An Introduction to Design Patterns Fabrizio Maria Maggi Institute of Computer Science (The java code and the material is taken from: https://www.tutorialspoint.com/design_pattern/design_pattern_overview.htm) Software Development MethodologyDomain Classes; Application Classes (e.g., Patterns); Attributes; Relations; Operations Domain (Class) Application (Class) Model Interaction Model Modelling Code 1 Systems modelling – Fabrizio Maria Maggi Application (Class) Model 2 Systems modelling – Fabrizio Maria Maggi Application (Class) Model 3 Systems modelling – Fabrizio Maria Maggi Application (Class) Model 4 Systems modelling – Fabrizio Maria Maggi Application (Class) Model 5 Systems modelling – Fabrizio Maria Maggi What is Gang of Four (GOF) Gamma, Helm, Johnson, Vlissides. Design Patterns - Elements of Reusable Object-Oriented Software Addison-Wesley 1994 Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides initiated the concept of Design Pattern in Software development These authors are collectively known as Gang of Four (GOF) . 6 Design Patterns Design patterns represent the best practices used by experienced object-oriented software developers Design patterns are solutions to general problems that software developers faced during software development These solutions were obtained by trial and error by numerous software developers over quite a substantial period of time Learning these patterns helps unexperienced developers to learn software design in an easy and faster way 7 Categories of Design Patterns 8 Categories of Design Patterns 9 Factory This type of design pattern comes under creational pattern In Factory pattern, we create object without exposing the creation logic to the client and refer to newly created object using a common interface 10 Systems modelling – Fabrizio Maria Maggi Factory 11 Systems modelling – Fabrizio Maria Maggi Factory 12 Systems modelling – Fabrizio Maria Maggi Factory 13 Systems modelling – Fabrizio Maria Maggi Factory 14 Systems modelling – Fabrizio Maria Maggi Abstract Factory Abstract Factory patterns work around a super-factory which creates other factories. This factory is also called as factory of factories This type of design pattern comes under creational pattern In Abstract Factory pattern an interface is responsible for creating a factory of related objects without explicitly specifying their classes. Each generated factory can give the objects as per the Factory pattern 15 Systems modelling – Fabrizio Maria Maggi Abstract Factory 16 Systems modelling – Fabrizio Maria Maggi Abstract Factory 17 Systems modelling – Fabrizio Maria Maggi Abstract Factory 18 Systems modelling – Fabrizio Maria Maggi Abstract Factory 19 Systems modelling – Fabrizio Maria Maggi Abstract Factory 20 Systems modelling – Fabrizio Maria Maggi Abstract Factory 21 Systems modelling – Fabrizio Maria Maggi Abstract Factory 22 Systems modelling – Fabrizio Maria Maggi Abstract Factory 23 Systems modelling – Fabrizio Maria Maggi Abstract Factory 24 Systems modelling – Fabrizio Maria Maggi Abstract Factory 25 Singleton Singleton pattern is one of the simplest design patterns in Java This type of design pattern comes under creational pattern This pattern involves a single class which is responsible to create an object while making sure that only single object gets created. This class provides a way to access its only object which can be accessed directly without need to instantiate the object of the class 26 Systems modelling – Fabrizio Maria Maggi Singleton 27 Systems modelling – Fabrizio Maria Maggi Singleton 28 Systems modelling – Fabrizio Maria Maggi Builder Builder pattern builds a complex object using simple objects and using a step by step approach This type of design pattern comes under creational pattern A Builder class builds the final object step by step. This builder is independent of other objects 29 Systems modelling – Fabrizio Maria Maggi Builder 30 Systems modelling – Fabrizio Maria Maggi Builder 31 Systems modelling – Fabrizio Maria Maggi Builder 32 Systems modelling – Fabrizio Maria Maggi Builder 33 Systems modelling – Fabrizio Maria Maggi Builder 34 Systems modelling – Fabrizio Maria Maggi Builder 35 Systems modelling – Fabrizio Maria Maggi Builder 36 Systems modelling – Fabrizio Maria Maggi Builder 37 Systems modelling – Fabrizio Maria Maggi Categories of Design Patterns 38 Adapter Adapter pattern works as a bridge between two incompatible interfaces This type of design pattern comes under structural pattern as this pattern combines the capability of two independent interfaces This pattern involves a single class which is responsible to join functionalities of independent or incompatible interfaces 39 Systems modelling – Fabrizio Maria Maggi Adapter 40 Systems