Composition over inheritance in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why prefer composition over inheritance?

Which of the following best explains why composition is often preferred over inheritance in system design?

A Inheritance avoids code duplication better than composition in every scenario.

B Composition allows changing behavior at runtime by swapping components, while inheritance fixes behavior at compile time.

C Composition requires fewer classes than inheritance in all cases.

D Inheritance always leads to faster code execution than composition.

💡 Hint

Think about flexibility and how behavior can be changed after the system is running.

Composition allows objects to contain other objects and change their behavior dynamically by swapping components. Inheritance fixes behavior in a class hierarchy, making it less flexible.

❓ Architecture

intermediate

Design choice for a payment system

You are designing a payment system that supports multiple payment methods (credit card, PayPal, crypto). Which design approach best uses composition over inheritance?

A Use inheritance to create subclasses for each payment type and hardcode the payment logic in each subclass.

B Create a PaymentMethod interface and have each payment type inherit from it, implementing their own logic.

C Create separate classes for each payment type with no shared interface or composition.

D Create a PaymentProcessor class that contains a PaymentMethod component which can be swapped to change payment type at runtime.

💡 Hint

Consider how you can change payment methods without changing the main processor class.

Using composition, the PaymentProcessor contains a PaymentMethod component that can be swapped dynamically, allowing flexible payment method changes without modifying the processor.

❓ scaling

advanced

Scaling a notification system with composition

You have a notification system that sends alerts via email, SMS, and push notifications. To scale and add new channels easily, which design is best?

A Use composition where a NotificationSender class contains a list of NotificationChannel components that can be added or removed dynamically.

B Hardcode all notification types inside one large Notification class using if-else statements.

C Use inheritance to create subclasses for each notification type and add new subclasses for new channels.

D Create separate monolithic services for each notification type without shared components.

💡 Hint

Think about how to add or remove notification channels without changing existing code.

Composition allows the NotificationSender to hold multiple NotificationChannel components, making it easy to add or remove channels dynamically and scale the system.

❓ tradeoff

advanced

Tradeoffs between composition and inheritance

Which of the following is a valid tradeoff when choosing composition over inheritance?

A Inheritance always results in simpler code and fewer classes than composition.

B Composition does not allow code reuse, unlike inheritance which always does.

C Composition can increase the number of objects and indirection, potentially impacting performance compared to inheritance.

D Inheritance allows changing behavior at runtime, while composition fixes behavior at compile time.

💡 Hint

Consider the overhead of managing multiple objects versus a class hierarchy.

Composition can introduce more objects and layers of delegation, which may add some performance overhead compared to inheritance's direct method calls.

❓ component

expert

Component interaction in a compositional design

In a compositional design for a media player, the Player class composes components: Decoder, Renderer, and Logger. Which sequence best describes the request flow when playing a media file?

A 1, 2, 3, 4

B 1, 3, 2, 4

C 2, 1, 3, 4

D 1, 4, 2, 3

💡 Hint

Think about the logical order of processing from request to output and logging.

The Player first receives the request, then the Decoder processes the file, Renderer outputs the media, and Logger records the event last.

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