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Design Patterns

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Design patterns

Advanced topics in software engineering
CSC 532

Submitted by:-
Harpreet Singh


Design patterns, a standard solution to problems in object oriented software engineering, are considered to be a well formed language to represent software design. Their benefits have been widely acknowledged by software professionals throughout the world. Design patterns can be classified according to multiple criteria the most common being the type of problem they solve. Till today many design patterns have been established and many more are being found as time passes. Patterns capture knowledge from various parts of software developing. Design patterns are helpful to the designers in a way that they represent the collective wisdom and experience of the community and their implementation leads to better quality software and also a novice designer does not find it difficult to understand the systems functionality. A very important advantage of design patterns is the fact that they speed up the development process by providing an almost ready-made solution that has been used earlier and proved to be efficient. Another advantage is that they allow for a generalized solution that does not depend on understanding a specific design problem from all its aspects, and thus ease reusing this solution. Apart from that they help the new developers to ignore traps and pitfalls which have earlier been learned by other developers by costly experience. In this paper I would explain about the design patterns in detail. Then I would try to elaborate upon the classification of design patterns into various existing pattern families and how to document patterns. Finally I would conclude my paper by providing the advantages of patterns and their critique .In this paper the term Pattern and Design Pattern is used interchangeably as it implies one and the same thing.

Design patterns provide a standard solution to various object oriented problems in software engineering. They ease the designing of new software projects by providing proven solutions to recurring problems. To know the definition of the term lets first look at the following fact :- Hundreds and thousands of software projects are made every single year , but the developers don’t invent a unique design every time . However, it could be the case that they are using some techniques which have earlier been used but not documented and they might not have the slightest idea about it. Not all of the software processes are completely different. So the developers should be able to use already invented designs instead of framing new ones every time they develop a project. And DESIGN PATTERNS are the solution to this problem. .“ Design patterns describe proven solutions to recurring problems” [1]. They are a well documented solution to a specific type of problem which has been proven to work in that specific problem situation. Not only do they reduce the time of the software development but also they provide better quality software. Till today many design patterns have been established and many more are being found as time passes. Patterns capture knowledge from various parts of software developing. They are helpful to the designers in a way that they provide the collective wisdom of the expertise of the and experience of the community and their implementation leads to better quality software.
One of the advantages of using design patterns is that novice developers find it easy to develop the software as they are now provided with a ready-made solution (design model) which they can implement without any worries as it has been used earlier and proved to be efficient in similar situations. Since design patterns have emerged they have gained widespread acceptence by the software parcatitioners as they cover wide areas of software development. However, one thing should be kept in mind that design patterns don’t solve all the problems originating during the software development , rather they play an important role in it. One major Advantage of design patterns is that they provide ready-made solutions which speed up the development process and their potential of being revised and improved over time make them perform better than home made designs. Design patterns should be specified in a more formal way otherwise it may give rise to ambiguity which may further limit the tool support and lead to incorrect usage. However most of the design pattern approaches used today are incomplete. The reason being that either they were not originally made to describe design patterns or they only specify one aspect from the behavioural or structural aspect and not both of them[1]. Patterns are discovered , not invented .It records the work done by various developers over the timespan of many years in order to solve a particular problem. Design patterns have become an important tool for the software development . “ they are an efficient tool to communicate , to capitalize on, and to understand solutions to common OO design problems ”[4].
Coming to the history of design patterns one thing that’s most extraordinary is that the idea of design patterns actually came from the field of architecture and not the software field itself. Christopher Alexender , who was an architect coined the term “ patterns ”. Then Ward Cunningham and Kent Back used some of the ideas of Alexender for User Interface design. However ,the most significant work was done by “ Erich Gamma, Richard Helm John Vlissides and Ralph Johnson ”[10] who wrote the book “Design Patterns: Elements of Reusable Object-Oriented Software popularly called Gang Of Four”. Within the software patterns community, patterns are mainly used to form solutions regarding the Designing part of software development. To be precise it means that the concrete form which recurs is that of a solution to a recurring problem. But a pattern is more than just a ready-made proven solution to a recurring problem. The problem occurs within a certain context, and in the presence of numerous scenarios and constraints. The proposed solution involves some kind of structure which fulfills these constraints in the manner most appropriate for the given context. Using the pattern form, the description of the solution tries to capture the essential insight which it embodies, so that others may learn from it, and make use of it in similar situations.
Patterns are usually concerned with some kind of architecture or organization of constituent parts to produce a greater whole. A pattern is a thing which happens in the world and at the same time the rule which tells us how to create that thing, and when we must create it. It is both a process and a thing; both a description of a thing which is alive, and a description of the process which will generate that thing. A pattern must be Useful because this would show how having the pattern in our minds may be transformed into an instance of the pattern in the real world, as something that adds value to our lives as developers and practitioners. A pattern should be Useable because this would show how a pattern described in literary form may be transformed into a pattern that we have in our minds. And a pattern must be used because this is how patterns that exist in the real world first became documented as patterns in literary form. A design pattern provides a scheme for improving the components of a software system or subsystems, or the relationships between them. It describes commonly recurring structure of communicating components that solves a general design problem within a particular context. It is a pattern whose

