Law of Demeter in LLD - System Design Exercise

Design: Law of Demeter Application in Software Design

Focus on the design principles and object interactions within a single module or small system. Out of scope are distributed systems, network communication, or database design.

Functional Requirements

FR1: Design a software module that follows the Law of Demeter to reduce coupling.

FR2: Ensure that objects only communicate with their immediate friends.

FR3: Avoid chaining calls that reach deep into object structures.

FR4: Demonstrate how to refactor a tightly coupled design into a loosely coupled one.

Non-Functional Requirements

NFR1: The design should minimize dependencies between modules.

NFR2: The system should be easy to maintain and extend.

NFR3: Latency and performance should not degrade significantly due to added abstraction.

NFR4: The design should be understandable by developers new to the project.

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

Key Components

Objects and their immediate collaborators

Interfaces or methods that expose internal structure

Encapsulation boundaries

Refactoring techniques to reduce coupling

Design Patterns

Facade pattern to hide complex subsystems

Mediator pattern to centralize communication

Tell, Don't Ask principle

Encapsulation and information hiding

Reference Architecture

Client
  |
  v
Module A (Facade)
  |
  v
Module B (Internal Objects)

- Client calls only Module A
- Module A communicates with Module B
- No direct calls from Client to Module B
- No chained calls like client.getA().getB().doSomething()

Components

Client

Any OO language

Uses the module through a simple interface without knowing internal details

Module A (Facade)

Any OO language

Provides a simplified interface and hides internal object structure

Module B (Internal Objects)

Any OO language

Contains detailed logic and data, accessed only by Module A

Request Flow

1. Client calls a method on Module A.

2. Module A performs necessary operations by calling its internal objects (Module B).

3. Module B executes logic and returns results to Module A.

4. Module A returns the final result to Client.

5. Client never calls methods on Module B directly or chains calls.

Database Schema

Not applicable as this design focuses on object interactions and coupling.

Scaling Discussion

Bottlenecks

Excessive method calls in facade can become complex.

Facade might become a god object if not carefully designed.

Performance overhead due to additional abstraction layers.

Difficulty in identifying direct collaborators if facade is too generic.

Solutions

Split facade into smaller facades for different responsibilities.

Use mediator pattern to manage complex interactions.

Profile and optimize critical paths to reduce overhead.

Maintain clear documentation of object relationships and interfaces.

Interview Tips

Time: Spend 10 minutes explaining the Law of Demeter and its importance, 15 minutes designing a simple example with a facade, and 10 minutes discussing scaling and trade-offs.

Explain how Law of Demeter reduces coupling and improves maintainability.

Show how to identify violations by spotting chained calls.

Demonstrate refactoring using facade or mediator patterns.

Discuss trade-offs between abstraction and performance.

Highlight benefits for team collaboration and code readability.

Practice

1.

What is the main idea behind the Law of Demeter in system design?

easy

A. An object should only communicate with its immediate friends.

B. An object can access any part of the system freely.

C. Objects should share all their internal details with others.

D. Objects must avoid any communication with other objects.

Solution

1. Step 1: Understand the Law of Demeter principle

   The Law of Demeter states that an object should only talk to its immediate friends, meaning it should not reach deep into other objects.

2. Step 2: Match the principle to the options

   An object should only communicate with its immediate friends. correctly describes this idea, while others contradict it.

3. Final Answer:

   An object should only communicate with its immediate friends. -> Option A

4. Quick Check:

   Law of Demeter = Talk only to immediate friends [OK]

Hint: Remember: Only talk to your immediate friends [OK]

Common Mistakes:

• Thinking objects can access any part of the system
• Believing objects should share all internal details
• Assuming no communication is allowed between objects

2.

Which of the following code snippets follows the Law of Demeter?

class Car {
  Engine engine;
  void start() {
    engine.start();
  }
}

easy

A. Calling engine.start() directly inside Car is allowed.

B. Accessing engine.fuel.tank.level inside Car is allowed.

C. Calling engine.getFuel().getTank().getLevel() is preferred.

D. Car should access engine.fuel.tank.level directly.

Solution

1. Step 1: Identify allowed communication

   According to the Law of Demeter, Car can call methods on its direct member engine.

