Introduction to Collection OR Java.util package

• What is Collection?
• Collection Framework.
• Benefits of java collection framework
• About Collection interface
• ArrayList
• LinkedList
• HashSet
• TreeSet
• The Map interface
• HashMap
• TreeMap
• Vector
• Dictionary
• HashTable
• Properties
• What is the difference between Enumeration & Iterator?
• Storing Customer objects as Collection
• Collection class and interface Hierarchy

---

What is Collection?

A collection is sometimes called container, is simply an object that groups a multiple elements into single unit.

• It is used to store, retrieve, manipulate and communicate aggregate data.

The java.util package also contains one of java’s most powerful subsystem:

• The Collection Framework is a sophisticated hierarchy of interfaces and classes that provide technology for managing groups of objects.

Collection Framework

It is a unified architecture for representing and manipulating collection.

It contains the followings :

• Interfaces
• Implementation
• Algorithm

Exception Thrown with Collection methods :

• ClassCastException
• NullPointerException
• IllegalArgumentException
• IllegalStateException – if it is full

Benefits of java collection framework

• Reduce Programming efforts
• Increases programming speed and quality
• Reduce efforts to learn new API’s
• Reduce efforts to design new API’s

Interfaces available in Collection:

• Collection
• List
• Set
• Sorted Set

Collection interface

Collection interface

• It enables u to work with group of objects.
• It is top of collection hierarchy

Collections and Arrays Classes from Object class Hierarchy

The Collection classes

ArrayList

• It implements the list Interface
• It supports the dynamic array that can grow as needed

LinkedList

• It implements the list interface
• It provides the link list data structure

It has some additional methods like :

• addFirst()
• addLast()
• removeFirst()
• removeLast()

HashSet

• It implements the set interface
• It creates the collection that uses the hash table for storage
• A hashtable stores a information by using a mechanism called Hashing.
• You cant directly index the hashtable

TreeSet

• It provides the implements the set interface that uses a tree for storage
• Objects are stored in sorted ascending order
• Access and retrieval time are quite fast which makes TreeSet an excellent choice when storing large amount of data that must be found quickly

The Map interface

• It maps unique key to value. A key is an object that use to retrieve a value at a later date.
• Given a key to value, you can store the value in a map object
• After the value is stored, you can retrieve it by using its key

The Map classes

• HashMap
• TreeMap

HashMap

• This class uses a hash table to implement the map interface
• To implement hash table it does not add any new methods of its own
• It does not guarantee the order of its elements

Tree Map

• It implements the map interface by using a tree
• It provides an efficient mean of sorting key/value pairs.
• A tree map guarantee that its elements will be sorted in ascending order key

Vector

• It implements a dynamic array
• It is similar to array list but with two differences
  1. Vector is synchronized
  2. It contain many legacy methods which are not a part of collection like addElement(), removeElements();

Note : The initial size of vector is 10

Dictionary

• It is an abstract class that represent key/value storage and operate much like map
• Given a key & value ,you can store the value in dictionary object
• Although not deprecated, Dictionary is classified as obsolete, because it is fully superseded by Map.

HashTable

• It is a concrete implementation of dictionary.
• It is similar to hash map but it is synchronized.
• While using a Hash table you specify an object that is used as a key and value that is linked to a key.
• Hash table store a information by using hashing.

Properties

• It is subclass of HashTable
• It is used to maintain list of values,in which the key is string and value is also a string
• The property class is used by many other java classes
• represents a persistent set of properties. { store & load methods }

store

public void store(OutputStream out, String comments) throws IOException


load

public void load(InputStream inStream)
//Reads a property list (key and element pairs) from the input stream.


What is the difference between Enumeration and Iterator?

• The functionality of Enumeration interface is duplicated by the Iterator interface. Iterator has a remove() method while Enumeration doesn’t.
• Enumeration acts as Read-only interface, because it has the methods only to traverse and fetch the objects.
• where as using Iterator we can manipulate the objects also like adding and removing the objects.

Storing Customer objects as Collection

Collection class and interface Hierarchy