http://java.sun.com/docs/books/vmspec/2nd-edition/ClassFileFormat-Java5.pdf

"Class and interface names that appear in class file structures are always represented
in a fully qualified form (§2.7.5)."

However, a little later this is modified:

"In this internal form, the ASCII periods ('.') that
normally separate the identifiers that make up the fully qualified name are
replaced by ASCII forward slashes ('/'). The identifiers themselves must be
unqualified names as discussed in section (§4.3.2) below."

Also, it seems that this fails to include classes which don't have
a fully qualified name, such as anonymous classes. However, appearances
are deceiving and it would probably be better to avoid terminology from
the JLS such as fully qualified name and binary name.