When comparing FLTK vs Java / Swing, the Slant community recommends Java / Swing for most people. In the question“What are the best cross-platform GUI toolkits?” Java / Swing is ranked 3rd while FLTK is ranked 7th.
Ranked in these QuestionsQuestion Ranking
Uses a limited and lightweight design and restricts itself to solely GUI functionality. Because of this restriction, the FLTK hello world example is only about 100 KiB.
It's simple design and lack of more advanced C++ features makes it easy for beginners.
Pro Several look and feels available
Pro Part of Java
Swing is part of the Java API.
Pro Very good and powerful API
E.g. separate models.
Pro Powerful controls
E.g. JTable which works fine on every platform with very large row counts, e.g. 100.000+.
Con Can be too simple for some projects
FLTK offers far fewer widgets than most other toolkits.
Con Does not look like a native application
Because it uses non-native widgets, it doesn't look like a native application on any platform.
Con No real native look
Though the importance of the native look seems to have dropped the last years by the raise of in-browser-applications.