https://www.javatpoint.com/singleton-design-pattern-in-java
https://www.tutorialspoint.com/java/java_using_singleton.htm
http://www.java2novice.com/java-interview-programs/java-singleton/
https://www.youtube.com/watch?v=NZaXM67fxbs
https://www.youtube.com/watch?v=SniLNfFejiU
https://www.javacodegeeks.com/2015/09/singleton-design-pattern.html
https://www.tutorialspoint.com/java/java_using_singleton.htm
http://www.java2novice.com/java-interview-programs/java-singleton/
https://www.youtube.com/watch?v=NZaXM67fxbs
https://www.youtube.com/watch?v=SniLNfFejiU
https://www.javacodegeeks.com/2015/09/singleton-design-pattern.html