So let's say you have coded your plugin and you want to give it to a colleague to use. You can give your .java files for all your classes, if you have more than one, but the final user will have to compile them. You can also send the .class files that will be placed in your plugins directory. One of the great things of Java is that a program compiled on a computer works on other systems as well. Just tell your colleague to copy the .class files to the plugins folder of his or her ImageJ installation.

Better yet: offer your plugin code through a popular code-sharing site, for example, https://github.com/. You will be helping the community and doing yourself a favor: someone might spot a bug in your code and report it, and in that case you can submit a corrected version.