11.5.3. Command Pattern

Encapsulates a request as an object, thereby letting users parameterize clients with different requests.

The Command Pattern is a behavioral design pattern that turns a request into a stand-alone object. This allows the parameters for executing a request to be stored and enables users to queue requests or log them. Instead of directly calling a method on an object, you create a command object that contains all the information needed to perform the action, which can then be executed at a later time.

Consider a remote control for a television. Each button on the remote (like 'power', 'volume up', or 'channel change') encapsulates a command. When you press a button, it sends a request to the TV to perform a certain action. You can think of the remote control as the invoker that holds various command objects (the buttons) that can be pressed anytime to perform different actions.

interface Command { void execute(); }

In the Command Pattern, a key component is the Command interface. This interface declares a method called 'execute'. This method will be implemented by concrete command classes. The intent is to define a common interface for all command objects so that they can be treated uniformly regardless of their specific actions.

Think of the Command interface as a job description for various professions. Just as different jobs (like 'teacher', 'doctor', 'engineer') implement the tasks defined in their job descriptions (teaching, healing, designing), different commands will implement the execute method, which embodies their specific actions.

class LightOnCommand implements Command { Light light; public LightOnCommand(Light light) { this.light = light; } public void execute() { light.turnOn(); } }

The Concrete Command is a specific implementation of the Command interface. In this example, the LightOnCommand class implements the Command interface and holds a reference to a Light object. The execute method in this class triggers the 'turnOn' method of the Light object, encapsulating the action of turning on a light as a distinct command.

Imagine a specific recipe for making a sandwich. The LightOnCommand is like a recipe that says 'To make a sandwich, take bread, add fillings, and put it together'. In this case, if you want to execute the command to turn on the light, you follow the recipe (execute method) to get the result (the light is turned on).

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