I&#39;m reading &#39;The gang of 4&#39; (Design Patterns) and I have come across this diagram.
<img src="https://res.cloudinary.com/hashnode/image/upload/v1478790451/lwp4uprk1o5ysqr0tqyw.png" alt="UML Diagram">
The hollow arrow indicates inheritance. This means that Door, Wall and Room are subclasses of MapSite. 
However what does the solid black arrow with the dot mean?

 I'm reading 'The gang of 4' (Design Patterns) and I have come across this diagram.

![UML Diagram](https://res.cloudinary.com/hashnode/image/upload/v1478790451/lwp4uprk1o5ysqr0tqyw.png)

The hollow arrow indicates inheritance. This means that Door, Wall and Room are subclasses of MapSite. 

However what does the solid black arrow with the dot mean?

What does this UML diagram mean? (The gang of 4)

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