ConcreteCommand

ConcreteCommand is a specific implementation of the Command pattern, a behavioral design pattern that encapsulates a request as an object, thereby allowing parameterization of clients with queues, logs, and undoable operations. In the pattern hierarchy, the abstract Command interface declares a method to execute an operation. ConcreteCommand implements this interface, linking a particular Receiver object and the action to perform.

The Receiver holds the business logic that actually manipulates the system state. ConcreteCommand stores a reference to the Receiver and defines the execute() method by invoking the proper Receiver methods. The Invoker holds a list of commands and triggers them, typically in response to user input or scheduled events, without knowing the details of what the commands do.

Typical use of ConcreteCommand appears in GUI toolkits: a menu item is a ConcreteCommand that, when executed, calls a particular method on a document Receiver. This decouples UI widgets from the underlying logic and facilitates features such as undo/redo stacks, macro recording, or remote command execution.

Implementing ConcreteCommand usually involves three classes: an abstract Command with an execute() method, a Receiver that performs the action, and the ConcreteCommand itself that holds a Receiver reference and implements execute() by delegating to the Receiver. This design promotes single responsibility—each command object knows exactly what it needs to do, and changes to the Receiver do not affect the Invoker.

ConcreteCommand provides modularity, extensibility, and testability: new commands can be added without altering existing code. However, excessive use may lead to a proliferation of command classes, increasing maintenance effort. The pattern is widely adopted in frameworks such as Swing, JavaFX, and many game engines for input handling and action scripting.