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    <p>Well the first thing you have to realize is the OOAD ("Object-oriented analysis and design") is a tool and not a means to an end. What you get out of that process is a model, not a true representation of what you're modelling. That model makes certain assumptions. The purpose of that model is to solve a problem you have.</p> <p>So how do you know how to design objects? How do you know if you've done it right? By the end result: has it solved your problem?</p> <p>So, for the Car example, in some models a car count could simply be an integer count, for example the car traffic through an intersection in a traffic model. In such a model rarely do you care about the make, model or construction of cars, just the number. You might care about the type of vehicle to the point of is it a truck or car (for example). Do you model that as a Vehicle object with a type of Car or Truck? Or just separate carCount and truckCount tallies?</p> <p>The short answer is: whichever works best.</p> <p>The normal test for something being an object or not is does it have behaviour? Remember that ultimately objects = data + behaviour.</p> <p>So you might say that cars have the following state:</p> <ul> <li><h1>of wheels;</h1></li> <li>Height of suspension;</li> <li>Left or right drive;</li> <li>Colour;</li> <li>Width;</li> <li>Weight;</li> <li>Length;</li> <li>Height;</li> <li><h1>of doors;</h1></li> <li>Whether it has a sunroof;</li> <li>Whether it has a stereo, CD player, MP3 player and/or satnav;</li> <li>Size of the petrol tank;</li> <li>Number of cylinders;</li> <li><h1>of turbo charges and/or fuel injection;</h1></li> <li>Maximum torque;</li> <li>Maximum brake-horsepower;</li> <li>and so on.</li> </ul> <p>Chances are you'll only care about a small subset of that: pick whatever is relevant. A racing game might go into more detail about the wheels, such as how hot they are, how worn, the width and tread type and so on. In such a case, a Wheel object could be said to be a collection of all that state (but little behaviour) because a Car has a number of Wheels and the Wheels are interchangeable.</p> <p>So that brings up the second point about objects: an object can exist because of a relationship such that the object represents a complete set of data. So a Wheel could have tread, width, temperature and so on. You can't divide that up and say a Car has tread but no wheel width so it makes sense for Wheel to be an object since a Wheel in it's entirety is interchangeable.</p> <p>But again, does that make sense for what're doing? That's the key question.</p>
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    1. CO+1 Very nice answer :)
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      1. USAndrew Hare
 

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