“Object-orientation” – what is it all about?

1. “Object-orientation” – what is it all about? Gill Harrison, Innovation North

2. Aims and Objectives this week To appreciate • what is meant by “object-orientation’ • how it arose • why it is a useful approach

3. What is Object-Orientation? • A method of writing programs and developing software that is “oriented” (or orientated or directed) towards the use of objects

4. What are objects? A software object is

5. What are objects? A software object issomething which holds both

6. What are objects? A software object is something which holds bothdata

7. What are objects? A software object is something which holds bothdata

8. What are objects? Stock object A software object is something which holds bothdata Stock CodeDescriptionNumber in StockUnit cost

9. What are objects? Stock object A software object is something which holds bothdataand the program instructions (organised into “methods”) for processing that data Stock CodeDescriptionNumber in StockUnit cost

10. What are objects? Stock object A software object is something which holds bothdataand the program instructions (organised into“methods”)for processing that data Stock CodeDescriptionNumber in StockUnit cost

11. What are objects? Stock object A software object is something which holds bothdataand the program instructions (organised into“methods”)for processing that data Stock CodeDescriptionNumber in StockUnit costMethod to change the Number in StockMethod to calculate value

12. How is a system built? One object can send a “message” to another asking it to perform one of its methods on its data

13. How is a system built? One object can send a “message” to another asking it to perform one of its methods on its data Stock object Stock CodeDescriptionNumber in StockUnit costMethod to change the Number in StockMethod to calculate value

14. How is a system built? One object can send a “message” to another asking it to perform one of its methods on its data Stock object Stock CodeDescriptionNumber in StockUnit costMethod to change the Number in StockMethod to calculate value an object dealing with goods received

15. How is a system built? One object can send a “message” to another asking it to perform one of its methods on its data Stock object Stock CodeDescriptionNumber in StockUnit costMethod to change the Number in StockMethod to calculate value increase your number in stock by 1000 an object dealing with goods received

16. If you’re interested (this is Java)... public class Stock { private String stockCode, description; private int numberInStock, unitCost; public void changeNumberInStock(.......) { some lines of program code here } public float calculateValue() { lines of code } } data methods

17. If you have met programs before.. You will realise that programs generally have • variables to hold data • procedures and functions, which seem to be like methods • calls to these procedures and functions, which seem like messages • So what’s new about objects?

18. Encapsulation The details of data and methods are shielded from outside view or interference. This is called “encapsulation”. Stock CodeDescriptionNumber in StockUnit costMethod to change the Number in StockMethod to calculate value

19. Encapsulation The details of data and methods are shielded from outside view or interference. This is called“encapsulation”. Stock CodeDescriptionNumber in StockUnit costMethod to change the Number in StockMethod to calculate value

20. Encapsulation The details of data and methods are shielded from outside view or interference. This is called“encapsulation”. Just send me messages and I’ll send replies

21. A bit of history • Objects arose via programming languages rather than via Analysis and Design • In the 1960s, the first language to use objects was Simula, a simulation language. A vehicle or a lift could be modelled more easily this way

22. A bit of history • in the 1970s, an important research team at Xerox Palo Alto Research Centre (Xerox PARC) produced • ideas about Graphical User Interfaces (GUIs) and • Smalltalk - the first pure O-O language • Programs with GUIs are “event-driven” - these are much easier to write if you use software objects

23. Object-oriented Programming languages • Simula - a simulation language • Smalltalk - the first pure O-O language • Eiffel • C++ (C with objects) • Java

24. Features of object-orientation • We define a general class of objects we are interested in, e.g. students, customers, orders • A specific individual of the class is called an object, or an instance • We define attributes of the class, which will have particular values for each instance • We define methods for the class, which any instance will be able to carry out in response to a message

25. Objects and Entities OBJECTS ENTITIES class is like entity type instance is like occurrence attribute is like attribute? method has no equivalent -

26. Objects and Entities Objects seem like entities, with processing added in. But ERDs show entities and DFDs show processing.

27. Looking at DFDs and ERDs

28. It is almost as though an entity...

29. absorbs bits of processing

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44. absorbs bits of processing Hey, I’m an object!

45. Evolution of Systems Analysis and Design methods • Methods using Structure Charts and Data Flow Diagrams • ..... plus Entity Relationship Diagrams • Object-Oriented Analysis and Design Methods

46. Relative maturity Mature Immature O-O Programming Languages O-O Databases O-O Analysis and Design

47. Object-Oriented Analysis and Design Methods Lots, often associated with the name of a person: • Rumbaugh - Object Modeling Technique (OMT) • Jacobsen - Object Oriented Software Engineering (OOSE) • Booch • Coad and Yourdon • Wirfs-Brock • Schlaer and Mellor

48. Object-Oriented Analysis and Design Methods Lots, often associated with the name of a person: • Rumbaugh - Object Modeling Technique (OMT) • Jacobsen- Object Oriented Software Engineering(OOSE) • Booch • Coad and Yourdon • Wirfs-Brock • Schlaer and Mellor have combined to create Unified Modeling Language (UML)

49. What use is it? • Stock balance now held as • balance at stock-taking • total number received since then • total number issued since then • Without objects - detailed changes to every program that deals with stock • With objects - just change stock methods: other objects send the same messages as before

50. What use is it? • Resilience to change - data is stable, processing is not • Small changes are localised • Far better chance of reusing objects than of reusing separate data and processes • Standard classes (e.g. for GUI features like buttons, list boxes) make software development faster and easier