public SparKonto erzeuge_spar_konto(String accountNumber, float zins_satz) throws BankException, RemoteException {
Konto konto = finde(accountNumber);
if(konto != null)
throw new BankException("account already exists!");
SparKonto sparKonto = new SparKontoImpl(accountNumber, zins_satz);
kontos.put(accountNumber, sparKonto);
return sparKonto;
}