I often use C-x h TAB and M-x untabify to format C, C++, and Java code.

If a document has an errant UTF-8 byte order mark (a UTF-8 BOM is EF
BB BF), Emacs cannot always format the source file.

For example, the attached Java file (JavaEncryptor.java-backup) has
1845 BOMs sprinkled throughout. I'm not sure what editor put them in,
but Emacs does not properly handle some operations with them present.
If I strip the errant BOMs with the attached program
(efbbbf-strip.cpp), Emacs will properly format the file.