What are Collections?
A collection - sometimes called a container - is simply an object
that groups multiple elements into a single unit. Collections are used
to store, retrieve, manipulate, and communicate aggregate data.
Typically, they represent data items that form a natural group, such as a
collection of strings, or a mapping of names to addresses.
How are collections (collection implementation classes) different from arrays?
Nope, they are not different. Array is nothing but a collection and
our definition above proves that. Infact the collection implementation
classes like ArrayList internally uses arrays to store objects. Vector,
Hashtable and array are collection implementations in earlier versions
(pre 1.2) of the Java Platform which unfortunately were not easy to
extend, and did not implement a standard member interface. Well, they
are not part of JDK collections framework but still they are
collections.
Why do we need Collections Framework?
All three of these collections viz, Vector, Hashtable, array have
different methods and syntax for accessing members: arrays use the
square bracket ([]) symbols, Vector uses the elementAt method, and
Hashtable uses get and put methods. These differences have forced
programmers to implement their own inconsistent collections - some
imitate the Vector access methods and some imitate the Enumeration
interface. To make it worse, most of the Vector methods are marked as
final; that is, you cannot extend the Vector class to implement a
similar sort of collection. We could create a collection class that
looked like a Vector and acted like a Vector, but it couldn't be passed
to a method that takes a Vector as a parameter. Finally, none of the
collections (array, Vector or Hashtable) implements a standard member
access interface. When programmers develop algorithms (like sorting) to
manipulate collections, what object should be passed to the algorithm -
is it an array or a Vector or implement both interfaces? A lot of such
questions pop up.
Thankfully, the Java Collections Framework (JCF) solves these
problems and offers a number of advantages over using no framework or
using the Vector and Hashtable
