Command.md

Command

Description

Command pattern turns a request into a stand-alone object that contains all information about the request. This transformation lets you parameterize methods with different requests, delay or queue a request's execution, and support undoable operations. It encapsulates a request as an object, thereby allowing for parameterization of clients with queues, requests, and operations.

Java Code Example

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

// Receiver class
class Light {
    public void on() {
        System.out.println("Light is on");
    }

    public void off() {
        System.out.println("Light is off");
    }
}

// Concrete command classes
class LightOnCommand implements Command {
    private Light light;

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

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

class LightOffCommand implements Command {
    private Light light;

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

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

// Invoker class
class RemoteControl {
    private Command command;

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

    public void pressButton() {
        command.execute();
    }
}

// Client code
public class CommandPatternDemo {
    public static void main(String[] args) {
        Light light = new Light();
        Command lightOn = new LightOnCommand(light);
        Command lightOff = new LightOffCommand(light);

        RemoteControl remote = new RemoteControl();

        remote.setCommand(lightOn);
        remote.pressButton();

        remote.setCommand(lightOff);
        remote.pressButton();
    }
}

In this example:

• Command is the command interface with a method execute that concrete command classes implement.
• Light is the receiver class that performs the actual operations (on and off).
• LightOnCommand and LightOffCommand are concrete command classes that implement Command and call the appropriate method on the Light receiver.
• RemoteControl is the invoker class that stores a command and calls its execute method.
• CommandPatternDemo is the client code that demonstrates the usage of the command pattern.

When you run the CommandPatternDemo class, it will use the RemoteControl to execute commands (lightOn and lightOff) on the Light receiver, demonstrating how the command pattern works by encapsulating a request as an object and allowing parameterization of clients with different requests.