Design Patterns in Scala

JavaDay 2013, http://pavelfatin.com/design-patterns-in-scala/

https://www.youtube.com/watch?v=F0AlT2kjE1k

2. Design pattern is: • Solution. • Template. • Best practice. • Obect-oriented. • Language extension. • Means of communication.
3. Creational patterns: Factory method Lazy initialization Singleton Structural patterns: Adapter Decorator Behavioral patterns: Value object Null Object Strategy Command Chain of responsibility Dependency injection
4. Factory method «Salty Oat Cookies» by Kimberly Vardeman from http://www.flickr.com/photos/kimberlykv/5061421858/, available under a Creative Commons Attribution 2.0 Generic license: http://creativecommons.org/licenses/by/2.0/
5. Factory method • Provides an interface for creating an object. • Encapsulates class instantiation in a method. • Lets subclasses decide which class to instantiate. Allows to: • extract complex object creation code, • select which class to instantiate, • cache objects.
6. Factory method
7. Factory method
8. Lazy initialization Photo by Viola Heusser from http://pixabay.com/en/hangover-cat-domestic-cat-black-173669/, available under a Creative Commons CC0 1.0 Universal license: http://creativecommons.org/publicdomain/zero/1.0/
9. Lazy initialization • A special case of lazy evaluation strategy. • Initializes a value/object on its first access. Allows to: • defer expensive computation.
10. Lazy initialization
11. Lazy initialization
12. Singleton «The Solitary Tree» by Bobbi Jones Jones from http://www.publicdomainpictures.net/view-image.php?image=22282, available under a Creative Commons CC0 1.0 Universal license: http://creativecommons.org/publicdomain/zero/1.0/
13. Singleton • Restricts the instantiation to one object. • Provides a global point of access to it. Allows to: • limit number of class objects, • implement interfaces, • use lazy initialization.
14. Singleton
15. Singleton
16. Adapter «Multiple jacks / adaptors» by Mihai Andoni from http://www.sxc.hu/photo/972429
17. Adapter • Converts an interface into expected interface. Allows to: • integrate incompatible classes, • adapt existing components.
18. Adapter
19. Adapter
20. Decorator «Colour Pencils» by George Hodan from http://www.publicdomainpictures.net/view-image.php?image=25646, available under a Creative Commons CC0 1.0 Universal license: http://creativecommons.org/publicdomain/zero/1.0/
21. Decorator • Extends functionality of a particular object. • Provides a flexible alternative to subclassing. Allows to: • decorate final classes, • arbitrarily combine extensions.
22. Decorator
23. Decorator
24. Value object «Dollars» by Виталий Смолыгин from http://publicdomainpictures.net/view-image.php?image=31593, available under a Creative Commons CC0 1.0 Universal license: http://creativecommons.org/publicdomain/zero/1.0/
25. Value object • A small immutable object. • Equality isn’t based on identity. Allows to: • represent numbers, dates, colors, etc, • implement inter-process communication, • simplify multi-threaded programing.
26. Value object
27. Value object Tuple: Tuple with a type alias: Case class:
28. Null Object «Mr. Invisible at the beach» by Carlos Koblischek from http://www.sxc.hu/photo/348295
29. Null Object • Represents the absence of an object. • Implements «do-nothing» behavior. Allows to: • avoid explicit checks of reference validity.
30. Null Object
31. Null Object
32. Strategy Photo by Nachrichten_muc from http://pixabay.com/en/chess-chess-board-black-play-white-140340/, available under Creative Commons CC0 1.0 Universal license http://creativecommons.org/publicdomain/zero/1.0/
33. Strategy • Defines a family of encapsulated algorithms. • Vary algorithms independently from clients. Allows to: • select an algorithm at runtime.
34. Strategy
35. Strategy
36. Command «Dog Stick Water» by Randen Pederson from http://www.flickr.com/photos/chefranden/2668370301, available under a Creative Commons Attribution 2.0 Generic license: http://creativecommons.org/licenses/by/2.0/
37. Command • Encapsulates information about a mehtod call. • Provides a way to call the method later. Allows to: • delay, sequence or log method calls.
38. Command
39. Command
40. Chain of responsibility «Things in motion: Colored Domino» by Sigurd Decroos from http://www.sxc.hu/photo/1077403
41. Chain of responsibility • Decouples the sender of a request from its receiver. • The request is processed by the chain until some object handles it. Allows to: • give more than one object a chance to handle the request.
42. Chain of responsibility
43. Chain of responsibility
44. Dependency injection «Tool Box & Levels» by Dylan Foley from http://www.flickr.com/photos/shoesfullofdust/3999374824/, available under a Creative Commons Attribution 2.0 Generic license: http://creativecommons.org/licenses/by/2.0/
45. Dependency injection • Removes hard-coded dependencies. • Provides a way to substitute dependencies. Allows to: • choose among multiple implementations of a particular component, • use mock implementations during unit testing.
46. Dependency injection
47. Dependency injection
48. «Cat And Dog» by Anna Langova from http://www.publicdomainpictures.net/view-image.php?image=24076, available under a Creative Commons CC0 1.0 Universal license: http://creativecommons.org/publicdomain/zero/1.0/
49. Thank you!