Java Developer 4 to 6 yrs exp Interview at Cognizant

Following were the questions which are asked to my colleagues at experience of 4 to 6 years.

1.  Tell me about your current project ?
2.  What were your roles and responsibilities in your project?
3.  Can you draw architecture diagram of your project?

Java 8 New Features

Java 8 comes with very exciting features of functional programming with lambda expression and streams.

Lets take a look at it features

Lambda Looping

Lambda expression can be used for looping over a Collection.

Custom Java HashMap - Internal Working

HashMap is a Collection class in Java that stores objects with the help of hascode and equals. It is said to be faster in retrieving objects.
Lets create a custom implementation of HashMap  and understand how it works. To understand HashMap you need to first understand Singly and DoublyLinkedList.

HashMap stores objects in form of Key and Value. A value can be fetched by passing the Key.

HashMap stores objects into different buckets(LinkedList). The object with same hashcode are stored in same LinkedList. Remember that the objects are stored in key-value pairs. The hashcode of key is used and not the hashcode of value. Only Key alone is required to put and get the object. Value is just a object placed against the key, So depending upon how many objects(Keys) are present with different hashcodes, that many LinkedList gets created.

Java Singleton Pattern

Singleton pattern is the most common types of patterns used in Java. It is also very common that interviewers ask questions to explain and implement singleton pattern

Lets first define Singleton
Singleton is a class whose only one object can be created.
There are couple of ways to implement Singleton class. Lets see the simplest one first.

Java Collections : Internal Working

Java Collections:

Going for an Job Interview? be prepared with Java Collections framework. Java Collection framework is the most preferred topic by interviewers. It gives them the idea of how much effort the interviewee as taken to understand this framework and clear the interview.

And guess what, similar questions are now also asked in US for H1B visa holders, or MS post graduates looking for job.

This blog explains internal working of the Collection framework. For Interview Question on Collection refer Java Collection Interview Questions. 

For Java Interview questions for 2 to 5 years Senior Developer refer this blog.

Recently Blockchain has also made place in Java Interviews. Blockchain internally is also a Data Structure. To know more click here : Implementing Blockchain using Java

Click Here for Core Java Objective Questions and Answers

Custom implementation of Collections and Data Structure:

Internal Working of Arraylist
Internal Working of LinkedList
Internal Working of HashMap
Internal Working of TreeMap

Java Collection internally uses the primitive and core elements like Arrays and datastructures like  Linked List, Tree etc. So if you are asked a question to explain the internal working of any of the Collection classes, don't be surprised. Be it an interview for an Junior Java developer or even for an Architect, Java Collection is always something that you will have on you plate.

Java provides many collection classes that can be used to store data. Knowing which collection class to use for best performance and optimum result is the key.

First the basics.
Below two image shows the complete hierarchy of interfaces and classes present in the Collection framework.

The Collections come in basic four flavors:
Lists : List of things ( Classes that implement List Interface)
Sets : Unique things ( Classes that implement Set Interface)
Maps : Things with unique id ( Classes that implement Map Interface)
Queues : Things arranged in order ( Classes that implement Queue Interface)

The blue ones are the Interfaces and the red ones are the implementation classes

Below is Map. A map is a special type of collection.

          Below table shows the different concrete classes implementing these interfaces

Maps	Sets	Lists	Queues	Utilities
HashMap	HashSet	ArrayList	PriorityQueue	Collections
Hashtable	LinkedHashSet	Vector		Arrays
TreeMap	TreeSet	LinkedList
LinkedHashMap

Data Structure : Binary Search Tree

This blog describes the Internal Working of TreeMap. This is a Binary Search Tree. Similar to java.util.TreeMap.
Binary Search Tree (BST) is a Data Structure which places the data in sorted order. Each Node has 2 pointers/references left and right. If the data to be inserted is smaller than the current node than the new node is attached at the left of the current node. If the data to be inserted is greater than the current node than the new node is attached at the right of the current node.