u are using
stmt.executeUpdate (" INSERT INTO Accounts (firstName, lastName, mailAdress, Zipcode) VALUES (accountID_sequence.NEXTVAL,'"+firstName+"','"+lastName+"','"+address+"','"+zipCode+"')");
But, the number of columns are diferent to the number of values, I mean,
you should use :
(id, firstName, lastName, mailAdress, Zipcode)
where id is the name of the column where you want to insert the value of the sequence.
Hope this helps.