You could create a list of Object like List<Object> list = new ArrayList<Object>(). As all classes implementation extends implicit or explicit from java.lang.Object class, this list can hold any object, including instances of Employee, Integer, String etc.
When you retrieve an element from this list, you will be retrieving an Object and no longer an Employee, meaning you need to perform a explicit cast in this case as follows:
List<Object> list = new ArrayList<Object>();
list.add("String");
list.add(Integer.valueOf(1));
list.add(new Employee());
Object retrievedObject = list.get(2);
Employee employee = (Employee)list.get(2); // explicit cast