Interface Segregation Principle

Clients should not be forced to depend upon interfaces that they do not use.

  • Make fine grained interfaces that are client specific
  • Many client specific interfaces are better than one “general purpose” interface
  • Keep your components focused and minimize dependencies between them
  • Notice relationship to the Single Responsibility Principle?
  • avoid ‘god’ interfaces

Liskov Subsitution Principle

If it looks like a duck and quacks like a duck but it needs batteries, you probably have the wrong abstraction.

  • By Barbara Liskov, in 1998
  • Objects in a program would be replaceable with instances of their subtypes WITHOUT altering the correctness of the program.
  • Violations will often fail the “Is a” test.
  • A Square “Is a” Rectangle
  • However, a Rectangle “Is Not” a Square

Open/Closed Principle

Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification.

  • Your classes should be open for extension
  • But closed for modification
  • You should be able to extend a classes behavior, without modifying it.
  • Use private variables with getters and setters – ONLY when you need them.
  • Use abstract base classes