What is instance variable and class variable in Java?

Class variables maintain a single shared value for all instances of the class, even if no instance object of that class exists. You would use the static keyword to change an instance variable into a class variable. Both instance and class variables are declared at the class level, not within methods.

Difference between Singleton Pattern vs Static Class in Java 

https://study.com/academy/lesson/static-class-in-java-definition-examples.html  

https://www.geeksforgeeks.org/static-class-in-java/

https://stackoverflow.com/questions/2765060/why-use-a-singleton-instead-of-static-methods

https://stackoverflow.com/questions/2798485/java-objects-shared-variables

Is Java really passing objects by value? - Stack Overflow

Shallow vs. Deep Copy in Java - DZone Java

How to Modify a String Without Creating a New One | Java Creed 

why use static function in java - Google Search

Java Enum Tutorial: 10 Examples of Enum in Java 

Java Programming Tutorial

Refer link

Development Setup:

Java 8 Installation on Ubuntu 16.04

Download Eclipse from: https://www.eclipse.org/efxclipse/install.html (http://efxclipse.bestsolution.at/)
4.4.2. Installing CDT | C/C++ Software Development with Eclipse

How to make a JAR file Linux executable (Example)

How to convert a .java or a .jar file into a Linux executable file ( without a .jar extension, which means it's not a .jar file ) - Stack Overflow

Creating Extensible Applications (The Java™ Tutorials > The Extension Mechanism > Creating and Using Extensions)

Java Interview Questions

http://www.developersbook.com/corejava/interview-questions/corejava-interview-questions-faqs.php
http://www.allapplabs.com/interview_questions/java_interview_questions.htm

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Explain different way of using thread?  
Describe synchronization in respect to multithreading.
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What is the difference between an Interface and an Abstract class?
Write the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers. 
What is the purpose of garbage collection in Java, and when is it used? 
Difference between Swing and Awt?