Question

I never know how to correctly model situations when there’s this data structure that I’ll want to later save on disk.

For instance, I’m currently working on a small code-generator. The general idea is that I’ll somewhere want to store both a class’ qualified name as well as its related contents, and I’ll later want to save the .java file to disk and to compile it via javac. It’s not entirely clear that I’ll want to do both actions, that is, saving to disk as .java and compiling via javac as just one action. I guess sometimes I might want to just write the .java file to disk without compiling.

class CompilationUnit {
    String qualifiedName;
    String contents;
}

Now, my question is on how to model this. Should I have a CompilationUnit that takes as arguments both an IFileSystem and IJavaCompiler, so every time I have “in my hand” a CompilationUnit I have everything needed to do both actions, or should I try to keep that compilation logic outside of the class, being a CompilationUnit just a data object?

One one hand, there’s this OO belief one should keep both state and behaviour on the same object, which would favor keeping the qualified name and its contents together with the possibility of writing the compilation unit to disk and compile it.

Despite of that fact, I cannot but feel uncomfortable with the idea of having the CompilationUnit taking both IFileSystem and IJavaCompiler dependencies, although I it’s a bit hard to explain why it is so.

My (idealist?) intuition is that a data object should be easy to instantiate, and in that view it cannot but feel awkward to have to pass in a IFileSystem and IJavaCompiler every time I want to store a qualified name and its contents together. This means that whoever is in charge of generating this data has to also have access to both IFileSystem and IJavaCompiler, which is kind of odd.

I’m not sure I’ve actually answered my own question or not.

This system will be tested, so a correct handling of dependencies issues is paramount.

Thanks!

Answer 1

If you inject Services into Entities or Value Objects, you are most likely breaking the Single Responsibility Principle. While I don’t know the details of this domain, IFileSystem and IJavaCompiler sounds a lot like Services to me – they’re not really part of the state of the object, but Services it can use.