I agree with the Qt option, as it clearly produces some nice & performant music applications. But you're still programming in C++ which is tedious because of memory management; also "hard realtime" often precludes the use of Garbage collectors. Of course Java has RT garbage collectors available: http://java.sun.com/developer/technicalArticles/Programming/rt_pt2/ although some may find "soft realtime" garbage collection as provided by the new G1 garbage collector sufficient: http://research.sun.com/jtech/pubs/04-g1-paper-ismm.pdf ... see also: http://blog.headius.com/2009/01/my-favorite-hotspot-jvm-flags.html

Lots of interesting Java-based music apps exist, with GUIs:

The good thing is you don't need to use Java anymore, but can take advantage of its portability, security, advanced garbage-collection options,  and just-in-time compiler advances that may give it a performance advantage, these days.... But you don't have to program in Java, you write in http://groovy.codehaus.org/ or http://clojure.org which means you can do really high-level things with just a few lines of groovy code ( http://lists.xwiki.org/pipermail/users/2009-August/016937.html ).

Fortunately, several million unfortunate souls have slogged through all the hard work and made a huge library of functionality for people to use, much of it open source.