okay so when I "see" library classes in my code, its IDE illusion?

Why?

When you add libraries to the compile-time classpath you're basically promising that binary-compatible versions of those libraries will be present at runtime as well.

Sure

oh i see, its slim / fat jar thing right

It’s actually apk + dex thing, but somehow similar to fat jar. Biggest difference there is no class files anymore, only one or multiple dex files

yea I know how it works on android, I sort of forgot that fat jar issue, since apks contain everything physically

exactly, and it’s responsibility of user of your library and user’s build system to provide compatible version, but not necessary exactly same version of your library dependencies

yes, thats why facebook dependencies burn me everytime I update appcompat 😄