Access type control (public, private, protected)
Access control modifiers include public (public, default), private (private), and protected (protected).
1.public
Members of the class have no access restrictions, and all external members can access the members of the class.
2.private
There are no access restrictions for all members of the same class, but external code of the class is not allowed to change or even operate, and for subclasses of the class, there is no access to private-decorated members.
3.protected
It has access to subclasses of this class and subclasses of subclasses, and can read and write properties and methods. However, it cannot be accessed by the external code of the class, nor does the external code of the subclass have access to its properties and methods.
