There is an ever-growing amount of information on the web. Even at JavaRanch there are gigabytes of content. Adding multiple topics only diversifies the content and makes it harder for people to sort through information for the answers needed. If there are existing topics (because you remembered to SearchFirst) try to see if those can answer your question, and if not, perhaps continuing that topic makes sense.
Take into account, however, that if you post a new question in someone else's topic, you are hijacking the topic and that isn't nice - especially if the question of the original poster isn't answered yet. Just start your own topic instead of hijacking someone else's.
Also, don't ask a new question in an existing topic if your new question is totally unrelated to the subject of the topic. If you do that, chances are smaller that people will see your new question, because the topic's title in the forum is about the original subject, so you're much less likely to get a good answer.