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

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