Getting MetaData for JDBC tables with getTables() does not work?

[Thomas Hansen]

When I access an Oracle database I can successfully select rows from a database
table. So at least one table must exist. But when I try to
access the MetaData with getTables() I get no data. Why?

I use the follwoing coding:

String dbname = "MYDB.TABLEA";
DatabaseMetaData md = cn.getMetaData();
ResultSet rsm = md.getTables(null, null, dbname, null);
if (rsm.next()) {
res = true;
System.out.println("tables found"); }
else
System.out.println("tables not found");

For what are the 4 parameters of getTables? I found some samples with
percentage signs as values. Others filled the last paramter and left the third
parameter to null. From my point of view putting null into parm 1 and 2
should return ALL tables for all users. Is this correct?
Thomas