Collection Framework Tutorial

Limitations of Arrays:

• Arrays are fixed in size. need to estimate the size of an array while declaration itself. once array created we can not increase the size of an array.
• Arrays can hold only homogeneous data elements. Means we can add same type of elements in an array. While declaring an array we need to mention the data type.
• int a[]= new int[10];
• By using Object array we can add heterogeneous elements to the array.
• Object o= new Object[10];
• There is no underlying data structure for arrays.
• Arrays are not recommended to use with respect to memory.
• Performance wise arrays are good to use.

Collections 

• Java Collection framework added in J2SE 1.2 release.
• Collections are set of classes and interfaces.
• By using Collections we can store and manipulate objects easily.
• Collections can hold heterogeneous data elements.
• Collections are no fixed in size and dynamically increase in size.
• Collection of objects. No primitives.
• All collection classes having underlying data structure.
• Collections are good with respect to memory. Bad with respect to performance.

Collection interface Hierarchy:

•  java.util package contains all collections classes and interfaces.
• Lets see collection interface hierarchy.
• under Collection. Set , List , Queue are the sub interfaces.

Collections Abstract classes and classes:

• Let us see  all the abstract classes implementing all the above interfaces and classes which extending these abstract classes.
• To Collect Objects in array format we choose Collection hierarchy classes.
• Main abstract class is AbstractCollection.
• AbstractSet
• AbstractList
• AbstractQueue
• AbstractSequentialList
• All the classes in Collection hierarchy are
• TreeSet
• HashSet
• LinkedHashSet
• LinkedList
• ArrayList
• Vector
• Stack
• PriorityQueue 
• To collect unique elements we must choose Set implemented classes
• To collect unique and duplicate elements in indexed order we choose List implemented classes.
• To retrieve elements in FIFO manner we choose Queue implemented classes.

Map Hierarchy:

• In this hierarchy Hashtable and properties classes are avilable since java 1.0.
• LinkedHashMap class is available since java 1.4
• NavigableMap is available since java 6 and all other classes available since java 1.2.
• SortedMap and NavigableMap are two main interfaces.
• TreeMap
• HashMap
• LinkedHashMap
• Hashtable
• Properties are the classes. 
• To collect objects in key, value pair format we choose Map hierarchy classes.