Wednesday, May 22, 2013

Good software design principles (part 5)

AS you have seen the two main interfaces of the SMF are:
- IInstrumentDefinition: an Abstract Factory that defines methods to get different classes to work with analyzers.
- IInstrument: a Strategy that implements all the steps needed to acquire data from an analyzer. Concrete instances of this are returned by concrete implementation of IInstrumentDefinition dynamically loaded from a .jar file.
The only class that needs to be public (exported) in the SM(I) .jar is the implementation of the IInstrumentDefinition
Now, how do you customize the behaviour of the client application (CA) to work properly with a given SM(I) ? After all, the steps in a analysis sequence of the CA might not all be appropriate for the data returned by a given SM(I).
Would having a method that defines an analyser family be a good idea ? Something like:
Family getFamily()
Where Family would be an enum. Something like:

public enum Family

Is this a good idea ? Think about this in light of the OCP.

Monday, May 13, 2013

Good software design principle part 4

Another thing that was a big success in the SMF was the use of List and Map(s) (Dictionary in .net). If a class looks like a List, smells like a List and sounds like a list then it probably is a List. This might sound trivial but I have seen project where a class with getters and setters was defined to handle a concept that obviously was a Map. People had to constantly update the interface when new properties where added and the whole thing really was a nightmare. Using data structures such as Lists and Maps makes programming in Java feel much more like dynamic programming. So the IInstrumentDefinition includes a number of methods that actually return unmodifiable Maps. The Maps are generic Map<String, INamedParameter> where the INamedParameter interface is a little one element heterogeneous container. The INamedParameter defines the name, class and value of a parameter or variable. A Map of those is a nice little package that can easily give access to the List<String> of INamedParameter names. Once you have the name you can get the actual INamedParameter from the Map.

Saturday, May 11, 2013

Good software design principle part 3

In the previous post I gave a summary of the problem to be solved using the Successful Module. I explained that in fact the problem is solved using a Successful Module Framework (SMF) and Successful Module Instances SM(i). The SMF is a library used by the client application (CA) and the SM(i) to interact with each other without building cumbersome circular references and dependencies. In Java all modules are .jar files. At compile time both the CA and the SM(i) know the SMF but they don't know each other. Of course at runtime the CA needs to know how to load the SM(i) but that information is limited to the file location. Once it is loaded the CA interacts with the SM(i) as if it consisted only of classes (more precisely interfaces) defined in the SMF. The CA loads the SM(i) .jar and looks for a class that implements an interface called IInstrumentDefinition that marks it as the point of entry into the module. In fact the CA knows only interfaces (pure abstract classes in C++) defined in the SMF. This is one of the key to the success of the whole project. No concrete classes in the API. To understand what the IInstrumentDefinition is we need to enumerate some of the interaction between the CA and an analyzer. The CA needs to:
1- Define the value of parameters used by the analyzer (resolution, number of scan, etc...)
2- Find the list of values that a particular analyzer can supply (the data it can return, etc...)
3- Start an acquisition
4- Read the status of the analyzer
5- Read the data at the end of an acquisition cycle
So. Does the IInstrumentDefinition need to supply operations (methods) related to all these interactions ? No, not at all. In fact it turns out that many of the operations listed above are defined in a separate class called an IInstrument. Things like operation 3, 4 and 5 are defined in the IInstrument. We found that in practice it was very useful to have a specific interface - the IInstrumentDefinition - to define the functions that would provide the more "static" information about an analyzer and that it made sense that this interface be the entry point into the module. We also identified another very important function of the IInstrumentDefinition: Abstract Factory.
The instrument definition is a factory for a host of classes used when interacting with a specific analyzer. Of course the most important of the factory method is the getInstrument() method that returns a IInstrument.