Understanding the Law of Demeter in Object-Oriented Design

When designing software, maintainability, modularity, and low coupling are critical. One principle that helps achieve this in object-oriented programming is the Law of Demeter (LoD). Despite its formal name, the concept is simple: “only talk to your immediate friends.”

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1. What is the Law of Demeter?

The Law of Demeter, also known as the Principle of Least Knowledge, is a guideline for reducing coupling between classes. It states:

A method of an object should only call methods of:

1. Itself
2. Its own fields (member objects)
3. Objects passed as arguments
4. Objects created locally within the method

In simple terms:
A class should only interact with objects it directly knows, not objects retrieved from other objects.

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2. Why Law of Demeter Matters in LLD

In Low-Level Design (LLD), LoD is crucial because it:

• Reduces tight coupling between classes
• Makes the system easier to maintain
• Simplifies unit testing
• Encourages encapsulation, hiding internal details from other classes

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3. Example: Violating the Law of Demeter

class Engine {
    public void start() { System.out.println("Engine started"); }
}

class Car {
    private Engine engine = new Engine();
    public Engine getEngine() { return engine; }
}

class Driver {
    public void drive(Car car) {
        car.getEngine().start(); // ❌ Violates LoD
    }
}

Problem:
Driver knows too much about Car’s internals (the Engine).
This creates tight coupling — any change in Car’s engine structure could break Driver.

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4. Correct Implementation (Following LoD)

class Engine {
    public void start() { System.out.println("Engine started"); }
}

class Car {
    private Engine engine = new Engine();
    public void startEngine() { engine.start(); } // LoD-friendly interface
}

class Driver {
    public void drive(Car car) {
        car.startEngine(); // ✅ Only talks to immediate friend
    }
}

Benefits:

• Driver only interacts with Car, not Engine.
• Car encapsulates its Engine internally.
• Changes inside Engine do not affect Driver.

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5. Common Patterns to Apply Law of Demeter

1. Encapsulation: Keep fields private and provide public methods for necessary actions.
2. Facade Pattern: Use a higher-level interface to interact with complex subsystems.
3. Avoid method chaining: Don’t write code like a.getB().getC().doSomething();
4. Dependency Injection: Pass objects as arguments to methods rather than accessing nested objects directly.

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6. Summary Table

Principle	Description	Benefit
Law of Demeter (LoD)	Only call methods of immediate friends	Reduces coupling, increases modularity
Violation	Accessing sub-objects of sub-objects	Tight coupling, harder to maintain

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7. Key Takeaways

• LoD is all about reducing unnecessary dependencies between classes.
• Following it makes your code cleaner, more modular, and maintainable.
• In LLD, it ensures modules interact in a controlled, predictable manner.
• Always think: “Am I talking to my immediate friend, or someone I just met through another friend?”

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✅ Conclusion:
The Law of Demeter is a simple yet powerful principle. When applied consistently, it prevents tight coupling, encourages encapsulation, and results in robust, maintainable systems.

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