OO Best Practice

Intro

• Exam focuses on recognizing tight encapsulation, loose coupling and high cohesion.

Coupling

• Coupling is the degree to which one class knows about the other class.
• Ideally a class should know about another class only through what has been exposed through its interface/API.
• If two classes know internal details about each other - they are said to be tightly coupled. This would mean that they cannot be modified independently.
• For e.g. if we use the generic type List in our public API, instead of ArrayList we can always change the internal implementation later to say, a LinkedList for performance reasons and clients of our API would need not change their code.

Cohesion

• Cohesion is all about how a single class is designed.
• Cohesion is used to indicate the degree to which a class has a single well-focused purpose.
• For e.g. in the below class has low cohesion, since it does a variety of things.

class TaxReport {

  void connectToDB();
  void generatePDF();
  void compute();

}

• The code should be split up to ensure that each class performs one functions and so is reusable:

class TaxReport {

  void compute();

}

class PDFUtil {

  void generatePDF();

}

class DBUtil {

  void connectToDB();

}