OO Design Principles

Principles: Why? Which?

  • Every handcraft has rules, on every single level, which everybody agrees upon

  • Our handcraft is no exception

  • On the design level: Design Principles

    • S: Single Responsibility

    • O: Open/Closed

    • L: Liskov Substitution

    • I: Interface Segregation

    • D: Dependency Inversion

  • ⟶ SOLID (for people who find it hard to remember rules)

  • Antipattern: a pattern that violates any of these principles

Single Responsibility

Every class must have responsibility over a single part of the program.

(Robert C. Martin, at around 2000)
Every class must do one thing and should do that well.

(Jörg Faschingbauer (Unix addict), all the time)


  • Understanding is easier

  • Documenting is easier

  • Defining/writing tests is easier


Software entities must be open for extension, but closed for modification.

(Bertrand Meyer, 1988)


  • Adding functionality not by modifying but by adding (e.g. plugins)

  • Heavy use of interfaces

Liskov Substitution

It must be possible in a program to exchange two implementations of
an interface without compromising the correctness of the program.

(Barbara Liskov, 1995)

Classical violation of Liskow’s principle: square/rectangle

  • A rectangle is defined as a pair (width, height), each of which is modifiable separately

  • Can a square be seen as a rectangle then?


  • No special cases in user code

  • Polished interfaces

Interface Segregation

No client of an interface should be forced to depend on methods it does not use.

(Robert C. Martin (again), at around 2000)

Dependency Inversion

* High-level modules should not depend on low-level modules. Both should depend on abstractions.
* Abstractions should not depend upon details. Details should depend upon abstractions.

(Robert C. Martin (again), at around 2000)