Composition over inheritance in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a class that uses composition to include a Logger component.

LLD

class Application:
    def __init__(self):
        self.logger = [1]()

Drag options to blanks, or click blank then click option'

ALogger

BInheritance

CComponent

DInterface

Attempts:

3 left

💡 Hint

Common Mistakes

Using inheritance keyword instead of creating an instance.

The Application class uses composition by creating an instance of the Logger class inside it.

2fill in blank

medium

Complete the code to delegate the log message to the composed Logger object.

LLD

class Application:
    def __init__(self):
        self.logger = Logger()
    def log(self, message):
        self.logger.[1](message)

Drag options to blanks, or click blank then click option'

Alog

Bwrite

Cprint

Dsend

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method name that does not exist in Logger.

The Application class calls the log method of the Logger instance to delegate logging.

3fill in blank

hard

Fix the error in the code to properly use composition instead of inheritance.

LLD

class Logger:
    def log(self, message):
        print(message)

class Application([1]):
    def __init__(self):
        self.logger = Logger()

Drag options to blanks, or click blank then click option'

AApplication

BLogger

Cobject

DComponent

Attempts:

3 left

💡 Hint

Common Mistakes

Inheriting from Logger instead of using it as a component.

The Application class should not inherit from Logger but from object to use composition properly.

4fill in blank

hard

Fill both blanks to complete the code that shows composition with two components.

LLD

class Application:
    def __init__(self):
        self.logger = [1]()
        self.database = [2]()

Drag options to blanks, or click blank then click option'

ALogger

BDatabase

CComponent

DService

Attempts:

3 left

💡 Hint

Common Mistakes

Using generic names instead of actual component classes.

The Application class composes Logger and Database components by creating their instances.

5fill in blank

hard

Fill all three blanks to complete the code that delegates methods to composed components.

LLD

class Application:
    def __init__(self):
        self.logger = Logger()
        self.database = Database()
    def log(self, message):
        self.logger.[1](message)
    def save(self, data):
        self.database.[2](data)
    def load(self, id):
        return self.database.[3](id)

Drag options to blanks, or click blank then click option'

Alog

Bsave

Cload

Dprint

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect method names that do not exist in components.

The Application class delegates log, save, and load calls to the respective component methods.

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