Tanmay Fadnavis

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Design Patterns Basics

27 Feb 2022 Design Principles Basics

Designing object-oriented software is hard, and designing 'reusable' object-oriented software is even harder. You must find pertinent objects, factor them into classes with the correct granularity, define class interfaces and hierarchy, etc. Your design should be specific with problem at hand, but also general enough to address future requriments. This is where design patterns come into picture. This blog is based on Design Patterns by GOF.

What is a Design Pattern?

"Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice" - Chrispother Alexander. The idea is not to solve every problem from first principles. In general a pattern has 4 elements:

  • Pattern name: handle used to describe a design problem, it's solutions and consequences.
  • Problem: describes when to apply the pattern. It explains the problem and it's context.
  • Solution: describes the elements that make up the design, their relationships, responsibilities, and collaborations.
  • Consequences: are results and trade-offs of applying the pattern.

Describing Design Pattern.

How to describe design patterns? Graphical notations while useful aren't sufficient. Hence the GOF came up with the below list for describing the DP(Design Patterns) in details:

  • Pattern name
  • Intent
  • Also known as
  • Motivation
  • Applicability
  • Structure
  • Participants
  • Collaborations
  • Consequences
  • Implementation
  • Sample Code
  • Known Uses
  • Related Patterns

How Design Pattern solve design problems.

Design patterns solve many of the day-to-day problems object-orinted designers face, in many different ways.
=> Finding appropriate objects - the hard part about OOD is decomposing systems into objects.
=> Determining object granularity.
Specifying object interfaces.
=> Programming to an interface, not an implementation. Client remains unaware of the specific types of objects they use, as long as the objects adhere to the interface that clients expect.
=> Putting reuse mechanism to work, inheritance vs Composition. Favor object composition over class inheritence.
=> Delegation is a way of making composition as powerful for reuse as inheritance. In delegation, two objects are involved in handling a request, e.g. Instead of window 'being' a rectangle, it would 'have' a rectangle.

Design Pattern Catalog.

  • Abstract Factory: Provide an interface for creating families of related or dependent objects without specifying their concrete class.
  • Adapter: Converts the interface of the class into another interface clients expect. Adapter lets classes work together that couldn't othwerside 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 change 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, there by 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. Decprators 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: Defines 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.
  • Prototype: Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.
  • Proxy: Provide a surrogate or placeholder for another object to control access to.
  • Singleton: Ensure a class only has one instance, and provide a global point to access it.
  • State: Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
  • Stategy: Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary indepdently from clients that use it.
  • Template Method: Define the skeleton of 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.

Common causes for re-design.

Below are the common causes for redesigning, and which DPs would be useful for the given cause.

  1. Creating an object by specifying a class explicitly.
    DP: Abstract factory, factory method, prototype.
  2. Dependence on specific operations.
    DP: COR, Command.
  3. Dependence on h/w and s/w platforms.
    DP: Abstract factory, bridge.
  4. Dependency on object representations or implemnetations.
    DP: Abstract factory, bridge, memento, proxy.
  5. Algorithmic dependencies.
    DP: Builder, Iterator, Strategy, Template method, Visitor.
  6. Tight coupling.
    DP: Abstract factory, builder, COR, Command, Facade, mediator, observer.
  7. Extending functionality by subclassing.
    DP: Bridge, COR, composite, decorator, observer, strategy.
  8. Inability to alter class conveniently.
    DP: Adapter, decorator, Visitor.