As I write more and more code, the necessity of some principles becomes apparent. At high level, I find Zen of Python quite pleasing. To learn more details, I got this book.

On Amazon, it has the ranking of

  • #1 in Books > Computers & Technology > Computer Science > AI & Machine Learning > Computer Vision & Pattern Recognition
  • #1 in Books > Computers & Technology > Programming > Software Design, Testing & Engineering > Software Reuse
  • #2 in Books > Textbooks > Computer Science > Object-Oriented Software Design

the principles

The authors advocate the following principles of object-oriented design

  • Program to an interface, not an implementation.
  • Favor object composition over class inheritance.

They also point out the common causes of redesign

  • Creating an object by specifying a class explicitly
  • Dependence on specific operations
  • Dependence on hardware and software platform
  • Dependence on object representations or implementations
  • Algorithmic dependencies
  • Tight coupling
  • Extending functionality by subclassing
  • Inability to alter classes conveniently

Here are some interesting quotes from it

  • One thing expert designers know not to do is solve every problem from first principles.
  • The choice of programming language is important because it influences one’s point of view.
  • Favoring object composition over class inheritance helps you keep each class encapsulated and focused on one task.
  • The main advantage of delegation is that it makes it easy to compose behaviors at run-time and to change the way they’re composed.
  • A design pattern should only be applied when the flexibility it affords is actually needed.
  • Encapsulate the concept that varies.
  • A class is more reusable when you minimize the assumtions other classes must make to use it.

the design patterns

The design patterns are summarized in the following table

Creational Structural Behavioral
Scope Class Factory Method Adapter Interpreter
Template Method
Object Abstract Factory
Chain of Responsibility
  • Abstract Factory: Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
  • Adapter: Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn’t otherwise because of incompatible interfaces.
  • Bridge: Decouple an abstraction from its implementation so that the two can vary independently.
  • Builder: Separate the construction of a complex object from its representation so that the same construction process can create different representations.
  • Chain of Responsibility: Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.
  • Command: Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
  • Composite: Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
  • Decorator: Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
  • Facade: Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.
  • Factory Method: Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.
  • Flyweight: Use sharing to support large numbers of fine-grained objects efficiently.
  • Interpreter: Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.
  • Iterator: Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
  • Mediator: Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.
  • Memento: Without violating encapsulation, capture and externalize an object’s internal state so that the object can be restored to this state later.
  • Observer: Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
  • Proxy: Provide a surrogate or placeholder for another object to control access to it.
  • Singleton: Ensure a class only has one instance, and provide a global point of access to it.
  • State: Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
  • Strategy: Define a family of algorithms, encapsulate each one, and make them interchaneable. Strategy lets the algorithm vary independently from clients that use it.
  • Template Method: Define the skeleton af an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm’s structure.
  • Visitor: Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.