Decorator pattern

• What problem does Decorator pattern solve?

Suppose we have a hierarchy of shape objects:

At some point we may need a Circle with a red border layout dynamically.

Is it correct to create another concrete class statically?

No, we would duplicate our code, for only one little adjustment of a red border.

In addition, we can't have an extra interface for our new requirement, because all Components will have to implement the red border.

We also want a dynamic change only. 

• How does Decorator pattern solve this?

The decorator pattern allows you to alter behavior dynamically, without affecting other objects of the same type.


We create an extra concrete class that 

- implements Component

- takes our to-be-decorated object as a constructor argument

- has generic functionality (add red border) for all Component objects

- has sole responsibility to add a red border

Code:

Use:

Only circle2 with have a red border

Note that ComponentWithRedBorder delegates execution to the original Component object, after having decorated it.