How to mock big ArrayList by Mockito?

I am going to make method which will analyze big ArrayList. and I want to write test method in JUnit. The size of ArrayList could reach up to couple of millions. I think that it is not good idea to connect to the database and get datas from there for analyzing because a test is not a unit test if it talks to the database. So how should I act correctly in this situation? Or how big datas are analyzed by unit tests generally?