About the bottom-up development🔗

Bottom-up development is a style of developing programs. With the bottom-up development, we develop in the reverse inside-out fashion; that is, we develop the service classes first. To test the service classes, we write a temporary dummy main class. After the service classes are done, we complete the top-level class that uses these service classes. The bottom-up development for this program implements the Loanclass first fully and then the LoanCalculatorclass.

Development steps🔗

In this example, we write the service class (Loanclass) and write the main function in the Loanclass to test the Loanclass. Next, we create a LoanCalculatorclass to handle the user’s input and output, and compute the monthly and total payments by calling the member method (getLoan()) of Loanclass.

Loan class🔗

Step 1🔗

First, we write a constructor (Loan()) to receive three form parameters (double amount, double rate and int period), and assign them to member variables (double amount, double rate and int period).

Second, we write two dummy function (double getMonPay() and double getTotalPay()) .

Third, we write a main function to test the Loanclass.

Source code:

class Loan {
    private final int MONTHS_IN_YEAR = 12;
    private double amount; // Loan amount
    private double rate; // Lnterest rate
    private int period; // Loan period

    // Constructor
    public Loan(double amount, double rate, int period){
        this.amount = amount;
        this.rate = rate;
        this.period = period;
    }

    // Calculating monthly payments
    public double getMonPay(){
        return 100.1;
    }

    // Calculating the total payments
    public double getTotalPay(){
        return 112312.1212;
    }


    // TEST
    public static void main(String[] args) {
        double amount = 100.3;
        double rate = 22;
        int period = 12;

        Loan loan = new Loan(amount, rate, period);

        System.out.println("Expected value: " + amount + " Actual value： " + loan.amount);
        System.out.println("Expected value: " + rate + " Actual value： " + loan.rate);
        System.out.println("Expected value: " + period + " Actual value： " + loan.period);

        System.out.println("Expected value: " + 100.1 + " Actual value： " + loan.getMonPay());

        System.out.println("Expected value: " + 112312.1212 + " Actual value： " + loan.getTotalPay());
    }
}

operation result:

Expected value: 100.3 Actual value： 100.3
Expected value: 22.0 Actual value： 22.0
Expected value: 12 Actual value： 12
Expected value: 100.1 Actual value： 100.1
Expected value: 112312.1212 Actual value： 112312.1212

Step 2🔗

Complete the logic in the dummy function to make it a true member method.

Use the real value in the main function to test whether the logic of the member method (double getMonPay() and double getTotalPay()) is correct.

Source code:

// Calculating monthly payments
public double getMonPay(){
    return (amount * (rate / 100.0)) / MONTHS_IN_YEAR;
}

// Calculating the total payments
public double getTotalPay(){
    return amount + ((amount * (rate / 100.0)) * period);
}

operation result:

Expected value: 100.3 Actual value： 100.3
Expected value: 22.0 Actual value： 22.0
Expected value: 12 Actual value： 12
Expected value: 1.838833 Actual value： 1.8388333333333333
Expected value: 365.092 Actual value： 365.092

So far, the Loanclass has been completed.

Loan Calculator class🔗

Step 1🔗

First, we write a member method (void getInput()) to handle input and another member method (void getOutput()) to handle output.

Sexond, we write a fake main function to test the input and output.

Source code:

import java.util.Scanner;

class LoanCalculator {
    private double amount; // Loan amount
    private double rate; // Lnterest rate
    private int period; // Loan period
    private double monPayment; // Monthly payment
    private double totalPayment; // Total payment

    public static void main(String[] args) {
        LoanCalculator loanCalculator = new LoanCalculator();

        loanCalculator.getInput();
        System.out.println("Input: " + loanCalculator.amount + " " + loanCalculator.rate + " " + loanCalculator.period);

        System.out.println();

        loanCalculator.monPayment = 100.1;
        loanCalculator.totalPayment = 200.2;
        loanCalculator.getOutput();
    }

    // Processing input
    private void getInput(){
        Scanner sc = new Scanner(System.in);

        System.out.print("Your loan amount: $");
        amount = sc.nextDouble();
        System.out.print("Your loan interest rate (%): ");
        rate = sc.nextDouble();
        System.out.print("Your loan period (year): ");
        period = sc.nextInt();

        sc.close();
    }

    // Processing output
    private void getOutput(){
        System.out.println("Your monthly payment is $" + monPayment);
        System.out.println("Your total payment is $" + totalPayment);
    }
}

operation result:

Your loan amount: $100.1
Your loan interest rate (%): 33.2
Your loan period (year): 12
Input: 100.1 33.2 12

Your monthly payment is $100.1
Your total payment is $200.2

Step 2🔗

Rewrite the main function, call the getMonPay() method of the Loan class to calculating the monthly payments, and call the getTotalPay() method of the Loan class to calculating the total payments.

Source code:

public static void main(String[] args) {
    LoanCalculator loanCalculator = new LoanCalculator();

    loanCalculator.getInput();

    Loan loan = new Loan(loanCalculator.amount, loanCalculator.rate, loanCalculator.period);
    System.out.println();

    loanCalculator.monPayment = loan.getMonPay();
    loanCalculator.totalPayment = loan.getTotalPay();
    loanCalculator.getOutput();
}

operation result:

Your loan amount: $100
Your loan interest rate (%): 22
Your loan period (year): 10

Your monthly payment is $1.8333333333333333
Your total payment is $320.0

Completed!!! ヾ(´▽‘)ﾉ.