private ArrayList<Employee> employee = new ArrayList<Employee>();

How do I write this attribute on a UML diagram?

Need a lot more information to put a whole UML diagram together. All you have here is the object 'Employee' in the many/* multiplicity of a Class or Object diagram. You don't just put a line of Java code into a diagram.

I advise some research on UML diagrams, and the types of UML diagrams that exist:

Use case diagrams
Class diagrams
Object diagrams
Sequence diagrams
Collaboration diagrams
Statechart diagrams
Activity diagrams
Component diagrams
Deployment diagrams


Tutorials:

http://www.smartdraw.com/resources/tutorials/uml-diagrams/
http://visualcase.com/tutorials/uml-tutorial.htm
http://edn.embarcadero.com/article/31863
http://www.objectmentor.com/resources/publishedArticles.html