One can compare the necessity to include and to define all methods in an interface when deciding to become part of one to the necessity to conform to certain writing conventions when deciding to write for a particular audience. In order for a program to call a method defined in an interface on a class which implements that interface, the class must define that method. In order for the work of a writer to appeal to and to be understood by a certain audience, that audience must be able to understand and to personally define the references within the written work. Without a basis for understanding references (or method calls), the message of a written work (or a program) fails, crashes.
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