Writing Code Is LifePapers about the SOLID Principles of the Object Oriented Design.
Single Responsibility Principle
“A class should only have a single responsibility, that is, only changes to one part of the software's specification should be able to affect the specification of the class."
Wikipedia (english)
Wikipedia (german)
PDF objectmentor.com / web.archive.org
Open/Closed Principle
“Software entities … should be open for extension, but closed for modification."
Wikipedia (english)
Wikipedia (german)
PDF objectmentor.com / web.archive.org
Liskov Substitution Principle
“Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program."
Wikipedia (english)
Wikipedia (german)
PDF objectmentor.com / web.archive.org
Interface Segregation Principle
“Many client-specific interfaces are better than one general-purpose interface."
Wikipedia (english)
Wikipedia (german)
PDF objectmentor.com / web.archive.org
Dependency Inversion Principle
One should “depend upon abstractions, [not] concretions."