When comparing Codename One vs Flutter, the Slant community recommends Flutter for most people. In the question“What are the best systems to deliver native Android and iOS apps from one code?” Flutter is ranked 2nd while Codename One is ranked 7th.
Ranked in these QuestionsQuestion Ranking
Pro High performance
Pro Open source
Pro Highly customizable
By utilizing lightweight component architecture and the built in graphical designer and theming options you are able to customize a good looking app on any platform.
Pro Java 8 support
Pro Works with all IDEs
Works with NetBeans, Eclipse, IntelliJ, etc.
Pro Multi-platform development environments (via build server)
You can even use Linux desktops to build native iOS apps, no need for a Mac.
Pro Vibrant & helpful community
Pro Attractive pricing
It is possible to build production grade apps even with the free version. Pricing is decent when compared to competitors like Xamarin.
Pro Great developer tools debug/hotreload/analyser
Pro Based on Dart language
Dart is a Java like language, easy to learn and startup fast for millions of Java developers.
Pro Open source
Pro A single codebase for iOS AND Android
The developer's goal is to allow people to make apps running at 120 FPS.
Con Still in development
According to the website, Flutter is still in its early stages of development.
Con Dart is unpopular and never gained serious community traction like Kotlin or Java
Con Does not support 32-bit iOS devices
If you plan on targeting iPhone 5, 5C or earlier, you can forget about Flutter.