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Clean Architecture & Design Patterns in Practice · Lesson

Designing Business Entities

Define your core business objects (Entities) that encapsulate enterprise-wide business rules and remain stable.

Designing Business Entities is a free Clean Architecture & Design Patterns in Practice lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Clean Architecture & Design Patterns in Practice learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

What are Business Entities?

In Clean Architecture, Entities are the heart of your application. They represent the core business concepts and rules that are crucial to your enterprise.

Think of them as the fundamental building blocks that define "what your business is" and "how it works," independent of how you store or display data.

Core Business Rules

Entities encapsulate enterprise-wide business rules. These are rules that would exist even if there were no software system, like "a product must have a positive price" or "an order must have a customer."

These rules are stable and apply across your entire organization, not just a specific part of your application.

Independent & Stable

A key principle of Entities is their independence. They should not depend on external layers like databases, web frameworks, or UI components.

This makes them the most stable part of your application, changing only when the core business rules themselves change, not when technology does.

Entity vs. DTO

It's important to distinguish an Entity from a simple Data Transfer Object (DTO). A DTO is just a bag of data, often used to move data between layers.

An Entity, however, contains both data and behavior. It actively enforces its own business rules through its methods.

Entity Structure

Typically, an Entity includes:

  • Attributes: The data that describes the business object (e.g., name, ID, price).
  • Methods: The operations that can be performed on the entity, enforcing its business rules and maintaining its integrity.

These methods ensure the entity always remains in a valid state according to your business logic.

Example: `Product` Entity

Let's consider a simple Product entity. What data and rules would it need?

  • A product needs an id, name, and price.
  • A core business rule might be: "A product's price must always be positive."

The entity itself should enforce this rule.

`Product` Entity Code

Here's how a basic Product entity might look in Java, ensuring the price rule:

public class Product {
  private String id;
  private String name;
  private double price;

  public Product(String id, String name, double price) {
    if (price <= 0) {
      throw new IllegalArgumentException("Price must be positive.");
    }
    this.id = id;
    this.name = name;
    this.price = price;
  }

  public String getId() { return id; }
  public String getName() { return name; }
  public double getPrice() { return price; }

  public static void main(String[] args) {
    try {
      Product laptop = new Product("P001", "Laptop", 1200.00);
      System.out.println(laptop.getName() + " costs $" + laptop.getPrice());
      // This would throw an error:
      // Product invalid = new Product("P002", "Free Item", 0.00);
    } catch (IllegalArgumentException e) {
      System.out.println("Error creating product: " + e.getMessage());
    }
  }
}

Entity Behavior & Rules

Entities aren't just data containers; they have methods that encapsulate behavior and enforce rules. For example, applying a discount might have its own business logic.

The entity's methods ensure its internal state remains consistent and valid according to business rules.

`Product` Discount Example

Let's add a method to our Product entity to apply a discount, with a rule that discounts can't make the price negative.

public class Product {
  private String id;
  private String name;
  private double price;

  public Product(String id, String name, double price) {
    if (price <= 0) {
      throw new IllegalArgumentException("Price must be positive.");
    }
    this.id = id;
    this.name = name;
    this.price = price;
  }

  public String getId() { return id; }
  public String getName() { return name; }
  public double getPrice() { return price; }

  // New method to apply discount
  public void applyDiscount(double percentage) {
    if (percentage < 0 || percentage > 100) {
      throw new IllegalArgumentException("Discount must be between 0-100%.");
    }
    double newPrice = this.price * (1 - percentage / 100);
    if (newPrice < 0) { // Ensure price doesn't go negative
      this.price = 0;
    } else {
      this.price = newPrice;
    }
  }

  public static void main(String[] args) {
    Product book = new Product("B001", "Clean Code Book", 40.00);
    System.out.println("Original: " + book.getName() + " $" + book.getPrice());
    book.applyDiscount(25); // 25% discount
    System.out.println("Discounted: " + book.getName() + " $" + book.getPrice());

    try {
      book.applyDiscount(150); // Invalid discount
    } catch (IllegalArgumentException e) {
      System.out.println("Error applying discount: " + e.getMessage());
    }
  }
}

Entity Properties Check

Based on what we've learned, which of the following are true characteristics of a Clean Architecture Entity?

Recap: Designing Entities

Great job! You've learned about Entities in Clean Architecture. Key takeaways:

  • Entities represent your core business concepts.
  • They encapsulate enterprise-wide business rules.
  • Entities contain both data and behavior, actively enforcing their rules.
  • They are independent and stable, not tied to frameworks or external layers.

Next, we'll explore how Use Cases interact with these robust entities to perform application-specific tasks.

Frequently asked questions

Is the “Designing Business Entities” lesson free?

Yes — the full text of “Designing Business Entities” is free to read here on the web, and the Clean Architecture & Design Patterns in Practice course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Clean Architecture & Design Patterns in Practice course, upgrade to CoddyKit PRO.

What will I learn in “Designing Business Entities”?

Define your core business objects (Entities) that encapsulate enterprise-wide business rules and remain stable. You practise Clean Architecture & Design Patterns in Practice with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Clean Architecture & Design Patterns in Practice?

No prior experience is required. Clean Architecture & Design Patterns in Practice on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Designing Business Entities” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Clean Architecture & Design Patterns in Practice lesson?

Yes. Every Clean Architecture & Design Patterns in Practice lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Designing Business Entities
  2. Implementing Use Cases (Interactors)
  3. Input and Output Ports
  4. Enforcing Business Rules with Invariants
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