Decorator vs. Inheritance

Decorator and inheritance both extend behavior, but they do it in very different ways.

Inheritance

Inheritance is a good fit when:

• the variation is fundamental to the type itself
• the behavior should be fixed by the subtype
• the hierarchy remains understandable over time

Decorator

Decorator is a good fit when:

• behavior is optional
• behavior should be combined in different ways
• wrapping order matters
• the same base implementation may appear with different added concerns

Java Rule Of Thumb

If the change is “this is a different kind of thing,” inheritance may be appropriate.

If the change is “this thing should also do X around the same contract,” decorator is often better.

Failure Mode

Teams sometimes use inheritance to create many variants such as:

• LoggingRenderer
• CachingRenderer
• LoggingCachingRenderer
• SecureLoggingCachingRenderer

That is exactly the kind of growth decorator is meant to avoid.