Suppose I want to create a blog focusing on a subject(say, Java), how to find out popular search keywords related to this subject?
Try using https://adwords.google.com/select/KeywordToolExternalSimilar this will give you great advice on number of searches and geographic specificity.
Yes you can use google adwords keyword toolbar for it, you can also use seomoz keyword toolbar for it, its really very good for searching good keyword.