When presence status changes for a user in the current users buddy list, an event number 307 (IMPS_IM_PRESENCE) will be passed to processEvents method on SnapEventListener implementing object.
If the users was logged in without setting a presence (parameter presenceAttrs in call to ServerComm.extendedLogin was "" or null), presence field in the event will NOT be present.
That means that you risk having NullPointerException in the event of calling something like:

ItemList list;
...
If(list.getItem("presence").equals("my presence data")) {

}

Instead always check whether the item is there like this:

ItemList list;

String presence = list.getItem("presence");
If(presence!=null&& presence.equals("my presence data")) {

}

Regards,
Ryan