The collection superclass provides the ability to manage various kinds of collections of objects. There are 20 system classes whose instances are able to contain other objects.
Collections can contain many different types of objects at the same time.
If you need a collection to hold only one type of object, then you must write your own subclass of the collection that has methods that only allow a certain type of object to be entered into the collection. Collection an abstract class, however, unlike magnitude, almost all its instance methods are defined at this level and inherited by subclasses. However, its elements are always kept in sorted order. New elements are put in position by comparing them with the pre-existing elements of the collection. It has no class or instance methods of its own.