Really intrigued by your comment. Can you please elaborate on the part of meta programming in java. I am trying to learn it in java. What will you suggest to someone who is working in java for a while to dive deep into it?
This is mostly used by libraries and is seldom used for actual applications (I mean, annotation processors themselves, ones implemented by libs are definitely common), for example MapStruct is a really cool library that generates mapping code between two classes, one can specify which field/property maps to which and thus making a common, error-prone operation very readable and easy to maintain.
With that said I disagree with the original statement that one should absolutely know/use these tools, similarly to macros in other languages these are very advanced/last resort mechanisms that are very great in that rare chance they are needed, but overuse of them can make code very hard to understand.
Annotations (like @Entity in JPA/Hibernate) and reflection is what I mean.
Several Java libraries use annotations in a good way and also once one masters Java, they aren't too bad to make oneselves.
Reflection is about taking decisions and even changing behavior at runtime, like "iterate over all classes in this package, filter the ones with name pattern/annotation (or whatever other information is available at runtime) and use/update them.
Reflection is very powerful and IMO somewhat more tricky than annotations.