Welcome to the third lesson of the "Applying Clean Code Principles" course. In our journey so far, we've discussed the importance of the DRY (Don't Repeat Yourself) principle to eliminate redundancy in code. We followed that with the KISS (Keep It Simple, Stupid) principle, which highlights the value of simplicity in software development. Today, our spotlight is on the Law of Demeter — a key guideline in object-oriented programming. By limiting the knowledge that an object has about other objects, this lesson will guide you in crafting more maintainable and modular code. 🤓

The Law of Demeter suggests that an object should only communicate with its immediate collaborators, avoiding the entire system. By reducing dependency between parts, you'll find your code easier to maintain and scale. In simple terms, a method in a class should only call methods of:

• The class itself
• An object created within the method
• An object passed as an argument to the method
• An object held in an instance variable of the class
• A class constant

With these principles, you control how parts of your application interact, leading to a more organized structure. Let's explore how this works with examples. 🚀

For the first point, a method should only access its own class's methods:

Ruby
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1class Car
2  def start
3    check_fuel
4    ignite
5  end
6
7  private
8
9  def check_fuel
10    puts "Checking fuel level..."
11  end
12
13  def ignite
14    puts "Igniting the engine..."
15  end
16end

In this example, the start method interacts solely with methods within the Car class itself. This shows how you maintain clear boundaries adhering to the Law of Demeter.

Next, a method can interact with the objects it creates:

Ruby
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1class Library
2  def borrow_book(title)
3    book = Book.new(title)
4    book.issue
5    book
6  end
7end
8
9class Book
10  def initialize(title)
11    @title = title
12  end
13
14  def issue
15    puts "Book issued: #{@title}"
16  end
17end

Here, the Library class creates a Book and calls the issue method on it. This usage pattern complies with the Law of Demeter, where Library interacts with the newly created Book. 📚

Continuing, let's look at interacting with objects passed as arguments:

Ruby
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1class Printer
2  def print(document)
3    document.send_to_printer
4  end
5end
6
7class Document
8  def send_to_printer
9    puts "Document is being printed..."
10  end
11end

The Printer class method print communicates with the Document object passed as an argument, aligning with the Law of Demeter by limiting communication to direct method parameters. 🖨️

Objects held in instance variables of a class can also be accessed:

Ruby
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1class House
2  def initialize
3    @door = Door.new
4  end
5
6  def lock_house
7    @door.close
8  end
9end
10
11class Door
12  def close
13    puts "Door is closed."
14  end
15end

In this example, the House class interacts with its @door through the lock_house method, showcasing compliance by interacting with an object it holds in an instance variable. 🏠

Finally, let's see a method interacting with constants. Constants should generally be used cautiously since they can lead to shared state issues in larger applications:

Ruby
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1class TemperatureConverter
2  CONVERSION_FACTOR = 9.0 / 5.0
3
4  def celsius_to_fahrenheit(celsius)
5    (celsius * CONVERSION_FACTOR + 32).to_i
6  end
7end

Here, CONVERSION_FACTOR is defined as a constant to indicate that it's a constant and to ensure correct calculations. Accessing constants like this is compliant with the Law of Demeter. 🌡️

Here's an example that violates the Law of Demeter:

Ruby
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1class Person
2  def initialize(address)
3    @address = address
4  end
5
6  def get_address_details
7    "Address: #{@address.first_name} #{@address.last_name}, #{@address.street}, " \
8    "#{@address.city}, #{@address.country}, ZipCode: #{@address.zip_code}"
9  end
10end
11
12class Address
13  attr_reader :first_name, :last_name, :street, :city, :country, :zip_code
14
15  # Assume initialization for attributes here
16end

In this case, Person is directly accessing multiple fields through Address, leading to tight coupling. Person relies on the internal structure of Address, which might result in fragile code.

Let's refactor the previous code to adhere to the Law of Demeter:

Ruby
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1class Person
2  def initialize(address)
3    @address = address
4  end
5
6  def get_address_details
7    @address.get_address_line
8  end
9end
10
11class Address
12  def get_address_line
13    "#{first_name} #{last_name}, #{street}, #{city}, #{country}, ZipCode: #{zip_code}"
14  end
15
16  private
17
18  # Assume private accessors or methods for attributes here
19end

By encapsulating all the address details within the get_address_line method in the Address class, the dependency is minimized, and Person no longer accesses Address's internals directly.

The Law of Demeter plays a vital role in writing clean, modular code by ensuring objects only interact with their closest dependencies. By understanding and implementing these guidelines, you enhance the modularity and maintainability of your code. As you move on to the practice exercises, challenge yourself to apply these principles and evaluate your code's interactions. Keep these lessons in mind as essential steps toward mastering clean code! 🌟