Question

I am making a program that takes input from files that I have called scripts. A script simply contains a series of information. In the future, I will need each script to contain more information, so I am associating each script with a version number.

The information in any script file can be completely contained into a particular class called EntityStatePdu (the details of which are not important for this question). So, I decided to make the following classes:

1) abstract class Script
2) class ScriptV1 extends Script
3) class ScriptV2 extends Script
4) …

I made each script file start with its version info. So, Version 1 scripts start with “scriptV1” and Version 2 scripts start with “scriptV2”. I would like to abstract the different script versions into the script class.

So, ideally, I want to have a factory method in the Script class called FromFile, such that calling Script.FromFile("scriptVersion1.txt") returns a valid ScriptV1 object and calling Script.FromFile("scriptVersion2.txt") returns a valid ScriptV1 object. I want the FromFile method to read the file, determine it’s version by looking at what the file starts with, and instantiate the appropriate Script subclass. This would let the user of the class use any script file, independent of version.

How would you go about writing this FromFile factory method? Would you suggest a different class design? Some sort of design pattern?

thanks!

Answer 1

Add a static variable to the Script class to contain a map from strings (eg. “scriptV1”) to instances of the appropriate classes. In the FromFile method, extract the string, look it up in the map, clone the instance and return it. As an initialization stage, the subclasses need to register an instance of themselves in the map under the appropriate string.