Java Interview Preparation article : OOPS concept

This is continuation of Java Interview Preparation . Here we will see OOPS concept. Encapsulation, Abstraction, Inheritance and Polymorphism are the basic pillars of object oriented programming language. And how Java adhere to these principle is crucial. These topics are covered in almost all the java book or can be found over internet also. Here is one link.

While answering abstraction and encapsulation it may sound overlapping. My explanation for encapsulation is, inside class all the members would be private to prohibit direct access of them from out side of the class. The public methods are for carry out various operation. And the caller of those methods need not know the internal implementation, only the meaning of required arguments to invoke the method is sufficient to achieve the goal. This is called abstraction.

In Java inheritance is achieved by "extends" and "implements" key word. It means either by extending another class or implementing a interface.
Child class is always a super set of parent class. Its also known as "is a" relation ship. There cloud be many questions under this topic. How the objects get created, the order of invocation of constructors. How a parent class reference can hold a child class object etc. Why java doesn't allow multiple inheritance. What are marker interface ? Several questions fall under this.

This is an nice article to understand polymorphism. There are two aspects of this compile time polymorphism and run time polymorphism.

https://www.geeksforgeeks.org/dynamic-method-dispatch-runtime-polymorphism-java/
https://www.dineshonjava.com/compile-time-polymorphism-in-java/

There could be other question like difference between aggregation vs composition.
"is a" vs "has a"
How can you achieve multiple inheritance in java ? ( By interface)

But I feel the major three concepts will put you in good spot to attempt the question around it.

Java Interview Preparation article :1

Time and again I get requests to write on preparation for interviews in Java. So I am planning to write series of post targeting interview preparation.
To start if the interview is for service based company the key is breadth. And for product based company its the depth. The exception for (breadth/depth) is directly proportional to years of experience.

Another point I want to make entry level roles (0 years to 5 years) you need to do well in java interview rounds to get selected. For more experience this round is kind of elimination round.

Topics in core Java

1. OOPs concept
2. Java key word, more priority to the latest released key words.
3. Inheritance, interface, marker interface, abstract class and is a has a relationship  
4. Enums
5. The details about java.lang.Object 
6. Strings 
7. Exception handling
8. Collections 
9. Threads and Concurrent Package
10. Design Pattern

All these topics will cover good amount of ground. There might be some here and there question about garbage collection, jvm architecture, java memory management etc. Those questions were more relevant when java was dominant. Now we see the increase popularity of python, Scala so its kind of ok to focus energy in understanding other topics.

Once you get thorough these topic next stage is Java in web development, which we will see in another post. 

BlockingQueue

Producer - consumer problem arises in multi thread environment. This can be defined when there are one or more threads generate data independently and same time one or many threads read those data to process independently.

To solve this we can have an array and there would be synchronized method to access. So when one thread only can either read/write at same time. If the array is full, producer threads need to wait. And if the array is empty consumer threads need to wait.

java.util.concurrent package comes with Blocking queue which is very handy to solve this problem.

Blocking queue is an interface. ArrayBlockingQueue  is an concrete implementation of interface. It allow both wait or fail for producer if the queue is full, same way for consumer it fails or waits if the queue is empty.

Some sample codes can be foundin https://github.com/pallabrath/myexpjava/tree/master/myexpjava/src/demo/blockingQueue

What we discussed here is in the scope of single instance. If we want scale then there would be more than one instance. In that case of distributed environment we need different kind of solution. Java message queue fit into this situation.

Again if we scale further and we want to process data in large volume, Kafka goes well in the distributed environment. Kafka itself run on one or many node to scale.

We will discuss Kafka in a separate thread.