command-pattern

🧩 Concept

The Command Pattern turns a request or action into an object.

👉 It decouples the object that issues a request (Invoker) from the object that performs it (Receiver).

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🧱 Key Roles

Role	Description
Command (Interface)	Declares an execute() method.
ConcreteCommand	Implements execute() and calls the receiver’s method.
Receiver	Knows how to perform the actual operation.
Invoker	Triggers the command’s execution.
Client	Configures commands and assigns them to the invoker.

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💻 Simple Java Example

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

// Receiver
class Light {
    void turnOn() { System.out.println("Light is ON"); }
    void turnOff() { System.out.println("Light is OFF"); }
}

// Concrete commands
class TurnOnCommand implements Command {
    private Light light;
    TurnOnCommand(Light light) { this.light = light; }
    public void execute() { light.turnOn(); }
}

class TurnOffCommand implements Command {
    private Light light;
    TurnOffCommand(Light light) { this.light = light; }
    public void execute() { light.turnOff(); }
}

// Invoker
class RemoteControl {
    private Command command;
    void setCommand(Command command) { this.command = command; }
    void pressButton() { command.execute(); }
}

✅ Usage Example:

Light light = new Light();
RemoteControl remote = new RemoteControl();

remote.setCommand(new TurnOnCommand(light));
remote.pressButton();  // Light is ON

remote.setCommand(new TurnOffCommand(light));
remote.pressButton();  // Light is OFF

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🧠 Flow Summary (Step-by-Step)

Step	Action	Invoker (Remote)	Command	Receiver (Light)	Output
1️⃣	setCommand(TurnOnCommand)	stores TurnOnCommand	TurnOnCommand	—	—
2️⃣	pressButton()	executes command	TurnOnCommand → light.turnOn()	"Light is ON"
3️⃣	setCommand(TurnOffCommand)	stores TurnOffCommand	TurnOffCommand	—	—
4️⃣	pressButton()	executes command	TurnOffCommand → light.turnOff()	"Light is OFF"

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🪜 Summary

• Encapsulates actions as objects.

• Allows undo, redo, macro, or queue of commands.

• Promotes loose coupling between Invoker and Receiver.

• Useful for UI buttons, transactions, task scheduling.

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💡 Real-world Analogy

🎮 Remote Control — each button (Invoker) is linked to a command (e.g., TurnOnCommand), which tells the device (Receiver) what to do.