If you want to learn how to use ImageJ and/or ImageJ2 (as opposed to write programs with ImageJ2), then visit ImageJ Tutorials.
The purpose of the notebooks is to give a "guided tour" for teaching ImageJ2.
The purpose of the java section of this repository is to give a collection of examples and howtos for different use cases.
To the extent possible under law, the ImageJ developers have waived all copyright and related or neighboring rights to this tutorial code.
See unlicense.org for details.