2. Step 2: Analyze the options

   Calling engine.start() directly inside Car is allowed. shows Car calling engine.start(), which is allowed. Options A, B, and C involve accessing deeper nested objects, which breaks the law.

3. Final Answer:

   Calling engine.start() directly inside Car is allowed. -> Option A

4. Quick Check:

   Direct member calls = Allowed [OK]

Hint: Only call methods on your direct members [OK]

Common Mistakes:

• Accessing nested objects directly
• Chaining multiple getters in one call
• Ignoring encapsulation principles

3.

Consider this code snippet:

class A {
  B b;
  void doSomething() {
    int x = b.getC().getValue();
    System.out.println(x);
  }
}

Which statement about this code is true regarding the Law of Demeter?

medium

A. It follows the Law of Demeter perfectly.

B. It violates the Law of Demeter by chaining calls.

C. It violates the Law of Demeter by not calling b directly.

D. It follows the Law of Demeter by accessing getValue().

Solution

1. Step 1: Analyze the method call chain

   The code calls b.getC().getValue(), which chains two calls to reach a nested object.

2. Step 2: Compare with Law of Demeter

   The Law of Demeter discourages chaining calls like this; an object should only call methods on its direct members.

3. Final Answer:

   It violates the Law of Demeter by chaining calls. -> Option B

4. Quick Check:

   Chained calls = Violation [OK]

Hint: Watch out for chained method calls [OK]

Common Mistakes:

• Thinking chained calls are allowed
• Confusing direct member calls with nested calls
• Ignoring encapsulation rules

4.

Identify the problem in this code snippet related to the Law of Demeter and suggest a fix:

class Order {
  Customer customer;
  void printCustomerCity() {
    System.out.println(customer.getAddress().getCity());
  }
}

medium

A. No problem; code follows the Law of Demeter.

B. Violation: Order should print city without any method calls.

C. Violation: Order should access Address directly. Fix by removing Customer.

D. Violation: Order accesses nested Address. Fix by adding getCity() in Customer.

Solution

1. Step 1: Identify Law of Demeter violation

   Order calls customer.getAddress().getCity(), chaining calls to nested objects.

2. Step 2: Suggest fix to follow Law of Demeter

   Add a getCity() method in Customer that returns the city, so Order calls only customer.getCity().

3. Final Answer:

   Violation: Order accesses nested Address. Fix by adding getCity() in Customer. -> Option D

4. Quick Check:

   Use direct member methods to avoid chaining [OK]

Hint: Add helper methods to avoid deep calls [OK]

Common Mistakes:

• Ignoring nested call chains
• Removing needed objects instead of fixing access
• Trying to print without method calls

5.

You are designing a messaging system where ChatRoom has User objects, and each User has a Profile. To follow the Law of Demeter, how should ChatRoom get a user's display name?

hard

A. Make Profile public and access displayName directly.

B. chatRoom.getUser().getProfile().getDisplayName() chaining calls is fine.

C. Add a method getDisplayName() in User and call chatRoom.getUser().getDisplayName().

D. Let ChatRoom access Profile objects directly.

Solution

1. Step 1: Identify Law of Demeter violation in chaining

   Chaining calls like getUser().getProfile().getDisplayName() breaks the Law of Demeter.

2. Step 2: Apply Law of Demeter by adding helper method

   Adding getDisplayName() in User lets ChatRoom call only one method on its direct friend User.

3. Final Answer:

   Add a method getDisplayName() in User and call chatRoom.getUser().getDisplayName(). -> Option C

4. Quick Check:

   Use helper methods to avoid deep chains [OK]

Hint: Use helper methods in immediate objects [OK]

Common Mistakes:

• Allowing chained calls
• Making internal objects public
• Accessing nested objects directly