🧑‍💻 Author: RK ROY

⚡ Command Pattern

Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

📋 Table of Contents

🎯 Intent
🚀 Problem & Solution
🏗️ Structure
💻 Implementation
🌟 Real-World Examples
✅ Best Practices
❌ Common Pitfalls
🎤 Interview Questions

🎯 Intent

The Command pattern encapsulates a request as an object, allowing you to parameterize clients with different requests, queue operations, log requests, and support undo/redo functionality. It decouples the object that invokes the operation from the object that performs it.

Key Characteristics

✅ Request Encapsulation: Commands are objects that encapsulate requests
✅ Decoupling: Invoker and receiver are decoupled
✅ Undo/Redo Support: Commands can be reversed
✅ Queuing & Logging: Commands can be stored and executed later
✅ Macro Commands: Multiple commands can be combined

🚀 Problem & Solution

🚨 Problem

Consider these scenarios:

GUI Applications: Button clicks need to trigger different actions
Remote Controls: Different buttons control various devices
Text Editors: Undo/Redo functionality for text operations
Task Queuing: Executing operations in background
Macro Recording: Recording and replaying user actions

💡 Solution

🏗️ Structure

UML Class Diagram

Sequence Diagram

💻 Implementation

1. 🔰 Basic Command Pattern (Java)

java

// Command interface
interface Command {
    void execute();
    void undo();
}

// Receiver classes
class Light {
    private boolean isOn = false;
    private int brightness = 0;

    public void turnOn() {
        isOn = true;
        brightness = 100;
        System.out.println("Light is ON (brightness: " + brightness + ")");
    }

    public void turnOff() {
        isOn = false;
        brightness = 0;
        System.out.println("Light is OFF");
    }

    public void setBrightness(int level) {
        if (isOn) {
            brightness = Math.max(0, Math.min(100, level));
            System.out.println("Light brightness set to: " + brightness);
        }
    }

    public boolean isOn() { return isOn; }
    public int getBrightness() { return brightness; }
}

class Fan {
    private boolean isOn = false;
    private int speed = 0;

    public void turnOn() {
        isOn = true;
        speed = 1;
        System.out.println("Fan is ON (speed: " + speed + ")");
    }

    public void turnOff() {
        isOn = false;
        speed = 0;
        System.out.println("Fan is OFF");
    }

    public void setSpeed(int speed) {
        if (isOn) {
            this.speed = Math.max(0, Math.min(5, speed));
            System.out.println("Fan speed set to: " + this.speed);
        }
    }

    public boolean isOn() { return isOn; }
    public int getSpeed() { return speed; }
}

// Concrete Commands
class LightOnCommand implements Command {
    private Light light;

    public LightOnCommand(Light light) {
        this.light = light;
    }

    @Override
    public void execute() {
        light.turnOn();
    }

    @Override
    public void undo() {
        light.turnOff();
    }
}

class LightOffCommand implements Command {
    private Light light;

    public LightOffCommand(Light light) {
        this.light = light;
    }

    @Override
    public void execute() {
        light.turnOff();
    }

    @Override
    public void undo() {
        light.turnOn();
    }
}

class FanOnCommand implements Command {
    private Fan fan;

    public FanOnCommand(Fan fan) {
        this.fan = fan;
    }

    @Override
    public void execute() {
        fan.turnOn();
    }

    @Override
    public void undo() {
        fan.turnOff();
    }
}

class FanSpeedCommand implements Command {
    private Fan fan;
    private int newSpeed;
    private int previousSpeed;

    public FanSpeedCommand(Fan fan, int speed) {
        this.fan = fan;
        this.newSpeed = speed;
    }

    @Override
    public void execute() {
        previousSpeed = fan.getSpeed();
        fan.setSpeed(newSpeed);
    }

    @Override
    public void undo() {
        fan.setSpeed(previousSpeed);
    }
}

// Null Object Pattern for empty slots
class NoCommand implements Command {
    @Override
    public void execute() {
        // Do nothing
    }

