Custom methods - Bringing it all together

Input/Output and Streams in Java

Alex Liu

Software Development Engineer

Integrating Concepts

Combine and use:
- Enums
- Date and Time
- Recursion
Goal: build clear, reusable, and maintainable methods
Example program:
- An e-commerce site
  - with practical OrderStatus system with order tracking methods
  - with loyalty discount algorithm.

Defining Enum with methods

Define OrderStatus Enum to represent order states

enum OrderStatus {
    PLACED("Order placed successfully"),
    DELIVERED("Order delivered");

    private final String message;
    OrderStatus(String message) {
        this.message = message;
    }

    public String getMessage() {
        return message;
    }
}

Custom method for Date operations

Use orderDate to estimate when the order will deliver to customer
- User .plusDays() method adds days to a given date

import java.time.LocalDate;

public class OrderUtils {
    public static LocalDate estimateDelivery(LocalDate orderDate, 
                                            int daysToDeliver) {
        return orderDate.plusDays(daysToDeliver);
    }
}

Recursive method for discount

Reward policy: Incrementing discounts month-by-month to reward return customers
- Use recursive method to calculate the cumulative discount:

public class DiscountCalculator {
    public static double cumulativeDiscount(int months) {
        if (months <= 0) return 0;
        return 5 + cumulativeDiscount(months - 1); // $5 discount per month
    }
}

Custom object

Create an Order class which contain all order related information
- Contain important information about order like id, date and current status.

class Order {
    int orderId;
    OrderStatus orderStatus;
    LocalDate orderDate;

    Order(int orderId, LocalDate orderDate) {
        this.orderId = orderId;
        this.orderDate = orderDate;
        this.orderStatus = orderStatus;
    }

Customer object with toString() method

We want to print the order in a way that the employee is easy to read or process
- Define toString() method to define how the Order object will be printed.

public String toString() {
        return "Order[id=" + orderId + ", date=" + orderDate + "]";
    }
}

Sample output by calling .toString():
```
Order[id=10015, date=2025-03-19]
```

Main method

public class Main {
    public static void main(String[] args) {
        Order order = new Order(101, LocalDate.now());
        System.out.println(order.toString());


        LocalDate delivery = OrderUtils.estimateDelivery(order.orderDate, 7);
        System.out.println("Estimated Delivery: " + delivery);
        System.out.println(OrderStatus.PLACED);


        double discount = DiscountCalculator.cumulativeDiscount(3);
        System.out.println("Total Discount: $" + discount);
    }
}

Main method - output

This main method is executed when a new order is placed with following information:
- The user has subscribed for 3 months already
- The estimate delivery time is 7 days
- Order status will initially be PLACED

Order[id=101, date=2025-03-09]
Estimated Delivery: 2025-03-16
Order placed successfully
Total Discount: $15.0

Let's practice!

Input/Output and Streams in Java