(1 + monthlyInterest, numberOfPayments))
                / (Math.pow(1 + monthlyInterest, numberOfPayments) - 1);
        return mortgage;
    }
}
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public class Main {
    final static byte MONTHS_IN_YEAR = 12;
    final static byte PERCENT = 100;

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

        printMortgage(principal, annualInterest, years);
        printPaymentSchedule(principal, annualInterest, years);
    }

    private static void printMortgage(int principal, float annualInterest, byte years) {
        double mortgage = calculateMortgage(principal, annualInterest, years);
        String mortgageFormatted = NumberFormat.getCurrencyInstance().format(mortgage);
        System.out.println();
        System.out.println("MORTGAGE");
        System.out.println("--------");
        System.out.println("Monthly Payments: " + mortgageFormatted);
    }

    private static void printPaymentSchedule(int principal, float annualInterest, byte years) {
        System.out.println();
        System.out.println("PAYMENT SCHEDULE");
        System.out.println("----------------");
        short numberOfPayments = (short) (years * MONTHS_IN_YEAR);
        for(short numberOfPaymentsMade = 1; numberOfPaymentsMade <= numberOfPayments; numberOfPaymentsMade++) {
            double balance = calculateBalance(principal, annualInterest, years, numberOfPaymentsMade);
            String balanceFormatted = NumberFormat.getCurrencyInstance().format(balance);
            System.out.println(balanceFormatted);
        }
    }

    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 number between " + min + " and " + max + ".");
        }
        return value;
    }

    public static double calculateMortgage(int principal,
                                           float annualInterest,
                                           byte years) {
        float monthlyInterest = annualInterest / MONTHS_IN_YEAR / PERCENT;
        short numberOfPayments = (short) (years * MONTHS_IN_YEAR);

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

        return mortgage;
    }

    public static double calculateBalance(int principal,
                                          float annualInterest,
                                          byte years,
                                          short numberOfPaymentsMade) {
        float monthlyInterest = annualInterest / MONTHS_IN_YEAR / PERCENT;
        short numberOfPayments = (short) (years * MONTHS_IN_YEAR);

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

        return balance;
    }
}