    @Override
    public void undo() {
        // Do nothing
    }
}

// Invoker
class RemoteControl {
    private Command[] onCommands;
    private Command[] offCommands;
    private Command undoCommand;

    public RemoteControl() {
        onCommands = new Command[7];
        offCommands = new Command[7];

        Command noCommand = new NoCommand();
        for (int i = 0; i < 7; i++) {
            onCommands[i] = noCommand;
            offCommands[i] = noCommand;
        }
        undoCommand = noCommand;
    }

    public void setCommand(int slot, Command onCommand, Command offCommand) {
        onCommands[slot] = onCommand;
        offCommands[slot] = offCommand;
    }

    public void onButtonPressed(int slot) {
        onCommands[slot].execute();
        undoCommand = onCommands[slot];
    }

    public void offButtonPressed(int slot) {
        offCommands[slot].execute();
        undoCommand = offCommands[slot];
    }

    public void undoButtonPressed() {
        undoCommand.undo();
    }

    @Override
    public String toString() {
        StringBuilder builder = new StringBuilder();
        builder.append("\n------ Remote Control ------\n");
        for (int i = 0; i < onCommands.length; i++) {
            builder.append("[slot ").append(i).append("] ")
                   .append(onCommands[i].getClass().getSimpleName())
                   .append("    ")
                   .append(offCommands[i].getClass().getSimpleName())
                   .append("\n");
        }
        builder.append("[undo] ").append(undoCommand.getClass().getSimpleName()).append("\n");
        return builder.toString();
    }
}

// Usage
public class RemoteControlDemo {
    public static void main(String[] args) {
        RemoteControl remote = new RemoteControl();

        Light livingRoomLight = new Light();
        Light kitchenLight = new Light();
        Fan ceilingFan = new Fan();

        // Create commands
        LightOnCommand livingRoomLightOn = new LightOnCommand(livingRoomLight);
        LightOffCommand livingRoomLightOff = new LightOffCommand(livingRoomLight);
        LightOnCommand kitchenLightOn = new LightOnCommand(kitchenLight);
        LightOffCommand kitchenLightOff = new LightOffCommand(kitchenLight);

        FanOnCommand ceilingFanOn = new FanOnCommand(ceilingFan);
        FanSpeedCommand ceilingFanMedium = new FanSpeedCommand(ceilingFan, 3);

        // Set commands
        remote.setCommand(0, livingRoomLightOn, livingRoomLightOff);
        remote.setCommand(1, kitchenLightOn, kitchenLightOff);
        remote.setCommand(2, ceilingFanOn, new NoCommand());
        remote.setCommand(3, ceilingFanMedium, new NoCommand());

        System.out.println(remote);

        // Test commands
        remote.onButtonPressed(0);  // Living room light on
        remote.offButtonPressed(0); // Living room light off
        remote.undoButtonPressed(); // Undo (light on again)

        remote.onButtonPressed(2);  // Fan on
        remote.onButtonPressed(3);  // Fan medium speed
        remote.undoButtonPressed(); // Undo (back to speed 1)
    }
}

2. 🎨 Text Editor with Undo/Redo (Java)

java

import java.util.*;

// Command interface
interface TextCommand {
    void execute();
    void undo();
    String getDescription();
}

// Receiver
class TextDocument {
    private StringBuilder content;
    private int cursorPosition;

    public TextDocument() {
        content = new StringBuilder();
        cursorPosition = 0;
    }

    public void insertText(String text, int position) {
        content.insert(position, text);
        cursorPosition = position + text.length();
    }

    public String deleteText(int start, int length) {
        String deleted = content.substring(start, Math.min(start + length, content.length()));
        content.delete(start, Math.min(start + length, content.length()));
        cursorPosition = start;
        return deleted;
    }

    public void replaceText(int start, int length, String newText) {
        content.replace(start, Math.min(start + length, content.length()), newText);
        cursorPosition = start + newText.length();
    }

    public String getContent() {
        return content.toString();
    }

    public int getCursorPosition() {
        return cursorPosition;
    }

    public void setCursorPosition(int position) {
        cursorPosition = Math.max(0, Math.min(position, content.length()));
    }

    public int getLength() {
        return content.length();
    }
}

// Concrete Commands
class InsertTextCommand implements TextCommand {
    private TextDocument document;
    private String text;
    private int position;

    public InsertTextCommand(TextDocument document, String text, int position) {
        this.document = document;
        this.text = text;
        this.position = position;
    }

