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Designing Good Classes

  • Classes bind related data and expose functions that operate on that data
    • This helps make the code more organized
  • There are many design principles and patterns that can help make our code better organized
    • Let's look at some rules which can help us design better classes
      • Organized and Encapsulated
      • Should be small and should do just one thing
      • Small number of instance variables

Organized and Encapsulated​

  • A class should keep all data attributes and utility functions private
    • Only the functions that are supposed to be exposed should be kept public
  • A class should be ordered like this
    • Start with the variables
      • public static constants
      • followed by private static attributes
      • followed by private instance attributes
    • followed by public functions
    • private utility function just after the public function that calls it

Should be small and should do just one thing​

  • A class should be very small. Different experts have different views on how small?
    • Based on most of the views, a class should be small enough that it does just one thing
    • In general, it should have less than 20 functions
  • If a class does more than one thing then it should be broken down into different classes each of which does a single thing

Small number of instance variables​

  • A class should have strong cohesion
    • i.e., the functions of a class should be strongly related in supporting a single central purpose
  • A maximal cohesive class is one in which all functions work with all the instance variables
    • Achieving that is pretty difficult
    • We should try to make our classes as cohesive as possible
    • This can be done by having a smaller number of instance variables such that each function in the class work with as many of the instance variables as possible
    • This can be done by splitting the class into multiple classes based on the responsibility of each class