Running:
equery depends jdk
Returns:
dev-lang/swig-1.3.31 (java? virtual/jdk)
media-libs/pdflib-7.0.2_p8 (java? >=virtual/jdk-1.4)
However, "-java" is set for pdflib:
emerge -pv pdflib
These are the packages that would be merged, in order:
Calculating dependencies... done!
[ebuild R ] media-libs/pdflib-7.0.2_p8 USE="perl python -cxx -doc -java -tcl" 0 kB
"java?" appears to mean that it depends on it IF the java USE flag is set. I couldn't find this documented anywhere, so I'm documenting it here.