How do copy contains from one arraylist to a new arraylist ?

A simple assignment of new_arraylist = arraylist only copies the reference to the object, there still is only one arraylist instance with two references to it.
To copy the items you need to create a new arraylist instance and copy the items one-by-one in a loop.
Also note...if the items in the arraylist have "reference semantics" (Class) then that is just copying the references to the objects.
For items with "value semantics" (e.g., numbers, strings, Structures), the two lists will end up with no overlap.