There actually is a tool that does exactly what you're asking called O2J. You can leverage your existing Objective-C codebase and convert it to Java with the O2J conversion tool.
It's a paid app available on the Mac App Store... O2J Objective-C to Java Converter
Most of the time it's preferable to have a native UI and native code for performance, working with the platform UI paradigms and to take advantage of platform specific APIs/services.
