What is an Object? OO Is An Organizational Principle. Process. The data doesn t move. Think Different! Allen I. Holub

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1 What is an Object? Process OO Is An Organizational Principle OO Systems have the same code as procedural systems, but that code is organized very differently. Allen I. Holub Holub Think Different! The data doesn t move Procedural: Data flows through the system, and intelligent agents act on it as it goes by. The data model is central. OO: A network of cooperating agents ask each other to do work. Data is hidden and data flow is minimized. The active behavior of the system is central. Object oriented: Data contained entirely within objects, which use it to do work. main Procedural: External data flows from function to function.!3!4

2 There s no spider What is an Object? OO systems are a network of intelligent agents, communicating via messages. Objects are black boxes that do work..!!5 So, what about attributes?!6 Suppose you need to represent the data as a spreadsheet over here, and a graph over there..." Without salaries, employees wouldn t be employees, they d be volunteers. How many times have you done that? If that was you re problem, you d use Excel An attribute is not a field Salary could be float, double, String, scaled long, SQL statement used to get salary... value be inferred from a title or pay grade. Numbers mean something. There s usually only one optimal UI for making the meaning clear. If there's more than one, then any will do. The only important attributes are the methods. There's no such thing as "data" An attribute is a distinguishing characteristic of a class of objects. are not data structures do communicate by messages do have responsibilities never expose their implementation.!7!8

3 Strings Should Not Export Characters Code like this: Byte[] b = my_string.getbytes(); cannot be internationalized. Characters must be represented as bytes. Fixing this is very difficult. You must find every call and modify all the code that surrounds the call. The object that has the information should do the work! System.out.println( somestring ); new PrintWriter( new BufferedWriter( new OutputStreamWriter( System.out ))).println( somestring ); somestring.print( System.out );!9!10 Employees should not reveal their identity What does getidentity() return? A String? A Name Object? An employee ID number? A picture, thumbprint, retinal scan? You shouldn't care. Use: e.compare_id(anemployee) // compare ids e.export_id_as_xml() // returns arbitrary XML e.printname(here) // or e.identity().print(here) e.change_name() // creates a user interface e.export( builder ) // use Builder pattern etc. The "Game of Life:" Each cell has 8 neighbors. If 1 or fewer are alive, die from loneliness. If 2 exactly, unchanged. If 3 exactly, cell comes alive. If 4 or more are alive, die from overcrowding. Otherwise, the cell is healthy. Cells have behavior, but are black boxes Cellular automata Each talks to surrounding cells only Each cell can ask neighbor "are you alive?!11!12

4 SimCity and Traffic Modeling A road (can pass cars to adjacent roads) Road to road: Accept these cars Draw yourself Interface, implemented by roadway, house (driveway), factory (parking lot.) A car (knows where it s going) Road to car: you re here, which way do you want to go? A map (tells the car how to get there) Car to map: I m here and want to go there, which way should I turn? Rules for Objects Ask the object that has the information to do the work. (delegation) A program is a group of intelligent, polite, and paranoid animals talking to each other. Ease of construction, testing, and maintenance is inversely proportional to the amount of data that flows through the system. Ease of modification and debugging is inversely proportional to the number of objects you talk to and the complexity of the conversation.!13!14 The Holub Replacement Principle Can you replace the implementation throw out all the fields and replace them with completely different fields replace the implementations of all the methods with radically different implementations and not impact the your object s users? the clients don t even know a change has happened, so don t need to be modified. Get/Set functions are EVIL The coupling relationships are too strong: If you return a field, you can't change that field's type without also changing all the places where the returned values are used. If you set a value from outside, you can't change the way the value is stored without changing all the places where the value is set. Might be okay to return an object. getname() is okay when it returns a Name object that doesn't expose its implementation and the notion of a name appears in the problem statement. (It's a "key" abstraction.)!15!16

5 Conceptually, Objects Must Create Their Own User Interfaces Objects that shun get/set methods must participate actively in the UI. OO UIs are composites of smaller UIs provided by the individual objects represented on the screen. Ask: What object manages this part of the UI? VB-like tools make procedural UIs -- don t use them! Various design patterns (e.g. Builder) solve this problem elegantly.? Allen Holub allen@holub.com!17!18