    @Override
    public void execute() {
        document.insertText(text, position);
    }

    @Override
    public void undo() {
        document.deleteText(position, text.length());
    }

    @Override
    public String getDescription() {
        return "Insert '" + text + "' at position " + position;
    }
}

class DeleteTextCommand implements TextCommand {
    private TextDocument document;
    private int start;
    private int length;
    private String deletedText;

    public DeleteTextCommand(TextDocument document, int start, int length) {
        this.document = document;
        this.start = start;
        this.length = length;
    }

    @Override
    public void execute() {
        deletedText = document.deleteText(start, length);
    }

    @Override
    public void undo() {
        if (deletedText != null) {
            document.insertText(deletedText, start);
        }
    }

    @Override
    public String getDescription() {
        return "Delete " + length + " characters from position " + start;
    }
}

class ReplaceTextCommand implements TextCommand {
    private TextDocument document;
    private int start;
    private int length;
    private String newText;
    private String originalText;

    public ReplaceTextCommand(TextDocument document, int start, int length, String newText) {
        this.document = document;
        this.start = start;
        this.length = length;
        this.newText = newText;
    }

    @Override
    public void execute() {
        String content = document.getContent();
        originalText = content.substring(start, Math.min(start + length, content.length()));
        document.replaceText(start, length, newText);
    }

    @Override
    public void undo() {
        if (originalText != null) {
            document.replaceText(start, newText.length(), originalText);
        }
    }

    @Override
    public String getDescription() {
        return "Replace text at position " + start + " with '" + newText + "'";
    }
}

// Macro Command
class MacroCommand implements TextCommand {
    private List<TextCommand> commands;
    private String description;

    public MacroCommand(List<TextCommand> commands, String description) {
        this.commands = new ArrayList<>(commands);
        this.description = description;
    }

    @Override
    public void execute() {
        for (TextCommand command : commands) {
            command.execute();
        }
    }

    @Override
    public void undo() {
        // Undo in reverse order
        for (int i = commands.size() - 1; i >= 0; i--) {
            commands.get(i).undo();
        }
    }

    @Override
    public String getDescription() {
        return description;
    }
}

// Command Manager (Invoker)
class TextEditor {
    private TextDocument document;
    private Stack<TextCommand> undoStack;
    private Stack<TextCommand> redoStack;
    private final int maxUndoLevels;

    public TextEditor(int maxUndoLevels) {
        document = new TextDocument();
        undoStack = new Stack<>();
        redoStack = new Stack<>();
        this.maxUndoLevels = maxUndoLevels;
    }

    public void executeCommand(TextCommand command) {
        command.execute();

        // Add to undo stack
        undoStack.push(command);

        // Limit undo stack size
        if (undoStack.size() > maxUndoLevels) {
            undoStack.remove(0);
        }

        // Clear redo stack when new command is executed
        redoStack.clear();

        System.out.println("Executed: " + command.getDescription());
        System.out.println("Document: \"" + document.getContent() + "\"");
        System.out.println();
    }

    public void undo() {
        if (!undoStack.isEmpty()) {
            TextCommand command = undoStack.pop();
            command.undo();
            redoStack.push(command);

            System.out.println("Undone: " + command.getDescription());
            System.out.println("Document: \"" + document.getContent() + "\"");
            System.out.println();
        } else {
            System.out.println("Nothing to undo");
        }
    }

    public void redo() {
        if (!redoStack.isEmpty()) {
            TextCommand command = redoStack.pop();
            command.execute();
            undoStack.push(command);

            System.out.println("Redone: " + command.getDescription());
            System.out.println("Document: \"" + document.getContent() + "\"");
            System.out.println();
        } else {
            System.out.println("Nothing to redo");
        }
    }

    public TextDocument getDocument() {
        return document;
    }

    public void printHistory() {
        System.out.println("=== Command History ===");
        System.out.println("Undo Stack (" + undoStack.size() + " commands):");
        for (int i = undoStack.size() - 1; i >= 0; i--) {
            System.out.println("  " + (undoStack.size() - i) + ". " + undoStack.get(i).getDescription());
        }

