I do a bit of teaching myself, and have recently been asked to teach a group of database oriented programmers Java, and Groovy has been mentioned as well. Being more comfortable with Java myself, I focused in on that, until I was reading your previous post about how nice Groovy is for JDBC. That is not my favorite bit of Java to teach, and if Groovy simplifies that code and makes it harder to make mistakes... well now I'm reconsidering thinking Groovy might be just what they need.
What do you think? With a group that has a mix of experienced mainframe and Microsoft developers do you think it's reasonable to mix in a little bit of both? (My understanding is they work with Java developers, so just teaching Groovy is probably not an option.) Do you think your book would be good for that audience or does it really rely on a more solid understanding of Java?
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I face that problem a lot. Normally I try to teach non-OO developers Java first, partly because Groovy uses Java and partly because most OO introductions use Java. For example, the Head First Java book does a great job of teaching object oriented programming. Most Groovy books are written for Java developers, so there's a ton of assumed knowledge in them that non-Java people don't have.
After teaching the Java part, though, then it's fun to show how much simpler Groovy can make things. Your example of groovy.sql.Sql vs raw JDBC is a great illustration.
It's ironic that Groovy can actually be harder to teach to non-OO people, because it doesn't completely replace Java. Sooner or later you need that Java background. I haven't yet tried to do Groovy first, though I'm tempted to. If you try it, please let me know how it works out.
My book assumes a Java background, so I don't discuss what abstract classes are, or how interfaces work, or the static keyword, or anything like that. The _Groovy in Action_ book pretty much makes the same assumptions.
I remember seeing a proposal for a _Hello, Groovy!_ book for Manning, but unfortunately nothing ever came of it. Maybe that's your market opportunity.