Adding initialization-on-demand idiom and noninstantiable class instead of interface constant idiom

[leogtzr]

Adding noninstantiable class instead of interface constant idiom.

[iluwatar]

In the Singleton example IvoryTower demonstrates eagerly initialized instance. In this pull request you have changed it implement Initialization-on-demand holder idiom. However, this has already been demonstrated in InitializingOnDemandHolderIdiom.

[iluwatar]

The second change concerning CustomerSchemaSql doesn't add any value since variables in interfaces are static and final by default.

[leogtzr]

I know they are static and final by default, the problem with the interface is that somebody can implement from it. We don't want that. Those classes were not designed to be instantiated. I am enforcing noninstantiability by using this idiom:

• interface -> class
• include a private constructor.

[iluwatar]

Ok, after reading more about the constant interface idiom I tend to agree that it is better style to declare final class instead. I wonder if this idiom has been used elsewhere in the codebase? If you update the pull request and comment on this thread I can accept it.

[leogtzr]

Added a little description, thanks :)

[leogtzr]

Added a few more changes, found that several java classes were using references to implementations instead of using abstractions/interfaces. Please review :D @iluwatar

[iluwatar]

@leogtzr looks good. Still the changes concerning IvoryTower need to be reverted for the reasons I mentioned before.

[iluwatar]

#532 (comment)

[leogtzr]

@iluwatar done, I have reverted the changes for IvoryTower.java, please take a look :)

[iluwatar]

@leogtzr thank you for the improvements!