Command pattern in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the core purpose of the Command pattern

Which of the following best describes the main purpose of the Command pattern in software design?

A To create a single instance of a class to control access to a resource.

B To encapsulate a request as an object, allowing parameterization and queuing of requests.

C To define a family of algorithms and make them interchangeable at runtime.

D To separate the construction of a complex object from its representation.

💡 Hint

Think about how commands can be stored and executed later.

The Command pattern encapsulates a request as an object, which allows you to parameterize clients with queues, requests, and operations. This supports undoable operations and logging.

❓ Architecture

intermediate

Identifying components in a Command pattern architecture

In a typical Command pattern architecture, which component is responsible for executing the actual operation?

A Receiver

B Client

C Invoker

D Command

💡 Hint

Consider which part knows how to perform the work.

The Receiver is the component that knows how to perform the operations associated with carrying out a request. The Command object calls methods on the Receiver to fulfill the request.

❓ scaling

advanced

Scaling command execution in distributed systems

You want to design a system using the Command pattern that supports executing commands asynchronously across multiple servers. Which approach best supports scalability and fault tolerance?

A Distribute commands to multiple worker nodes via a message queue, allowing parallel processing and retries on failure.

B Execute commands synchronously on the client side to avoid network overhead.

C Store commands in a local file and process them in batches once a day.

D Use a centralized command queue with a single worker processing commands sequentially.

💡 Hint

Think about how to handle many commands efficiently and recover from failures.

Distributing commands via a message queue to multiple workers allows parallel processing, improving scalability. Message queues also support retries and fault tolerance, making the system more robust.

❓ tradeoff

advanced

Tradeoffs of using the Command pattern for undo functionality

When implementing undo functionality using the Command pattern, what is a key tradeoff to consider?

A The Command pattern cannot support undo operations effectively.

B Undo functionality requires commands to be stateless, simplifying design but limiting features.

C Commands must store state to reverse actions, increasing memory usage and complexity.

D Undo operations are automatically handled by the Invoker without extra code.

💡 Hint

Think about what is needed to reverse an action.

To support undo, commands often need to store the state before execution or implement reverse operations. This adds memory overhead and complexity to the command objects.

❓ estimation

expert

Estimating command queue capacity for a high-traffic system

A system uses the Command pattern with a command queue to handle user requests. The system expects 10,000 commands per second, each command averaging 5 KB in size. The queue should hold commands for up to 10 seconds to handle spikes. What is the minimum queue storage capacity required?

A 50 MB

B 500 KB

C 5 GB

D 500 MB

💡 Hint

Calculate total commands in 10 seconds and multiply by command size.

10,000 commands/sec * 10 seconds = 100,000 commands. Each command is 5 KB, so total = 100,000 * 5 KB = 500,000 KB = 500 MB.

Practice

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

easy

A. To create multiple instances of a class efficiently

B. To ensure only one instance of a class exists

C. To define a family of algorithms and make them interchangeable

D. To encapsulate a request as an object, allowing parameterization and queuing of requests

Solution

1. Step 1: Understand the Command pattern role

   The Command pattern encapsulates a request as an object, which allows you to parameterize clients with queues, requests, and operations.

2. Step 2: Compare with other patterns

   Creating multiple instances relates to Prototype or Factory, a family of algorithms to Strategy, and a single instance to Singleton; these are not Command.

3. Final Answer:

   To encapsulate a request as an object, allowing parameterization and queuing of requests -> Option D

4. Quick Check:

   Command pattern = encapsulate request [OK]

Hint: Command pattern = wrap action as object for flexibility [OK]

Common Mistakes:

• Confusing Command with Singleton or Factory patterns
• Thinking Command creates instances instead of encapsulating actions
• Mixing Command with Strategy pattern

2. Which of the following is the correct method signature for the execute method in a Command interface?

easy

A. void execute(String[] args);

B. void execute();

C. boolean execute(String commandName);

D. int execute(int commandId);

Solution

1. Step 1: Recall Command interface basics

   The Command interface typically defines a simple execute() method without parameters to perform the action.