form is described by means of software design constructs, for example objects, classes, inheritance, aggregation and use-relationship. Design patterns are documented using a “PATTERN LANGUAGE” which is a structured method of describing good design practises within a particular domain. It is characterised by :-
( Naming the problem in a specific field
( Describing the solution which would be most effective in solving that problem in that specific problem domain.
( Giving help to the developer in moving from one problem to another.

Classification of Design Patterns
Design patterns can be classified according to two criteria * Purpose * Scope
By the term purpose we mean what a pattern does while the term Scope means where the pattern is Applied.

PURPOSE is further divided into :-
( Creational
Design Patterns that deal with Object creation mechanisms, trying to create objects in a manner suitable to the situation. The basic form of object creation could result in design problems or added complexity to the design. These patterns solve this problem by somehow controlling this object creation. These patterns allow you to add new subclasses to your system without changing your existing code. Some of the examples of these type of patterns are Singleton method and Factory method patterns.
( Structural
A design pattern which eases the design by identifying a simple way to realize relationships between the entities that are used in the software development. This design pattern is all about Class and Object composition. Structural class-creation patterns use inheritance to compose interfaces. Structural object-patterns define ways to compose objects to obtain new functionality. Some of the examples of these type of methods are Proxy , Adapter and Facade.
( Behavioural
Design patterns that identify common communication patterns between the objects and realise these patterns. These patterns increase the flexibility in carrying out this communication . These design patterns teach the developers how classes can be combined to solve algorithms and how polymorphism is used. The patterns Strategy and State serve to eliminate long conditional code (multiple if-else and switch statement) that makes code hard to maintain. Some of the examples of these kind of patterns are Strategy , State , Command and Iterator patterns.
SCOPE is also further divided into two criteria
( Class
Patterns in this category mainly deal with relationships between classes and their subclasses. These classes are static as they are established through inheritance.
( Object
Patterns in this category deal with object relationships . These relationships are dynamic in nature and hence can be changed at runtime.

Documenting the Design Patterns
Documenting the design patterns is one of the toughest task. Patterns should not just tell the facts rather they should provide complete information about the problem for which the pattern is being used , the situation or context in which it is used and the solution that it provides. A pattern should help its users to investigate the existing systems, customize systems to fit user needs and construct new systems. A commonly used format ,the one provided by Gang Of Four contains the following things :- * Pattern name and classification
Every pattern name should be unique and it should be classifies among one of the existin patterns * Intent
Tell the goal of designing and using the pattern. * Motivation
Provides the scenario in which the pattern could be used * Applicability
Represent the context part and tells where the pattern could be applied. * Structure
This is basically a graphical representation of pattern usin class diagrams and interaction diagrams * Participants
It is a listing of classes and objects that would be used in the pattern * Collaboration
Describes how the classes and objects used in the pattern interact with each other. * Implementation
It describes how the pattern could be implemented and represents the solution part of pattern. * Sample code
Tells how this pattern could be used in programming language * Known use
This is one of the most important parts . It tells us where all this pattern has been implemented * Related patterns
This section tell us if there are any other related patterns which could also be used.

