What is Collection framework in Java? Why we need Collection? Collection hierarchy in Java

Guide to Collection framework in Java with Examples | Guide for choose which collection to use

In simple term, The Java Collections Framework is a collection of interfaces and classes which helps in storing and processing the data efficiently.

Java collections refer to a collection of individual objects that are represented as a single unit.

Whoever class or interface implements or extends Collection interface, they can use all method of collection interface. Some of frequently used methods :

• add()
• addAll()
  
• remove()
• removeAll()
  
• size()
• clear()
  
• isEmpty()
• contains() 
  

Why we need Collection in Java?

As we all know we can use java Arrays for store, alter, delete, sort data then why we still need Collection?

• Arrays are not resizable and Collection is resizable.
  
• Java Collections Framework provides lots of different useful data types, like Linkedlist allows insertion anywhere in constant time.
• There are different implementation you can choose from for the same set of services : like, ArrayList or LinkedList, HashSet or SortedSet, etc.
• Collection allows programmers to write readable and maintainable code.  

Collection framework hierarchy :

List Interface :

• List is child interface of Collection
• Duplicates are allowed and insertion order must be preserved

Classes under List interface :

• ArrayList (Class) (1.2 version)
• LinkedList (Class) (1.2 v)
• Vector (Class) (1.0 v)
• Stack (Class) (1.0 v)

Set Interface :

• List is child interface of Collection
• Duplicates are not allowed and insertion order does not preserved

Classes and Interfaces under Set interface :

• HashSet (Class) (1.2 v)
  
• LinkedHashSet (Class) (1.4 v)
• SortedSet (Interface) (1.2 v)
• NavigableSet (Interface) (1.6 v)
• TreeSet (Class) (1.2 v)

Queue Interface :

• Queue follows First In First Out (FIFO) order.

Classes and Interfaces under Set interface :

• PriorityQueue (Class) (1.5 v)
• BlockingQueue (Interface) (1.5 v)

Map Interface :

If we want to represent group of object as key-value pair then we should use Map interface. 

Map is not child interface of Collection.

• Both keys and values are Objects only 
• Duplicates keys are not allowed but values can be duplicated

Classes and Interfaces under Map interface :

• HashMap (Class) (1.2 v)
• LinkedHashMap (Class) (1.4 v)
• SortedMap (Interface) (1.2 v)
• NavigableMap (Interface) (1.6 v)
• TreeMap (Class) (1.2 v)

 

How to choose which collection we have to use in our code?

List :

    ArrayList : 

• It is combination of Array and List.
• Use when does not want to use Static array.
• Insertion order preserve and Duplication of data does not matter. Arraylist is sequential list.
• Better for storing and retrieval data frequently. 

    LinkedList :

• Insertion order preserve and Duplication of data does not matter.
  
• Better for data manipulation (insertion, deletion).

    Vector :

• Vector is same as ArrayList except that all the Vector class methods are synchronized. Means vector class is Thread safe. 
  

    Stack :

• Want to use Last In First Out (LIFO) order of data. 
  

Set :

    HashSet :

• Duplication is not allowed.
• Insertion order does not preserve.

    LinkedHashSet :

• Want to maintain insertion order.
• Duplicates are not allowed.
  

    TreeSet :

• Want to sorted data.
  
• Duplicates are not allowed. 

Queue :

    PriorityQueue :

• Want to use First In First Out (FIFO) order. first element is removed first and last element is removed at last. 

Map :

    HashMap :

• When you want to store data as key-value pair. (Name -> Mobile No.).
• Does not maintain insertion order.
• Can store only one NULL key and many NULL value.
  
• Key contains only unique data, but value can be duplicates.
  

    LinkedHashMap :

• Same as HashMap.
• Insertion order is maintain.  
  

    TreeMap :

• Also store Key-value pair.
• Want to sorted elements. Elements sorted by ascending order.
• Does not contains any NULL key. but value can.
• When read/write of elements is frequent, then TreeMap is good choice.   
  

Happy learning... Happy coding...

Other articles :

• A Guide to Iterator in Java | hasNext(), next(), remove(), forEachRemaining() Methods  
• Object Oriented Programming (OOP) concept in Java 
• Most asked interview questions and answers for Java developers on Core Java