All methods that are declared in a classdef file need to be implemented as well.
All methods that are declared in a classdef file need to be implemented as well, and vice versa. The declaration should match the implementation, i.e. do not declare varargin but have a method with x, y, z input arguments.
Correctly declaring and implementing methods increases the readability of your code. Inconsistency between declaration and implementation may lead to misconception.