        System.out.println("Redo Stack (" + redoStack.size() + " commands):");
        for (int i = redoStack.size() - 1; i >= 0; i--) {
            System.out.println("  " + (redoStack.size() - i) + ". " + redoStack.get(i).getDescription());
        }
        System.out.println();
    }
}

// Usage
public class TextEditorDemo {
    public static void main(String[] args) {
        TextEditor editor = new TextEditor(10);
        TextDocument doc = editor.getDocument();

        System.out.println("=== Text Editor Command Pattern Demo ===\n");

        // Execute various commands
        editor.executeCommand(new InsertTextCommand(doc, "Hello", 0));
        editor.executeCommand(new InsertTextCommand(doc, " World", 5));
        editor.executeCommand(new InsertTextCommand(doc, "!", 11));

        // Create a macro command
        List<TextCommand> macroCommands = Arrays.asList(
            new DeleteTextCommand(doc, 5, 6),  // Delete " World"
            new InsertTextCommand(doc, " Java", 5)  // Insert " Java"
        );
        editor.executeCommand(new MacroCommand(macroCommands, "Replace 'World' with 'Java'"));

        // Test undo/redo
        editor.undo();
        editor.undo();
        editor.redo();

        // Show command history
        editor.printHistory();

        // More operations
        editor.executeCommand(new ReplaceTextCommand(doc, 0, 5, "Hi"));
        editor.executeCommand(new DeleteTextCommand(doc, 2, 5));

        System.out.println("Final document: \"" + doc.getContent() + "\"");
    }
}

3. 🐍 Task Queue System (Python)

python

from abc import ABC, abstractmethod
from typing import List, Dict, Any, Optional
import time
import threading
from enum import Enum
from datetime import datetime, timedelta
import json

# Command interface
class Command(ABC):
    @abstractmethod
    def execute(self) -> Any:
        pass

    @abstractmethod
    def undo(self) -> Any:
        pass

    @abstractmethod
    def can_undo(self) -> bool:
        pass

    @abstractmethod
    def get_description(self) -> str:
        pass

    @abstractmethod
    def serialize(self) -> Dict[str, Any]:
        pass

# Task status enumeration
class TaskStatus(Enum):
    PENDING = "pending"
    RUNNING = "running"
    COMPLETED = "completed"
    FAILED = "failed"
    CANCELLED = "cancelled"

# File operations receiver
class FileManager:
    def __init__(self):
        self.files: Dict[str, str] = {}

    def create_file(self, filename: str, content: str) -> bool:
        if filename not in self.files:
            self.files[filename] = content
            print(f"Created file: {filename}")
            return True
        return False

    def delete_file(self, filename: str) -> Optional[str]:
        if filename in self.files:
            content = self.files.pop(filename)
            print(f"Deleted file: {filename}")
            return content
        return None

    def update_file(self, filename: str, new_content: str) -> Optional[str]:
        if filename in self.files:
            old_content = self.files[filename]
            self.files[filename] = new_content
            print(f"Updated file: {filename}")
            return old_content
        return None

    def read_file(self, filename: str) -> Optional[str]:
        return self.files.get(filename)

    def list_files(self) -> List[str]:
        return list(self.files.keys())

# Concrete Commands
class CreateFileCommand(Command):
    def __init__(self, file_manager: FileManager, filename: str, content: str):
        self.file_manager = file_manager
        self.filename = filename
        self.content = content
        self.executed = False

    def execute(self) -> Any:
        result = self.file_manager.create_file(self.filename, self.content)
        self.executed = result
        return result

    def undo(self) -> Any:
        if self.executed:
            self.file_manager.delete_file(self.filename)
            self.executed = False
            return True
        return False

    def can_undo(self) -> bool:
        return self.executed

    def get_description(self) -> str:
        return f"Create file '{self.filename}' with {len(self.content)} characters"

