[Extract Interface] initial codes
[Extract Interface] create interface and make employee implement it
[Extract Interface] change the usage place to use Interface instead of implementation class
[ReplaceInheritanceWithDelegation] initial codes
[ReplaceInheritanceWithDelegation] add a new field in sub-class and assign this to it
[ReplaceInheritanceWithDelegation] use the newly-added field to call methods
[ReplaceInheritanceWithDelegation] remove Inheritance