Design principles

A brief overview over common design principles

SOLID

• Single Responsibility Principle (SRP): 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.
• Open/Closed Principle (OCP): "Software entities ... should be open for extension, but closed for modification."
• Liskov Substitution Principle (LSP): "Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program."
• Interface Segregation Principle (ISP): "Many client-specific interfaces are better than one general-purpose interface."
• Dependency Inversion Principle (DIP): One should "depend upon abstractions, [not] concretions."

DRY (Don’t Repeat Yourself)

"Every piece of knowledge must have a single, unambiguous, authoritative representation within a system"

KISS principle

most systems work best if they are kept simple rather than made complicated; therefore, simplicity should be a key goal in design, and unnecessary complexity should be avoided

Law of Demeter (LoD)

a given object should assume as little as possible about the structure or properties of anything else (including its subcomponents), in accordance with the principle of "information hiding"

Design by contract (DbC)

software designers should define formal, precise and verifiable interface specifications for software components, which extend the ordinary definition of abstract data types with preconditions, postconditions and invariants

Encapsulation

bundling of data with the methods that operate on that data, or the restricting of direct access to some of an object's components. Encapsulation is used to hide the values or state of a structured data object inside a class, preventing unauthorized parties' direct access to them.

Command-Query-Separation(CQS)

“Functions should not produce abstract side effects...only commands (procedures) will be permitted to produce side effects.” - Bertrand Meyer: Object Oriented Software Construction

Principle of least astonishment (POLA)

a component of a system should behave in a way that most users will expect it to behave. The behavior should not astonish or surprise users

Linguistic-Modular-Units

“Modules must correspond to syntactic units in the language used.” - Bertrand Meyer: Object Oriented Software Construction

Self-Documentation

“The designer of a module should strive to make all information about the module part of the module itself.” - Bertrand Meyer: Object Oriented Software Construction

Uniform-Access

“All services offered by a module should be available through a uniform notation, which does not betray whether they are implemented through storage or through computation.” - Bertrand Meyer: Object Oriented Software Construction

Single-Choice

“Whenever a software system must support a set of alternatives, one and only one module in the system should know their exhaustive list.” - Bertrand Meyer: Object Oriented Software Construction

Persistence-Closure

“Whenever a storage mechanism stores an object, it must store with it the dependents of that object. Whenever a retrieval mechanism retrieves a previously stored object, it must also retrieve any dependent of that object that has not yet been retrieved.” - Bertrand Meyer: Object Oriented Software Construction