    def serialize(self) -> Dict[str, Any]:
        return {
            "type": "CreateFileCommand",
            "filename": self.filename,
            "content": self.content,
            "executed": self.executed
        }

class DeleteFileCommand(Command):
    def __init__(self, file_manager: FileManager, filename: str):
        self.file_manager = file_manager
        self.filename = filename
        self.deleted_content: Optional[str] = None

    def execute(self) -> Any:
        self.deleted_content = self.file_manager.delete_file(self.filename)
        return self.deleted_content is not None

    def undo(self) -> Any:
        if self.deleted_content is not None:
            result = self.file_manager.create_file(self.filename, self.deleted_content)
            if result:
                self.deleted_content = None
            return result
        return False

    def can_undo(self) -> bool:
        return self.deleted_content is not None

    def get_description(self) -> str:
        return f"Delete file '{self.filename}'"

    def serialize(self) -> Dict[str, Any]:
        return {
            "type": "DeleteFileCommand",
            "filename": self.filename,
            "deleted_content": self.deleted_content
        }

class UpdateFileCommand(Command):
    def __init__(self, file_manager: FileManager, filename: str, new_content: str):
        self.file_manager = file_manager
        self.filename = filename
        self.new_content = new_content
        self.old_content: Optional[str] = None

    def execute(self) -> Any:
        self.old_content = self.file_manager.update_file(self.filename, self.new_content)
        return self.old_content is not None

    def undo(self) -> Any:
        if self.old_content is not None:
            self.file_manager.update_file(self.filename, self.old_content)
            return True
        return False

    def can_undo(self) -> bool:
        return self.old_content is not None

    def get_description(self) -> str:
        return f"Update file '{self.filename}'"

    def serialize(self) -> Dict[str, Any]:
        return {
            "type": "UpdateFileCommand",
            "filename": self.filename,
            "new_content": self.new_content,
            "old_content": self.old_content
        }

# Batch Command (Composite)
class BatchCommand(Command):
    def __init__(self, commands: List[Command], description: str):
        self.commands = commands
        self.description = description
        self.executed_commands: List[Command] = []

    def execute(self) -> Any:
        results = []
        self.executed_commands = []

        for command in self.commands:
            try:
                result = command.execute()
                self.executed_commands.append(command)
                results.append(result)
            except Exception as e:
                print(f"Error executing command {command.get_description()}: {e}")
                # Rollback executed commands
                self.undo()
                raise e

        return results

    def undo(self) -> Any:
        # Undo in reverse order
        for command in reversed(self.executed_commands):
            if command.can_undo():
                command.undo()
        self.executed_commands = []
        return True

    def can_undo(self) -> bool:
        return len(self.executed_commands) > 0

    def get_description(self) -> str:
        return self.description

    def serialize(self) -> Dict[str, Any]:
        return {
            "type": "BatchCommand",
            "description": self.description,
            "commands": [cmd.serialize() for cmd in self.commands],
            "executed_count": len(self.executed_commands)
        }

# Task wrapper for commands
class Task:
    def __init__(self, task_id: str, command: Command, priority: int = 0, scheduled_time: Optional[datetime] = None):
        self.task_id = task_id
        self.command = command
        self.priority = priority
        self.scheduled_time = scheduled_time or datetime.now()
        self.status = TaskStatus.PENDING
        self.created_at = datetime.now()
        self.started_at: Optional[datetime] = None
        self.completed_at: Optional[datetime] = None
        self.result: Any = None
        self.error: Optional[str] = None

    def execute(self) -> Any:
        self.status = TaskStatus.RUNNING
        self.started_at = datetime.now()

        try:
            self.result = self.command.execute()
            self.status = TaskStatus.COMPLETED
            self.completed_at = datetime.now()
            return self.result
        except Exception as e:
            self.status = TaskStatus.FAILED
            self.error = str(e)
            self.completed_at = datetime.now()
            raise e

    def cancel(self):
        if self.status == TaskStatus.PENDING:
            self.status = TaskStatus.CANCELLED
            return True
        return False

    def get_info(self) -> Dict[str, Any]:
        return {
            "task_id": self.task_id,
            "description": self.command.get_description(),
            "status": self.status.value,
            "priority": self.priority,
            "scheduled_time": self.scheduled_time.isoformat(),
            "created_at": self.created_at.isoformat(),
            "started_at": self.started_at.isoformat() if self.started_at else None,
            "completed_at": self.completed_at.isoformat() if self.completed_at else None,
            "error": self.error,
            "can_undo": self.command.can_undo() if self.status == TaskStatus.COMPLETED else False
        }

