My first introduction to a "smart" Java IDE that did source code checking on-the-fly was IBM's Visual Age for Java. That was in the mid to late '90s working Wall Street where Java development was taking over everything and access to high-end tools was the norm. IBM doesn't offer VAJ anymore (still available for SmallTalk, COBOL, and others) but pushing more and more software checking earlier and earlier into the coding activities has become the norm of modern software development.
I've been fortunate to use the big-name (and often expensive) source checking tools from Together and ParaSoft but there are plenty of modestly (and even free!) tools available as well. Since I'm a big fan of del.icio.us links, naturally I've collected some of this in a links list - see http://del.icio.us/brianlawler/Java+Testing