How does the buffer work?

FileOutputStream fos = new FileOutputStream(fileComposite);//if I do not set the "append" to true

       //iterate over the map with filenames
       for (String fileName : folder.values()) {
           //create inputStream to read content of each file
           FileInputStream fis = new FileInputStream(fileName);//gives a nullpointer exception as the files do not exist

           //create a buffer for all bytes read available() gives number of bytes to read
           byte[] buffer = new byte[fis.available()];

           //read the bytes from a file and add it to the buffer
           while (fis.available() > 0) {
               //reads all the bytes and put them in the buffer?
               int byteRead = fis.read(buffer);
               //write the buffer to outputStream file
               fos.write(buffer, 0, byteRead);//why doesn't oit verwrite the previous part?
           }
           fis.close();
       }
       fos.close();