modelling – Fabrizio Maria Maggi Adapter 41 Systems modelling – Fabrizio Maria Maggi Adapter 42 Systems modelling – Fabrizio Maria Maggi Adapter 43 Systems modelling – Fabrizio Maria Maggi Adapter 44 Systems modelling – Fabrizio Maria Maggi Filter Filter pattern or Criteria pattern is a design pattern that enables developers to filter a set of objects using different criteria and chaining them in a decoupled way through logical operations This type of design pattern comes under structural pattern as this pattern combines multiple criteria to obtain single criteria 45 Systems modelling – Fabrizio Maria Maggi Filter 46 Systems modelling – Fabrizio Maria Maggi Filter 47 Systems modelling – Fabrizio Maria Maggi Filter 48 Systems modelling – Fabrizio Maria Maggi Filter 49 Systems modelling – Fabrizio Maria Maggi Filter 50 Systems modelling – Fabrizio Maria Maggi Filter 51 Systems modelling – Fabrizio Maria Maggi Filter 52 Systems modelling – Fabrizio Maria Maggi Filter 53 Systems modelling – Fabrizio Maria Maggi Composite Composite pattern composes objects in term of a tree structure to represent part as well as whole hierarchy This type of design pattern comes under structural pattern as this pattern creates a tree structure of group of objects This pattern creates a class that contains group of its own objects: This class provides ways to modify its group of same objects 54 Systems modelling – Fabrizio Maria Maggi Composite 55 Systems modelling – Fabrizio Maria Maggi Composite 56 Decorator Decorator pattern allows a user to add new functionality to an existing object without altering its structure This type of design pattern comes under structural pattern as this pattern acts as a wrapper to existing class This pattern creates a decorator class which wraps the original class and provides additional functionality keeping class methods signature intact 57 Systems modelling – Fabrizio Maria Maggi Decorator 58 Systems modelling – Fabrizio Maria Maggi Decorator 59 Systems modelling – Fabrizio Maria Maggi Decorator 60 Systems modelling – Fabrizio Maria Maggi Decorator 61 Systems modelling – Fabrizio Maria Maggi Decorator 62 Systems modelling – Fabrizio Maria Maggi Facade Facade pattern hides the complexities of the system and provides an interface to the client using which the client can access the system This type of design pattern comes under structural pattern as this pattern adds an interface to existing system to hide its complexities This pattern involves a single class which provides simplified methods required by client and delegates calls to methods of existing system classes 63 Systems modelling – Fabrizio Maria Maggi Facade 64 Systems modelling – Fabrizio Maria Maggi Facade 65 Systems modelling – Fabrizio Maria Maggi Facade 66 Systems modelling – Fabrizio Maria Maggi Facade 67 Systems modelling – Fabrizio Maria Maggi Categories of Design Patterns 68 Command Command pattern is a data driven design pattern and falls under behavioral pattern category A request is wrapped under an object as command and passed to invoker object: Invoker object looks for the appropriate object which can handle this command and passes the command to the corresponding object which executes the command 69 Systems modelling – Fabrizio Maria Maggi Command 70 Systems modelling – Fabrizio Maria Maggi Command 71 Systems modelling – Fabrizio Maria Maggi Command 72 Systems modelling – Fabrizio Maria Maggi Command 73 Systems modelling – Fabrizio Maria Maggi Command 74 Systems modelling – Fabrizio Maria Maggi Iterator Iterator pattern is very commonly used design pattern in Java and .Net programming environment This pattern is used to get a way to access the elements of a collection object in sequential manner without any need to know its underlying representation Iterator pattern falls under behavioral pattern category 75 Systems modelling – Fabrizio Maria Maggi Iterator 76 Systems modelling – Fabrizio Maria Maggi Iterator 77 Iterator 78 Observer Observer pattern is used when there is one-to-many relationship between objects such as if one object is modified, its dependent objects are to be notified automatically Observer pattern falls under behavioral pattern category 79 Systems modelling – Fabrizio Maria Maggi Observer 80 Systems modelling – Fabrizio Maria Maggi Observer 81 Systems modelling – Fabrizio Maria Maggi Observer 82 Systems modelling – Fabrizio Maria Maggi Observer 83 Systems modelling – Fabrizio Maria Maggi Observer 84 Systems modelling – Fabrizio Maria Maggi Observer 85 Systems modelling – Fabrizio Maria Maggi Strategy In Strategy pattern, a class behavior or its algorithm can be changed at run time This type of design pattern comes under behavioral pattern In Strategy pattern, we create objects which represent