The format specified above is just one format . But most of the times Authors use their own formats so that they can specify more details which they think is relevant for their pattern . Even the names of the sections could be different from the above given format.

Advantages of using pattern designs
Pattern designs are now being used very widely in software development. They have not one or two but many advantages. These are :- * They provide ready made solutions to various problems and hence speed up the development process. * The risk associated with using these problems is very less as they have been proven to work in similar areas. * Novice developers can use these patterns to develop projects without any worries of major problems creeping up during development. * They capture the knowledge and expertise of many professionals and hence increase the quality of software being made. * Since they are revised and improved over time so they are more reliable then the patterns made by a developer himself which might have some problems. * They allow for a generalized solution that does not depend on understanding a specific design problem from all its aspects, and thus ease reusing this solution.
Critique of design patterns
We know that everything has positive points as well as negative points. So do design patterns. The critique of this approach feel that the need for design patterns only arises from using computer languages and techniques with insufficient abstraction. Further it has been found that design patterns don’t always provide the relevant solutions. They sometime make things even more complex. Experiments performed on “OBSERVER, COMPOSITE, VISITOR” [6] are a few examples. So we cannot say that design patterns are always useful.

In this paper I have explained in detail about the design patterns. They provide standard ready-made and proven solutions to various problems related to Object Oriented software development. Using them provides many advantages as they can speed up the development process, help novice developers learn from the ideas of the experienced developers from the documentation provided by them for various design patterns. Their classification according to the work they perform helps us to make a decision that which design pattern would suit our need. Despite several advantages a few disadvantages make us feel that their use should be avoided if there are some simpler solutions available. But still I would recommend that unless there is a clear reason to use simpler solution it would be wise to use the flexibility provided by the Design Pattern in order to avoid more problems later on.

References :-
1. Formal specification of design patterns: a comparison
Taibi, T.; Ling, D.N.C.;
Computer Systems and Applications, 2003. Book of Abstracts. ACS/IEEE International Conference on , 14-18 July 2003 2. Evolutionary patterns of design and design patterns
Aoyama, M.;
Principles of Software Evolution, 2000. Proceedings. International Symposium on , 1-2 Nov 2000
3. Precise modeling of design patterns in UML
Mak, J.K.H.; Choy, C.S.T.; Lun, D.P.K.;
Software Engineering, 2004. ICSE 2004. Proceedings. 26th International Conference on , 23-28 May 2004
4. Instantiating and detecting design patterns: putting bits and pieces together
Albin-Amiot, H.; Cointe, P.; Gueheneuc, Y.-G.; Jussien, N.;
Automated Software Engineering, 2001. (ASE 2001). Proceedings. 16th Annual International Conference on , 26-29 Nov. 2001
5. Design patterns-essentials, experience, Java case study
Pree, W.; Sikora, H.;
Software Engineering Conference, 1997. Asia Pacific ... and International Computer Science Conference 1997. APSEC '97 and ICSC '97. Proceedings , 2-5 Dec. 1997
6. A controlled experiment in maintenance: comparing design patterns to simpler solutions
Prechelt, L.; Unger, B.; Tichy, W.F.; Brossler, P.; Votta, L.G.;
Software Engineering, IEEE Transactions on ,Volume: 27 , Issue: 12 , Dec. 2001
7. Precise specification and automatic application of design patterns
Eden, A.H.; Yehudai, A.; Gil, J.;
Automated Software Engineering, 1997. Proceedings., 12th IEEE International Conference , 1-5 Nov. 1997
8. Visualizing design patterns with a UML profile
Jing Dong; Sheng Yang;
Human Centric Computing Languages and Environments, 2003. Proceedings. 2003 IEEE Symposium on , Oct. 28-31, 2003
9. A UML-based pattern specification technique
France, R.B.; Kim, D.-K.; Sudipto Ghosh; Song, E.;
Software Engineering, IEEE Transactions on , Volume: 30 , Issue: 3 , March 2004
10. patterns in java (volume 1) 2nd addition by :- Mark Grand published by :- Wiley

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