Facade pattern in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the Facade class constructor.

LLD

class Facade {
    constructor() {
        this.subsystem = new [1]();
    }
}

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

AFacade

BSubsystem

CClient

DController

Attempts:

3 left

💡 Hint

Common Mistakes

Using the Facade class itself instead of the Subsystem.

Using unrelated class names like Client or Controller.

The Facade class creates an instance of the Subsystem to simplify client interaction.

2fill in blank

medium

Complete the code to implement a Facade method that calls a subsystem method.

LLD

class Facade {
    operation() {
        return this.subsystem.[1]();
    }
}

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

Aexecute

Bstart

Cprocess

Drun

Attempts:

3 left

💡 Hint

Common Mistakes

Using method names not defined in the subsystem.

Using generic names that do not exist in the subsystem.

The Facade calls the 'run' method of the subsystem to perform the operation.

3fill in blank

hard

Fix the error in the Facade method to correctly call two subsystem methods.

LLD

class Facade {
    complexOperation() {
        this.subsystem.[1]();
        this.subsystem.[1]();
    }
}

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

Ainitialize

Bstart

Cexecute

Drun

Attempts:

3 left

💡 Hint

Common Mistakes

Using different method names for the two calls.

Using method names that do not exist in the subsystem.

The Facade should call the 'execute' method twice to perform the complex operation.

4fill in blank

hard

Fill both blanks to complete the Facade method that coordinates subsystem methods.

LLD

class Facade {
    coordinate() {
        this.subsystem.[1]();
        this.subsystem.[2]();
    }
}

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

Ainitialize

Bstart

Cexecute

Dshutdown

Attempts:

3 left

💡 Hint

Common Mistakes

Reversing the order of initialization and start.

Using shutdown instead of start.

The Facade coordinates by first initializing then starting the subsystem.

5fill in blank

hard

Fill all three blanks to implement a Facade method that manages subsystem lifecycle.

LLD

class Facade {
    lifecycle() {
        this.subsystem.[1]();
        this.subsystem.[2]();
        this.subsystem.[3]();
    }
}

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

Ainitialize

Bstart

Cshutdown

Dexecute

Attempts:

3 left

💡 Hint

Common Mistakes

Incorrect order of method calls.

Using initialize instead of execute.

The Facade manages lifecycle by executing, starting, then shutting down the subsystem.

Practice

1. What is the main purpose of the Facade pattern in system design?

easy

A. To make the system slower by adding extra layers

B. To increase the number of classes in a system

C. To provide a simple interface to a complex system

D. To replace all subsystem classes with one class

Solution

1. Step 1: Understand the Facade pattern role

   The Facade pattern is designed to simplify interactions by providing a single interface to a complex system.

2. Step 2: Compare options with the pattern's purpose

   To provide a simple interface to a complex system correctly states the purpose. Options A, B, and C describe incorrect or harmful effects.

3. Final Answer:

   To provide a simple interface to a complex system -> Option C

4. Quick Check:

   Facade pattern = Simple interface [OK]

Hint: Facade means simple front for complex system [OK]

Common Mistakes:

• Thinking Facade replaces all classes
• Believing Facade adds complexity
• Confusing Facade with Singleton pattern

2. Which of the following is the correct way to define a Facade class method in a typical object-oriented language?

easy

A. private void complexOperation() { subsystem1.action(); }

B. public int complexOperation() { subsystem1.action(); subsystem2.action(); return 0; }

C. void complexOperation() { return subsystem1.action(); }

D. public void complexOperation() { subsystem1.action(); subsystem2.action(); }

Solution

1. Step 1: Identify Facade method visibility and calls

   Facade methods are usually public to expose simplified operations and call multiple subsystems internally.

