Java Database Connectivity (JDBC)

Hi there, i am trying the following query to extract a specific date and idno from a database. I keep getting the error message: "data type mismatch in criteria expression". I'm not sure why i am getting this error. Please help. Thanks a lot

String a = victSnameTxt.getText().trim();
String b = victFnameTxt.getText().trim();

String query = "SELECT VSurname, VFirstName, VDateOfBirth, OSurname, OFirstName, ODateOfBirth, IdNo, IDate, CollarNo, OffName FROM CompleteIncidents WHERE IdNo = '" + a + "' AND IDate = '" + b + "' ";

ResultSet rs = stmt.executeQuery(query);