## Decorator pattern

### Introduction

Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

### Participants:

#### Component

- defines the interface for objects that can have responsibilities added to them dynamically.
- this participant can not be read-only.

#### Concrete component

- defines an object to which additional responsibilities can be attached.
- this participant can not be read-only.

#### Decorator

- maintains a reference to a Component object and defines an interface that conforms to Component's interface.
- this participant can not be read-only.
- this participant can not be an interface.

In the "Attribute" field you can specify the name of a Decorator's attribute where it keeps a reference to a Component object.

The "Initialization variant" option defines the kind of initialization of that attribute.

#### Concrete decorator

- adds responsibilities to the component.
- this participant can not be read-only.
- this participant can not be an interface.

The "Copy documentation" option controls whether to copy JavaDoc comments from methods in interfaces participating in the pattern to the stubs of these methods made by the pattern in classes implementing such interfaces.

The "Create pattern links" option controls whether to generate additional links which can be used by this pattern later to determine classes and interfaces participating in the pattern. That means that if you selected this option and used the pattern to create a set of classes and interfaces, the pattern invoked for some participant later (via the popup menu item "Choose Pattern...") will automatically find (if it is possible) all other participants and fill in participant fields with their names.

Also, if you applied the pattern with this option on and invoked the pattern via the popup menu item "Choose Pattern..." for some participant later, the pattern will have additional field called "Use selected class as", containing possible roles for the selected element.

This option is very useful when you are planning to change something in the classes/interfaces participating in the pattern. For example, if this option is on and after creation of the classes and interfaces you added several methods to certain interface-participant (and this change must be reflected somehow in other participants), all you have to do is to select this changed interface, invoke the "Choose Pattern..." dialog for this element and select the original pattern. After that the pattern will determine other participants and you will only have to press "Finish". The pattern will modify all other classes and interfaces accordingly to your changes.

### Applicability

Use Decorator

- to add responsibilities to individual objects dynamically and transparently, that is, without affecting other objects.
- for responsibilities that can be withdrawn.
- when extension by subclassing is impractical. Sometimes a large number of independent extensions are possible and would produce an explosion of subclasses to support every combination. Or a class definition may be hidden or otherwise unavailable for subclassing.