java - Scanning non-txt files with unspecified filename -

i want make tool 2 following things::

• open files not .txt can opened .txt , return them string. returns empty string @ moment.
• the filenames unknown, file extension @ end , the yyyymmdd number in front same, therefore i'd app scan every file in same folder (not same file twice, obviously). how can done?

that's i've got far. java code:

public string readfile(string filename) throws filenotfoundexception {     scanner scanner = null;     file file = new file(filename);     string output = "";     try{         scanner = new scanner(file);     }catch(filenotfoundexception e){         system.out.println("error: file " + filename + " not found!");     }     while (scanner.hasnextline()){         output=output+scanner.nextline();     }     return output; }