Yes, but it's probably easier with the text file.

   Scan through the file and compare every date listed with the current
date. When they match, display our pop up window with a label initialized
with the appropriate message.

   - Oliver

There are many ways to skin this cat.  If the file is huge, I would do
it with a file of strings written with DataOutputStream.writeUTF, one
after the other.

You create an in RAM lookup table that indexes by message number to
get the offset in the RandomAccessFile, then do a
DataInputStream.readUTF to get your string back.

Alternatively use a HashMap to index by message code string to get the
offset.

If the file were smaller, I would use a ResourceBundle.  See
http://mindprod.com/jgloss/resourcebundle.html

If it were no internationalisation concerns, I might just use a
Properties file which turns into a RAM-based Hashtable.
see http://mindprod.com/jgloss/properties.html