FileInputStream and FileOutputStream to the same file: Is a read() guaranteed to see all write()s that "happened before"?

Assuming you are using a posix file system, then yes.

FileInputStream and FileOutputStream on *nix use the read and write system calls internally. The documentation for write says that reads will see the results of past writes,

After a write() to a regular file has successfully returned:

Any successful read() from each byte position in the file that was modified by that write shall return the data specified by the write() for that position until such byte positions are again modified.

I'm pretty sure ntfs on windows will have the same read() write() guarantees.

You can't talk about "happens-before" relationship in terms of the Java memory model between your FileInputStream and FileOutputStream objects since they don't share any memory or thread. VM is free to reorder them just honoring your synchronization requirements. When you have proper synchronization between reads and writes without application level buffering, you are safe.

However FileInputStream and FileOutputStream share a file, which leaves things up to the OS which in main stream ones you can expect to read after write in order.