ConcreetComponent

ConcreetComponent is the concrete implementation of a component interface within the Decorator design pattern. It represents the core object that provides the primary functionality that other objects may augment at runtime through decoration. In design pattern terms, the Component defines the common interface for objects that can have responsibilities added dynamically, while ConcreetComponent provides the standard behavior.

ConcreetComponent serves as the object that can be wrapped by one or more Decorator objects. Decorators maintain a reference to a Component and delegate method calls, potentially adding behavior before or after the delegation. The ConcreetComponent itself remains independent of any decorators, providing the base functionality that decoration extends.

In implementation terms, ConcreetComponent implements the Component interface or extends the abstract component class and exposes the methods defined there. It is designed to be used as the underlying target for decoration, enabling flexible extension without modifying existing code.

Example: in a text rendering system, there is a Renderable interface with a render() method. A ConcreetComponent such as BasicTextRenderer implements Renderable and outputs plain text. Decorators like BoldDecorator or ItalicDecorator wrap a Renderable instance and augment the render output, adding formatting without changing the underlying component.

History and context: the pattern originates from the design patterns book by Gamma, Helm, Johnson, and Vlissides. It is commonly used to add responsibilities to objects dynamically and to avoid an explosion of subclasses through composition.

See also: Decorator pattern, Component, ConcreteDecorator, AbstractComponent.