What are the key components of the Command pattern?

What are the key components of the Command pattern?

• A. Involves direct method calls between components.
• B. Defines an algorithm family only.
• C. Command, Receiver, Invoker, and optionally Undo.
• D. Story, Actor, Scene, and Editor.

Answer: (C)

The Command pattern hinges on wrapping a request as an object so the sender of the request is decoupled from the object that performs it. The essential pieces are a Command, which encapsulates the action and any parameters; a Receiver, which knows how to carry out the action; and an Invoker, which invokes the command without needing to know the details of what happens when it runs. Often there’s support for undo, by making the command store enough state to reverse the action or by keeping a history of commands. This setup enables powerful flexibility: you can queue commands, log them, or trigger them remotely, and you can implement undo by having commands expose an undo operation. Direct method calls bypass this encapsulation, which loses the decoupling the pattern provides. Defining an algorithm family isn’t the main aim here—that’s more aligned with patterns focused on choosing among algorithms. And a narrative-like set of terms (Story, Actor, Scene, and Editor) isn’t related to this design pattern.

The Command pattern hinges on wrapping a request as an object so the sender of the request is decoupled from the object that performs it. The essential pieces are a Command, which encapsulates the action and any parameters; a Receiver, which knows how to carry out the action; and an Invoker, which invokes the command without needing to know the details of what happens when it runs. Often there’s support for undo, by making the command store enough state to reverse the action or by keeping a history of commands.

This setup enables powerful flexibility: you can queue commands, log them, or trigger them remotely, and you can implement undo by having commands expose an undo operation. Direct method calls bypass this encapsulation, which loses the decoupling the pattern provides. Defining an algorithm family isn’t the main aim here—that’s more aligned with patterns focused on choosing among algorithms. And a narrative-like set of terms (Story, Actor, Scene, and Editor) isn’t related to this design pattern.