# Task Queue Manager (Invoker)
class TaskQueueManager:
    def __init__(self, max_workers: int = 3):
        self.task_queue: List[Task] = []
        self.completed_tasks: List[Task] = []
        self.max_workers = max_workers
        self.workers: List[threading.Thread] = []
        self.running = False
        self.lock = threading.Lock()
        self.task_counter = 0

    def add_task(self, command: Command, priority: int = 0, scheduled_time: Optional[datetime] = None) -> str:
        with self.lock:
            self.task_counter += 1
            task_id = f"task_{self.task_counter:04d}"
            task = Task(task_id, command, priority, scheduled_time)

            # Insert task based on priority (higher priority first)
            inserted = False
            for i, existing_task in enumerate(self.task_queue):
                if existing_task.priority < priority:
                    self.task_queue.insert(i, task)
                    inserted = True
                    break

            if not inserted:
                self.task_queue.append(task)

            print(f"Added task {task_id}: {command.get_description()}")
            return task_id

    def start_processing(self):
        if self.running:
            return

        self.running = True
        for i in range(self.max_workers):
            worker = threading.Thread(target=self._worker, name=f"Worker-{i+1}")
            worker.daemon = True
            worker.start()
            self.workers.append(worker)

        print(f"Started task queue with {self.max_workers} workers")

    def stop_processing(self):
        self.running = False
        for worker in self.workers:
            worker.join()
        self.workers = []
        print("Stopped task queue processing")

    def _worker(self):
        while self.running:
            task = self._get_next_task()
            if task:
                try:
                    print(f"[{threading.current_thread().name}] Executing: {task.command.get_description()}")
                    task.execute()

                    with self.lock:
                        self.completed_tasks.append(task)

                    print(f"[{threading.current_thread().name}] Completed: {task.task_id}")
                except Exception as e:
                    print(f"[{threading.current_thread().name}] Failed: {task.task_id} - {e}")
                    with self.lock:
                        self.completed_tasks.append(task)
            else:
                time.sleep(0.1)  # Wait for new tasks

    def _get_next_task(self) -> Optional[Task]:
        with self.lock:
            current_time = datetime.now()
            for i, task in enumerate(self.task_queue):
                if task.status == TaskStatus.PENDING and task.scheduled_time <= current_time:
                    return self.task_queue.pop(i)
            return None

    def cancel_task(self, task_id: str) -> bool:
        with self.lock:
            for task in self.task_queue:
                if task.task_id == task_id:
                    return task.cancel()
            return False

    def undo_task(self, task_id: str) -> bool:
        with self.lock:
            for task in self.completed_tasks:
                if task.task_id == task_id and task.command.can_undo():
                    try:
                        task.command.undo()
                        print(f"Undone task {task_id}: {task.command.get_description()}")
                        return True
                    except Exception as e:
                        print(f"Failed to undo task {task_id}: {e}")
                        return False
            return False

    def get_status(self) -> Dict[str, Any]:
        with self.lock:
            pending_tasks = len([t for t in self.task_queue if t.status == TaskStatus.PENDING])
            running_tasks = len([t for t in self.completed_tasks if t.status == TaskStatus.RUNNING])
            completed_tasks = len([t for t in self.completed_tasks if t.status == TaskStatus.COMPLETED])
            failed_tasks = len([t for t in self.completed_tasks if t.status == TaskStatus.FAILED])

            return {
                "running": self.running,
                "workers": len(self.workers),
                "pending_tasks": pending_tasks,
                "running_tasks": running_tasks,
                "completed_tasks": completed_tasks,
                "failed_tasks": failed_tasks,
                "total_tasks": self.task_counter
            }

    def list_tasks(self, status_filter: Optional[TaskStatus] = None) -> List[Dict[str, Any]]:
        with self.lock:
            all_tasks = self.task_queue + self.completed_tasks
            if status_filter:
                all_tasks = [t for t in all_tasks if t.status == status_filter]

            return [task.get_info() for task in sorted(all_tasks, key=lambda t: t.created_at, reverse=True)]

