Composition over inheritance in LLD - Architecture Diagram

System Overview - Composition over inheritance

This system demonstrates the principle of composition over inheritance in software design. Instead of creating deep class hierarchies, it composes objects with reusable components to achieve flexible and maintainable behavior.

The key requirement is to allow objects to share behaviors by composing small, focused components rather than inheriting from large base classes.

Architecture Diagram

User
  |
  v
+-----------------+
|   Client Code   |
+-----------------+
         |
         v
+-----------------+       +-----------------+       +-----------------+
|  Vehicle Object |<----->|  Engine Module  |<----->|  Fuel Module    |
+-----------------+       +-----------------+       +-----------------+
         |
         v
+-----------------+
| Navigation Module|
+-----------------+

Components

Client Code

client

Creates and uses composed objects instead of relying on inheritance

Vehicle Object

composed_object

Main object composed of multiple behavior modules

Engine Module

component

Provides engine-related behavior to the vehicle

Fuel Module

component

Manages fuel-related behavior independently

Navigation Module

component

Adds navigation capabilities to the vehicle

Request Flow - 5 Hops

Client Code → Vehicle Object

Vehicle Object → Engine Module

Vehicle Object → Fuel Module

Vehicle Object → Navigation Module

Vehicle Object → Client Code

Failure Scenario

Component Fails:Engine Module

Impact:Vehicle cannot start engine but other modules like Navigation and Fuel still function

Mitigation:Replace or fix Engine Module independently without affecting other components

Architecture Quiz - 3 Questions

Why does the system prefer composition over inheritance?

A To make the system slower but more complex

B To force all objects to share the same base class

C To allow flexible reuse of behaviors without deep class hierarchies

D To avoid using any reusable code

Design Principle

This design shows how composing small, focused components creates flexible and maintainable systems. It avoids rigid inheritance trees by assembling behaviors at runtime, enabling easier updates and independent failure handling.

Practice

1. What is the main advantage of using composition over inheritance in system design?

easy

A. It allows combining simple parts to build complex behavior flexibly.

B. It forces a strict class hierarchy that is easier to follow.

C. It eliminates the need for interfaces or contracts.

D. It automatically improves system performance.

Solution

1. Step 1: Understand composition concept

   Composition means building complex systems by combining smaller, reusable parts.

2. Step 2: Compare with inheritance

   Inheritance creates rigid class hierarchies, while composition offers flexibility and modularity.

3. Final Answer:

   It allows combining simple parts to build complex behavior flexibly. -> Option A

4. Quick Check:

   Composition = Flexible combination [OK]

Hint: Composition means combining parts, inheritance means extending classes [OK]

Common Mistakes:

• Thinking inheritance is always more flexible
• Confusing composition with interface implementation
• Believing composition removes need for design contracts

2. Which of the following code snippets correctly demonstrates composition in a class design?

easy

A. class Car extends Engine { }

B. class Engine { } class Car { Engine engine; }

C. class Car implements Engine { }

D. class Engine extends Car { }

Solution

1. Step 1: Identify composition syntax

   Composition means a class contains an instance of another class as a member.

2. Step 2: Analyze options

   class Engine { } class Car { Engine engine; } shows Car has an Engine instance, which is composition. Others use inheritance or interface incorrectly.

3. Final Answer:

   class Engine { } class Car { Engine engine; } -> Option B

4. Quick Check:

   Composition = class has member object [OK]

Hint: Composition means 'has-a' relationship, not 'is-a' [OK]

Common Mistakes:

• Confusing inheritance (extends) with composition
• Using implements keyword incorrectly for composition
• Assuming inheritance is composition

3. Given the following code, what will be the output?

class Battery {
  int capacity = 100;
}
class Phone {
  Battery battery = new Battery();
  int getBatteryCapacity() {
    return battery.capacity;
  }
}
Phone p = new Phone();
System.out.println(p.getBatteryCapacity());

medium

A. Compilation error

B. 0

C. NullPointerException

D. 100

Solution

1. Step 1: Understand composition usage

   Phone class contains a Battery instance initialized with capacity 100.

2. Step 2: Trace method call

   Calling getBatteryCapacity returns battery.capacity which is 100.

3. Final Answer:

   100 -> Option D

4. Quick Check:

   Composition returns member value correctly [OK]

Hint: Composition accesses member objects directly [OK]

Common Mistakes:

• Assuming battery is null without initialization
• Confusing inheritance with composition access
• Expecting compilation error due to missing constructor

4. Identify the error in the following code that tries to use composition:

class Screen { }
class Laptop {
  Screen screen;
  Laptop() {
    screen = null;
  }
  void display() {
    System.out.println(screen.toString());
  }
}

medium

A. screen is not initialized before use, causing NullPointerException

B. Screen class should extend Laptop for composition

C. display method should return a value, not print

D. Laptop constructor should not assign null to screen

Solution

1. Step 1: Check initialization of composed object

   screen is assigned null in constructor, so it is not a valid object.

2. Step 2: Analyze method usage

   Calling screen.toString() on null causes NullPointerException at runtime.

3. Final Answer:

   screen is not initialized before use, causing NullPointerException -> Option A

4. Quick Check:

   Null object in composition causes runtime error [OK]

Hint: Always initialize composed objects before use [OK]

Common Mistakes:

• Thinking inheritance is needed for composition
• Ignoring null initialization issues
• Confusing method return type with composition error

5. You are designing a payment system where different payment methods (credit card, PayPal, crypto) share common steps but have unique processing. How does using composition over inheritance improve your design?

hard

A. Composition requires duplicating code for each payment method, increasing maintenance.

B. Inheritance forces all payment methods to share the same code, reducing flexibility.

C. By composing payment method objects, you can add or change methods without altering a deep class hierarchy.

D. Inheritance allows runtime swapping of payment methods more easily than composition.

Solution

1. Step 1: Understand payment system requirements

   Different payment methods share steps but differ in processing details.

2. Step 2: Analyze composition benefits

   Composition lets you build payment methods as separate objects combined in the system, enabling easy addition or change without deep inheritance.

3. Step 3: Contrast with inheritance

   Inheritance creates rigid hierarchies that are hard to modify and extend for new payment types.

4. Final Answer:

   By composing payment method objects, you can add or change methods without altering a deep class hierarchy. -> Option C

5. Quick Check:

   Composition = flexible, modular payment design [OK]

Hint: Use composition to add features without changing base classes [OK]

Common Mistakes:

• Assuming inheritance is always simpler for new features
• Believing composition duplicates code unnecessarily
• Confusing runtime flexibility between inheritance and composition