As tar archives don't have an index, commons-compress can't tell whether another file in the most recently unpacked directory will occur later (without decompressing the whole file). Thus your question is really about the behavior of the compressing program, not your decompressor.
In general, there's no restriction on the order of entries in a tar file (or even their uniqueness -- later entries may overwrite earlier ones). My command-line tar
will pack files into the archive in the order they're passed on the command line, so I can alternate like a/foo b/bar a/baz b/quux
and that's the order they're packed in. I might do this, for example, to keep similar files nearby each other in the archive, for better compression with dictionary-based (sliding window) algorithms like gzip.
You can assume all files in a directory are listed contiguously in a tar archive only if you have special knowledge of the archiver which created the files you're processing.