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