You are not the first person to make this request, and I suspect you won't be the last before we have a tag-targeted offering available.
Currently, we do not have plans to offer tag-specific targeting. There are technical limitations of our self-serve partner (BuySellAds) that make the hurdles a bit high right now. They've been wonderful in implementing geo-targeting, but as we have a staggering number of tags, it won't be easy.
Ultimately, I see something akin to targeting types of developers (Front End or Mobile developers, for instance), rather than individual tags, as our next step in self-serve. This is, however, just my opinion. I've been supporting our engagement team for more than five years and seen a lot of changes, but expanding our self-serve options has always been a cause close to my heart. I know there are many people out there with smaller budgets that can't hit our minimums but still want to engage with our users.
In short: no, we don't currently offer tag-targeting on self-serve. But we hope to offer something to better reach more specific audiences in the future.
As an aside: java is one of the largest on Stack Overflow. General run-of-site targeting will likely deliver a large portion of your impressions to that tag. Of course, it's not perfect, but it is something. Email me at steve@ or submit an inquiry to adops@ and we can possibly provide more context. However, if the above is clear enough to have you pass for now, I'll be sure to update this answer when we can offer more robust targeting that may help you accomplish what you're trying to accomplish.