What is the Command pattern used for?

The Command pattern is used to encapsulate a request as an object, allowing parameterization of clients with different requests, queuing or logging requests, and supporting undo operations.

Main components of the pattern:

• Command: An interface or abstract class declaring a method for executing a request.
• ConcreteCommand: Classes implementing Command, linking the Receiver with an action. Each ConcreteCommand contains a reference to the Receiver.
• Invoker: A class that initiates the command. It holds a reference to a Command but does not know the details of the operation.
• Receiver: A class that performs the actual action. ConcreteCommand calls methods on the Receiver.

Example of usage for implementing media player control buttons:

// Receiver
class MediaPlayer {
    public void play() {
        System.out.println("Playing media...");
    }

    public void stop() {
        System.out.println("Stopping media...");
    }
}

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

// Concrete Commands
class PlayCommand implements MediaCommand {
    private MediaPlayer player;

    public PlayCommand(MediaPlayer player) {
        this.player = player;
    }

    @Override
    public void execute() {
        player.play();
    }
}

class StopCommand implements MediaCommand {
    private MediaPlayer player;

    public StopCommand(MediaPlayer player) {
        this.player = player;
    }

    @Override
    public void execute() {
        player.stop();
    }
}

// Invoker
class MediaController {
    private MediaCommand command;

    public void setCommand(MediaCommand command) {
        this.command = command;
    }

    public void pressButton() {
        if (command != null) {
            command.execute();
        }
    }
}

// Client Code
class Client {
    public static void main(String[] args) {
        MediaPlayer player = new MediaPlayer();
        PlayCommand playCommand = new PlayCommand(player);
        StopCommand stopCommand = new StopCommand(player);

        MediaController controller = new MediaController();

        controller.setCommand(playCommand);
        controller.pressButton(); // Output: Playing media...

        controller.setCommand(stopCommand);
        controller.pressButton(); // Output: Stopping media...
    }
}

Advantages:

• Separates the sender of a request from its execution.
• Easily adds new commands without changing the Invoker.
• Supports undo operations (if implemented).
• Can be used to create macros (sequence of commands) and log operations.