Hash#has_key? is deprecated in favor of Hash#key?
Fix access modifier indentation and keep at same depth as method definitions
Replacing {…} with do…end for multiline blocks
Should not use :: for regular method invocation
Remove extra empty lines
Add empty lines around access modifiers

Although not explicity required to have an empty line after the modifier rubocop
offers no way to change the behaviour.
Prefer literal array and hash creation notation

Unless you need to pass parameters to their constructors.
No need to introduce new global variables
Prefer %w to the literal array syntax when you need an array of strings