Design Principles to design Patterns

Design Principles to design Patterns

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  P R ES E N T E D B Y F A I Z A N H A I D E R Design Principles to Design Patterns
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  What are DesignPrinciples  Design principles are the desirable goals that one aims to achieve. Design patterns are tools one can use to realize those goals.  It's therefore better to learn design principles first because then you can easily understand what (and why) a pattern is trying to achieve.  Design principles are guidelines to be followed throughout the software development process. Design patterns are well accepted solutions to recurring design problems.  In other words: design patterns employ design principles.
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  Why Use DesignPrinciples?  Avoid Bad Code like  Rigidity - It is hard to change because every change affects too many other parts of the system.  Fragility - When you make a change, unexpected parts of the system break.  Immobility - It is hard to reuse in another application because it cannot be disentangled from the current application.
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  Some Desing Principles Open close Principle  Dependency Inversion Principle  Interface Segregation Principle  Single Responsibility Principle  Liskov's Substitution Principle
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  1. Open closePrinciple  Software entities like classes, modules and functions should be open for extension but closed for modifications.
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  2. Dependency InversionPrinciple  High-level modules should not depend on low-level modules. Both should depend on abstractions.  Abstractions should not depend on details. Details should depend on abstractions.
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  3. Interface SegregationPrinciple  Clients should not be forced to depend upon interfaces that they don't use.  Avoid fat interfaces like if we code interface worker then should keep in mind that all the workers like worker, manager, director and robots would implement that interface.  Fat interfaces can be segregated using the Adapter pattern.  Instead of one fat interface use small ones with sub tasks
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  4. Single ResponsibilityPrinciple  A class should have only one reason to change.
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  5. Liskov's SubstitutionPrinciple  Derived types must be completely substitutable for their base types.  Extension of the Open Close Principle in terms of behavior meaning that we must make sure that new derived classes are extending the base classes without changing their behavior.
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  What are DesignPatterns?  Patterns focus on different types of problems. Related patterns are grouped together and assigned a type  1. Creational Patterns 2. Structural Patterns 3. Behavioral Patterns
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  1. Creational Patterns All about the class instantiation  Uses class-creation patterns and object-creational patterns.  Class-creation pattern uses inheritance effectively in the instantiation process while object-creation pattern uses delegation to get the job done  1. Factory Pattern  2. Abstract Factory Pattern  3. Builder Pattern  4. Prototype Pattern  5. Singleton Pattern
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  2. Structural Patterns How objects and classes can be combined to form larger structures  Class patterns describe abstraction using inheritance and describe how it can be used to provide more useful program interface.  Object patterns describe how objects can be associated and composed to form larger, more complex structures.  1. Adapter Pattern  2. Bridge Pattern  3. Composite Pattern  4. Decorator Pattern  5. Facade Pattern  6. Flyweight Pattern  7. Proxy Pattern
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  3. Behavioral Patterns specifically concerned with communication (interaction) between the objects.  Object communicating with each other and loosely coupled. The loose coupling is the key to n-tier architectures  The implementations and the client should be loosely coupled in order to avoid hard-coding and dependencies.  1. Chain of Resposibility Pattern  2. Command Pattern  3. Interpreter Pattern  4. Iterator Pattern  5. Mediator Pattern  6. Momento Pattern  7. Observer Pattern  8. State Pattern  9. Strategy Pattern  10. Template Pattern  11. Visitor Pattern
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  Conclusion  Design principlesrepresent a set of guidelines that helps us to avoid having a bad design.  While Design patterns are solution to specific problems.  We use Design patterns to implement a design principle.  If Design principles are End Game then Design Patterns are the way to get There.