I was teaching a class last week that introduced the attendees to a BPM tool. Since this particular BPM environment leverages the use of OO paradigms, basing the organization of its rules and workflow on classes, it is central to the tool’s operation to have an understanding of Object Oriented Design (OOD). When I teach this course I always spend a day covering OO terminology and concepts. This gives the class a common basis to proceed as we learn how to navigate, design, build and test within the BPM tool.
At one point during the week we were discussing an example of some code within the BPM tool that included a variable whose declaration (type) was an interface and whose definition, obviously, was a class. Although not key to the example, I pointed out the use of the interface declaration as a reminder regarding good practices, noting that developers should always “program to the interface”.
I continued on but a student’s raised hand caught my eye. The individual asked me to repeat and elaborate on what I had mentioned. Although we had spent a day discussing OOD, it occurred to me that I may not have used that expression before. Even though we had discussed interfaces, heterogeneous collections, Liskov’s Substitution Principle, and so forth, apparently I had not effectively stressed the relationship to using interfaces as data types in order to decouple our code from implementation.
As we discussed the value of using interfaces as our data types the students caught on and were able to describe the advantages of this “rule”. Certainly Spring developers are familiar with the concept since Inversion of Control (IoC) is specifically used to decouple our code from other implementations.
Developers should favor the use of interfaces for data types rather than the implementation type even if it is not clear that another implementation will ever be leveraged or created. The practice of coding to interfaces will aid in consistently decoupling modules. The approach simplifies refactoring and allows for much more flexibility in future releases.