2. Step 2: Analyze options

   The options with parameters (String[], int commandId, String commandName) or return types are not standard in Command pattern interfaces; the command object itself holds necessary data.

3. Final Answer:

   void execute(); -> Option B

4. Quick Check:

   Command execute method = void execute() [OK]

Hint: Command execute usually has no parameters [OK]

Common Mistakes:

• Adding parameters to execute method unnecessarily
• Confusing Command with other patterns that require arguments
• Assuming execute returns a value

3. Given the following code snippet implementing the Command pattern, what will be the output?

class Light {
  turnOn() { console.log('Light is ON'); }
  turnOff() { console.log('Light is OFF'); }
}

class TurnOnCommand {
  constructor(light) { this.light = light; }
  execute() { this.light.turnOn(); }
}

class TurnOffCommand {
  constructor(light) { this.light = light; }
  execute() { this.light.turnOff(); }
}

class RemoteControl {
  setCommand(command) { this.command = command; }
  pressButton() { this.command.execute(); }
}

const light = new Light();
const remote = new RemoteControl();
remote.setCommand(new TurnOnCommand(light));
remote.pressButton();
remote.setCommand(new TurnOffCommand(light));
remote.pressButton();

medium

A. Light is ON\nLight is OFF

B. Light is OFF\nLight is ON

C. Light is ON\nLight is ON

D. Light is OFF\nLight is OFF

Solution

1. Step 1: Trace first command execution

   The remote sets the command to TurnOnCommand and calls execute, which calls light.turnOn(), printing 'Light is ON'.

2. Step 2: Trace second command execution

   The remote sets the command to TurnOffCommand and calls execute, which calls light.turnOff(), printing 'Light is OFF'.

3. Final Answer:

   Light is ON\nLight is OFF -> Option A

4. Quick Check:

   TurnOn then TurnOff commands print ON then OFF [OK]

Hint: Follow command set and execute calls step-by-step [OK]

Common Mistakes:

• Mixing order of commands
• Assuming commands execute immediately without setting
• Confusing method names turnOn and turnOff

4. In the following code, what is the main issue that prevents the Command pattern from working correctly?

class Light {
  turnOn() { console.log('Light is ON'); }
}

class TurnOnCommand {
  constructor() { }
  execute() { this.light.turnOn(); }
}

const light = new Light();
const command = new TurnOnCommand();
command.execute();

medium

A. The execute method should return a value

B. The Light class is missing the turnOff method

C. The TurnOnCommand constructor does not receive or store the Light object

D. The command object is not instantiated properly

Solution

1. Step 1: Check TurnOnCommand constructor

   The constructor does not accept or assign the Light object to this.light, so this.light is undefined.

2. Step 2: Analyze execute method call

   Calling this.light.turnOn() fails because this.light is undefined, causing an error.

3. Final Answer:

   The TurnOnCommand constructor does not receive or store the Light object -> Option C

4. Quick Check:

   Missing light reference in command = error [OK]

Hint: Ensure command stores receiver object before execute [OK]

Common Mistakes:

• Ignoring missing receiver object in command
• Thinking missing turnOff method causes error here
• Assuming execute must return a value

5. You are designing a text editor with undo functionality using the Command pattern. Which design choice best supports undo operations efficiently?

hard

A. Store a history stack of Command objects and call an undo() method on the last command

B. Keep a log of all text changes as strings and replay them to undo

C. Use a single Command object that modifies text directly without history

D. Implement undo by reloading the entire document from disk

Solution

1. Step 1: Understand undo with Command pattern

   Each Command object should implement both execute() and undo() methods to reverse its action.

2. Step 2: Evaluate design choices

   Storing a history stack of Command objects allows calling undo() on the last command efficiently. Other options either lack command encapsulation or are inefficient.

3. Final Answer:

   Store a history stack of Command objects and call an undo() method on the last command -> Option A

4. Quick Check:

   Undo = command history stack with undo() [OK]

Hint: Undo needs command history with undo method [OK]

Common Mistakes:

• Using string logs instead of command objects
• Not implementing undo in commands
• Reloading entire document is inefficient