2. Step 2: Check each option's correctness

   public void complexOperation() { subsystem1.action(); subsystem2.action(); } is public and calls multiple subsystems, matching Facade pattern. private void complexOperation() { subsystem1.action(); } is private, so not accessible. void complexOperation() { return subsystem1.action(); } returns a value but calls only one subsystem. public int complexOperation() { subsystem1.action(); subsystem2.action(); return 0; } returns int unnecessarily.

3. Final Answer:

   public void complexOperation() { subsystem1.action(); subsystem2.action(); } -> Option D

4. Quick Check:

   Facade method = public + multiple calls [OK]

Hint: Facade methods are public and call multiple subsystems [OK]

Common Mistakes:

• Making Facade methods private
• Returning values unnecessarily
• Calling only one subsystem inside Facade

3. Given this Facade code snippet, what will be the output?

class SubsystemA {
  void operationA() { print('A'); }
}
class SubsystemB {
  void operationB() { print('B'); }
}
class Facade {
  SubsystemA a = SubsystemA();
  SubsystemB b = SubsystemB();
  void doWork() {
    a.operationA();
    b.operationB();
  }
}
void main() {
  Facade f = Facade();
  f.doWork();
}

medium

A. BA

B. AB

C. A B

D. No output

Solution

1. Step 1: Trace Facade's doWork method calls

   doWork calls operationA() then operationB(), which print 'A' and 'B' respectively.

2. Step 2: Determine output sequence

   Since operationA prints 'A' first, then operationB prints 'B', output is 'AB' without spaces.

3. Final Answer:

   AB -> Option B

4. Quick Check:

   Facade calls order = AB [OK]

Hint: Follow method calls in order to find output [OK]

Common Mistakes:

• Assuming output is 'BA' reversing calls
• Expecting spaces between prints
• Thinking no output due to missing return

4. Identify the error in this Facade implementation:

class Subsystem {
  void action() { print('Action'); }
}
class Facade {
  Subsystem subsystem;
  void doTask() {
    subsystem.action();
  }
}
void main() {
  Facade f = Facade();
  f.doTask();
}

medium

A. Subsystem object is not initialized before use

B. Facade class should not call subsystem methods

C. doTask method should return a value

D. Subsystem class should be abstract

Solution

1. Step 1: Check subsystem object usage in Facade

   The Facade has a subsystem field but never initializes it before calling action(), causing a null reference error.

2. Step 2: Validate other options

   Facade class should not call subsystem methods is wrong because Facade should call subsystem methods. doTask method should return a value is not required. Subsystem class should be abstract is unnecessary.

3. Final Answer:

   Subsystem object is not initialized before use -> Option A

4. Quick Check:

   Uninitialized object = runtime error [OK]

Hint: Always initialize objects before calling methods [OK]

Common Mistakes:

• Ignoring null object initialization
• Thinking Facade should not call subsystems
• Expecting return values unnecessarily

5. You are designing a home automation system with many devices: lights, thermostat, security cameras, and music system. How would applying the Facade pattern help in this scenario?

hard

A. Create a single HomeAutomationFacade class that provides simple methods like turnOnAll() and setTemperature() to control all devices

B. Replace all device classes with one large class containing all device logic

C. Make each device class inherit from a common Facade superclass

D. Use Facade pattern to split each device into smaller classes

Solution

1. Step 1: Understand Facade's role in simplifying complex systems

   Facade groups multiple subsystem calls into simple methods, hiding complexity from users.

2. Step 2: Apply this to home automation

   Create a single HomeAutomationFacade class that provides simple methods like turnOnAll() and setTemperature() to control all devices correctly describes creating a Facade class with simple methods controlling multiple devices. Options A, B, and D misunderstand Facade's purpose.

3. Final Answer:

   Create a single HomeAutomationFacade class that provides simple methods like turnOnAll() and setTemperature() to control all devices -> Option A

4. Quick Check:

   Facade = simple interface for complex devices [OK]

Hint: Facade groups complex calls into simple methods [OK]

Common Mistakes:

• Merging all logic into one big class
• Confusing Facade with inheritance
• Splitting devices further instead of simplifying