J2ObjC is an open-source command-line tool from Google that translates Java code to Objective-C for the iOS (iPhone/iPad) platform. This tool enables Java code to be part of an iOS application's build, as no editing of the generated files is necessary. The goal is to write an app's non-UI code (such as data access, or application logic) in Java, which is then shared by web apps (using GWT), Android apps, and iOS apps.
J2ObjC supports most Java language and runtime features required by client-side application developers, including exceptions, inner and anonymous classes, generic types, threads and reflection. JUnit test translation and execution is also supported.
J2ObjC is currently between alpha and beta quality. Several Google projects rely on it, but when new projects first start working with it, they usually find new bugs to be fixed. Apparently every Java developer has a slightly different way of using Java, and the tool hasn't translated all possible paths yet. It's initial version number is 0.5, which hopefully represents its release status correctly.
Project Home: http://code.google.com/p/j2objc/