# Usage example
if __name__ == "__main__":
    # Create file manager and task queue
    file_manager = FileManager()
    task_queue = TaskQueueManager(max_workers=2)

    print("=== Task Queue Command Pattern Demo ===\n")

    # Start processing
    task_queue.start_processing()

    # Add individual tasks
    task1_id = task_queue.add_task(
        CreateFileCommand(file_manager, "document.txt", "Hello World!"),
        priority=1
    )

    task2_id = task_queue.add_task(
        CreateFileCommand(file_manager, "readme.md", "# Project README"),
        priority=2
    )

    # Add batch command
    batch_commands = [
        UpdateFileCommand(file_manager, "document.txt", "Hello Python!"),
        CreateFileCommand(file_manager, "config.json", '{"setting": "value"}')
    ]
    batch_task_id = task_queue.add_task(
        BatchCommand(batch_commands, "Update document and create config"),
        priority=3
    )

    # Add scheduled task (2 seconds from now)
    scheduled_time = datetime.now() + timedelta(seconds=2)
    scheduled_task_id = task_queue.add_task(
        DeleteFileCommand(file_manager, "readme.md"),
        scheduled_time=scheduled_time
    )

    # Wait for tasks to complete
    time.sleep(3)

    # Show status
    print("\n=== Task Queue Status ===")
    status = task_queue.get_status()
    for key, value in status.items():
        print(f"{key}: {value}")

    # List all tasks
    print("\n=== All Tasks ===")
    tasks = task_queue.list_tasks()
    for task_info in tasks:
        print(f"Task {task_info['task_id']}: {task_info['status']} - {task_info['description']}")

    # Test undo functionality
    print("\n=== Testing Undo ===")
    task_queue.undo_task(task1_id)
    task_queue.undo_task(batch_task_id)

    # Show final file state
    print(f"\n=== Final File State ===")
    print(f"Files: {file_manager.list_files()}")
    for filename in file_manager.list_files():
        print(f"{filename}: {file_manager.read_file(filename)}")

    # Stop processing
    task_queue.stop_processing()

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✅ Best Practices

🎯 When to Use Command Pattern

✅ Undo/Redo functionality needed
✅ Queuing operations for later execution
✅ Logging requests for audit trails
✅ Decoupling invoker from receiver
✅ Macro commands combining multiple operations

📊 Command Pattern Benefits

❌ Common Pitfalls

🚨 Anti-Patterns to Avoid

Commands with Side Effects

java

// BAD: Command with uncontrollable side effects
class BadEmailCommand implements Command {
    public void execute() {
        sendEmail(); // What if this fails? How to undo?
        logToDatabase(); // Multiple side effects
        updateFileSystem(); // Hard to reverse
    }
}

// GOOD: Atomic command with proper undo
class GoodEmailCommand implements Command {
    private EmailService emailService;
    private String emailId;

    public void execute() {
        emailId = emailService.sendEmail(recipient, subject, body);
    }

    public void undo() {
        if (emailId != null) {
            emailService.markAsRecalled(emailId);
        }
    }
}

Missing Undo State

java

// BAD: No state preservation for undo
class BadDeleteCommand implements Command {
    private File file;

    public void undo() {
        // Cannot restore - file content is lost!
    }
}

// GOOD: Preserve state for undo
class GoodDeleteCommand implements Command {
    private File file;
    private String fileContent;

    public void execute() {
        fileContent = readFile(file);
        deleteFile(file);
    }

    public void undo() {
        if (fileContent != null) {
            writeFile(file, fileContent);
        }
    }
}

🎤 Interview Questions

📝 Fundamental Questions

Q1: What is the Command pattern and when would you use it?

