SE Reference

Cohesion

Signifies the togetherness of components of module. high cohesion is good, it makes modules reusable and readable.

low cohesion

class calculator{
  input();
  add();
  multiply();
  display();
}

high cohesion

class calculator{
  add();
  multiply();
}

class display{
  input();
  display();
}

Coupling

Signifies dependency of module on other modules. Low coupling is good.

https://www.youtube.com/watch?v=-XJk27254KA

Cyclomatic Complexity

Used to measure complexity of program, calculated with a help of control flow diagram.

Cyclomatic complexity = E - N + 2*P 
where,
  E = number of edges in the flow graph.
  N = number of nodes in the flow graph.
  P = number of nodes that have exit points

Lower the Program’s cyclomatic complexity, lower the risk to modify and easier to understand

sauce

SOLID

Single Responsibility

Every class should only have a single responsibility. e.g calculator class in low cohesion example has 2 responsibilities - calculating stuff and displaying results.

Open Close

Classes should be open for extension but closed for modification. We should avoid changing an old class.

Liskov’s Substitution Principle

Instances of any subtype of a parent type, should be replacable / substitutable with instance of parent type not just syntactically but also semantically (logically) .

e.g if there’s a class fish and its child class star-fish. fish class has a method swim. but star fish doesn’t swim. so this voilates Liskov’s principle.

Interface Segregation

Interface should not be bulky. class should not implement interfaces that consists of methods they(classes) don’t need. Common methods should be seperated in a root interface and other interfaces should extend this root interface.

Dependency Inversion

High lvl modules should not depend on low level modules . rather both should depend on abstraction (interfaces). this reduces coupling.