sqlite - java database interface transaction layer -

so have write application (databaselayer, transactionlayer, not applicationlayer) , part of code given me (and i'm supposed keep likt that). however, there interfaces used

packages:

• databag
• database
• exception
• generators
• transactie (transaction)

code i've written myself right now:

  package database, class klantdb:     import exception.dbexception;   import java.util.*;   import databag.klant;   import java.sql.connection;   import java.sql.preparedstatement;   import java.sql.resultset;   import java.sql.sqlexception;       public class klantdb {     public integer toevoegenklant(klant nieuweklant) throws dbexception {      try (connection conn = connectionmanager.getconnection();) {        try (preparedstatement stmt = conn.preparestatement(         "insert klant(naam, voornaam, geboortedatum, opmerking,                    debetstand_limiet, actief)             values(?,?,?,?,?,?)",preparedstatement.return_generated_keys);) {         stmt.setstring(1, nieuweklant.getnaam());         stmt.setstring(2, nieuweklant.getvoornaam());         stmt.setstring(3, nieuweklant.getgeboortedatum().tostring());         stmt.setstring(4, nieuweklant.getopmerking());         stmt.setdouble(5, nieuweklant.getdebetstandlimiet().doublevalue());         byte b;         if (nieuweklant.isactief() == true){             b = 1;          }         else{             b = 0;         }         stmt.setbyte(6, b);         stmt.execute();         resultset rs = stmt.getgeneratedkeys();         return rs.getint(1);        } catch (sqlexception sqlex) {         throw new dbexception("sql-exception in toevoegenklant - statement"+           sqlex);      }      } catch (sqlexception sqlex) {       throw new dbexception(         "sql-exception in toevoegenklant - connection"+ sqlex);     }         }} 

then noticed:

package transactie class klanttrans

package transactie;  import databag.klant; import exception.applicationexception; import exception.dbexception;   public interface klanttrans {  /**  * klant toevoegen.  *  * @param k klant info has added  * @return de klant made (with filled in id)  * @throws applicationexception  * @throws dbexception  */ public klant klantaanmaken(klant k) throws applicationexception, dbexception; 

the goal write simple bank application using database, whilst using data layer , transaction layer. don't have interface, or why interface in first place. how implement interface? supposed use in klantdb class? , if so, why? it's database layer, not transaction layer don't quite understand. sorry if confusing...