A: The Command pattern encapsulates requests as objects, allowing you to:

Parameterize clients with different requests
Queue or log requests
Support undo operations
Decouple invokers from receivers

Use it for undo/redo functionality, task queues, GUI actions, and macro recording.

Q2: How does Command pattern support undo/redo?

A: Commands store the necessary state to reverse their operations:

java

interface Command {
    void execute();
    void undo();     // Reverse the execute operation
}

class TextInsertCommand implements Command {
    private TextEditor editor;
    private String text;
    private int position;

    public void execute() {
        editor.insert(text, position);
    }

    public void undo() {
        editor.delete(position, text.length());
    }
}

Q3: What's the difference between Command and Strategy patterns?

Command: Encapsulates requests/operations as objects, supports undo
Strategy: Encapsulates algorithms, focuses on interchangeable behavior
Command stores state and can be executed later; Strategy is stateless

🚀 Advanced Questions

Q4: How would you implement a macro command?

java

class MacroCommand implements Command {
    private List<Command> commands;

    public void execute() {
        for (Command cmd : commands) {
            cmd.execute();
        }
    }

    public void undo() {
        // Undo in reverse order
        for (int i = commands.size() - 1; i >= 0; i--) {
            commands.get(i).undo();
        }
    }
}

Q5: How do you handle command failures in a macro?

A: Implement transactional behavior:

java

class TransactionalMacro implements Command {
    private List<Command> commands;
    private List<Command> executed = new ArrayList<>();

    public void execute() throws Exception {
        try {
            for (Command cmd : commands) {
                cmd.execute();
                executed.add(cmd);
            }
        } catch (Exception e) {
            // Rollback executed commands
            rollback();
            throw e;
        }
    }

    private void rollback() {
        for (int i = executed.size() - 1; i >= 0; i--) {
            executed.get(i).undo();
        }
        executed.clear();
    }
}

🎯 Summary

Aspect	Details
Intent	Encapsulate requests as objects
Problem	Need for undo/redo, queuing, logging operations
Solution	Command objects with execute() and undo() methods
Benefits	Decoupling, undo/redo, queuing, macro commands
Drawbacks	Increased number of classes, complexity
Use When	GUI actions, undo/redo, task queues, transaction logs

📚 Key Takeaways

Encapsulate requests as command objects
Store state necessary for undo operations
Support macro commands for complex operations
Handle failures gracefully with rollback mechanisms
Consider queuing and scheduling capabilities

← Back: Strategy Pattern | Next: Decorator Pattern →

⚡ Command Pattern ​

📋 Table of Contents ​

🎯 Intent ​

Key Characteristics ​

🚀 Problem & Solution ​

🚨 Problem ​

💡 Solution ​

🏗️ Structure ​

UML Class Diagram ​

Sequence Diagram ​

💻 Implementation ​

1. 🔰 Basic Command Pattern (Java) ​

2. 🎨 Text Editor with Undo/Redo (Java) ​

3. 🐍 Task Queue System (Python) ​

✅ Best Practices ​

🎯 When to Use Command Pattern ​

📊 Command Pattern Benefits ​

❌ Common Pitfalls ​

🚨 Anti-Patterns to Avoid ​

🎤 Interview Questions ​

📝 Fundamental Questions ​

🚀 Advanced Questions ​

🎯 Summary ​

📚 Key Takeaways ​

⚡ Command Pattern

📋 Table of Contents

🎯 Intent

Key Characteristics

🚀 Problem & Solution

🚨 Problem

💡 Solution

🏗️ Structure

UML Class Diagram

Sequence Diagram

💻 Implementation

1. 🔰 Basic Command Pattern (Java)

2. 🎨 Text Editor with Undo/Redo (Java)

3. 🐍 Task Queue System (Python)

✅ Best Practices

🎯 When to Use Command Pattern

📊 Command Pattern Benefits

❌ Common Pitfalls

🚨 Anti-Patterns to Avoid

🎤 Interview Questions

📝 Fundamental Questions

🚀 Advanced Questions

🎯 Summary

📚 Key Takeaways