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.


  • 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.

Collection interface Hierarchy

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.

Collection interview Questions

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.

Collection Map Hierarchy tutorial

Instance Of Java

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