Transforming Legacy Code: Simplify with Extract Class!

Legacy code can often feel like a tangled web of confusion, making it difficult to understand, modify, or enhance. Whether you're a seasoned developer or a beginner, you've likely experienced the challenges associated with managing legacy systems. The Extract Class refactoring technique is one powerful tool that can help simplify your code and make it more maintainable. In this blog post, we will delve into the Extract Class technique, discussing when and how to use it effectively.

What is the Extract Class Refactoring Technique?

The Extract Class refactoring technique involves taking a portion of a class that has too many responsibilities and moving it to a new class. This often leads to a cleaner codebase and better organization, allowing for easier testing, maintenance, and understanding.

Why Use Extract Class?

There are several reasons why you might want to use the Extract Class technique:

Single Responsibility Principle: According to this principle, a class should have only one reason to change. By extracting a class, you can adhere to this principle more closely, making changes easier and more predictable.
Improved Readability: Large classes can be overwhelming. By splitting functionality into smaller classes, others can more easily grasp the purpose and behavior of each class.
Enhanced Reusability: Smaller, well-defined classes can often be reused across different parts of an application, reducing code duplication and promoting DRY (Don’t Repeat Yourself) principles.
Simplified Testing: Smaller classes are easier to test individually. By isolating functionalities, you can write more focused unit tests.

When to Use Extract Class

Consider using the Extract Class technique when you observe any of the following signs:

A class has too many responsibilities and behaviors. This often leads to difficulty managing changes.
You're frequently changing or adding functionality in a specific section of a class.
Some methods and properties of the class might better belong in a different class.

Example Scenario: A Legacy Class

Let’s look at an example. Suppose we have the following legacy class Order that handles both order details and customer information.

public class Order {
    private String productId;
    private int quantity;
    private String customerName;
    private String customerAddress;

    public Order(String productId, int quantity, String customerName, String customerAddress) {
        this.productId = productId;
        this.quantity = quantity;
        this.customerName = customerName;
        this.customerAddress = customerAddress;
    }

    public void processOrder() {
        // Process the order
        System.out.println("Processing order for " + customerName + "...");
    }

    public void printCustomerDetails() {
        System.out.println("Customer Name: " + customerName);
        System.out.println("Customer Address: " + customerAddress);
    }
}

Problem Analysis

In this example, the Order class has two distinct responsibilities: managing order details and handling customer information. This violates the Single Responsibility Principle and makes the class more difficult to work with overall.

Refactoring with Extract Class

To refactor this class using Extract Class, we will create a new class named Customer.

Step 1: Create the Customer Class

public class Customer {
    private String name;
    private String address;

    public Customer(String name, String address) {
        this.name = name;
        this.address = address;
    }

    public String getName() {
        return name;
    }

    public String getAddress() {
        return address;
    }

    public void printDetails() {
        System.out.println("Customer Name: " + name);
        System.out.println("Customer Address: " + address);
    }
}

Step 2: Update the Order Class

Next, we'll modify the Order class to use this new Customer class:

public class Order {
    private String productId;
    private int quantity;
    private Customer customer;

    public Order(String productId, int quantity, Customer customer) {
        this.productId = productId;
        this.quantity = quantity;
        this.customer = customer;
    }

    public void processOrder() {
        // Process the order
        System.out.println("Processing order for " + customer.getName() + "...");
    }

    public void printCustomerDetails() {
        customer.printDetails();
    }
}

Commentary on the Refactored Code

Separation of Concerns: The Order class now solely handles order details, while the Customer class manages customer information. This separation makes both classes easier to modify and understand.
Improved Readability: Each class has a clear purpose. A developer can quickly see that Order deals with orders and Customer contains customer-related logic.
Reusability: The Customer class can now be reused wherever customer information is needed without duplicating code.

Testing the Refactored Classes

To test the refactored classes, we can create instances of Customer and Order.

public class Main {
    public static void main(String[] args) {
        Customer customer = new Customer("John Doe", "123 Elm Street");
        Order order = new Order("P001", 2, customer);
        
        order.processOrder();
        order.printCustomerDetails();
    }
}

Expected Output

When running the above code, you should see:

Processing order for John Doe...
Customer Name: John Doe
Customer Address: 123 Elm Street

Final Thoughts

The Extract Class technique is a vital tool for refactoring legacy code into more manageable components. By adhering to principles like Single Responsibility, improving readability, and enabling enhanced reusability, developers can significantly increase the maintainability of their applications.