The Collections Framework
In this section you should be able to:
1. Make appropriate selection of collection
classes/interfaces to suit specified behaviour requirements.
2. Distinguish between correct and incorrect
implementations of hashcode methods.
The
Collections framework
A collection allows a group of objects to be
treated as a single unit. Collections define a set of core interfaces. These
are:-
·
Collection
·
Set
·
List
·
SortedSet
·
Map
·
SortedMap
Collections also provide implementation for
these interfaces.
Core
interfaces
The object hierarchy of Core interfaces
defined in Collections is given below.
Figure: Core Interfaces
of Collections
Collection
Interface
The Collection interface is the root of
Collection hierarchy, and is used for common functionality across all collections.
There is no direct implementation of the Collection interface.
Set
Interface
The Set interface is used to represent a
group of unique elements. It extends the Collection interface. The class HashSet implements the Set interface.
SortedSet Interface
The SortedSet
interface extends the Set interface. It provides extra functionality of keeping
the elements sorted. So SortedSet interface is used to
represent collections consisting of unique, sorted elements. The class TreeSet is an implementation of interface SortedSet.
List
Interface
The List interface extends the Collection
interface to represent sequence of numbers in a fixed order. Classes ArrayList, Vector and LinkedList are implementations of List interface.
Map
Interface
The Map interface is a basic interface that
is used to represent mapping of keys to values. Classes HashMap and Hashtable are implementations of Map interface.
SortedMap Interface
The SortedMAp
interface extends Map interface and maintains their mappings in key order. The
class TreeMap implements SortedMap interface.
The table below give a list of Collection
interfaces and the classes that implement them.
|
Interface |
Class Implementation |
|
Set |
HashSet |
|
SortedSet |
TreeSet |
|
List |
ArrayList, Vector, Linked List |
|
Map |
HashMap,
Hashtable |
|
SortedMap |
TreeMap |