* Declared 'Map' instead of 'HashMap' fields.
* Switched from 'protected' to 'private' fields and added needed getter methods.
* Added a 'final' modifier to methods not overridden.
* Removed string constant duplications.
* Removed local variable declarations when they are immediately returned.
* Avoided to catch Throwable.
* Declared a protected constructor for utility classes.
* Removed unnecessary 'public' modifier for interface methods.
* Added 'final' modifier to all constants.
* Avoided to throw Error instances.
* Added brackets for if, if/else, while statements.