/**
     * Compone una chiave composta da dim caratteri di cui alfa sono caratteri dell'alfabeto italiano
     * @param dim lunghezza della chiave
     * @param alfa numero di caratteri della chiave
     * @return chiave
     */
    private String getRandomKey(int dim,int alfa) {

        if (dim<0) return null;

        if (dim<alfa) dim = alfa;


        Random rnd = new Random();
        int num = dim-alfa;
        int appo=0;
        String pass="";
        int alterna=0;
        int contnum=0;
        int contalfa=0;
        for (int i=1;i<=dim;i++) {
            alterna = rnd.nextInt(2);
            if (alterna==1 && contalfa>=alfa){
                alterna=0;
            }else{
                if (contnum>=num && alterna==0) {
                    alterna=1;
                }
            }

            if (alterna==1)
            {
                contalfa++;
                appo = rnd.nextInt(25);
                switch (appo)
                {
                    case 0 : pass = pass+"A";break;
                    case 1 : pass = pass+"B";break;
                    case 2 : pass = pass+"C";break;
                    case 3 : pass = pass+"D";break;
                    case 4 : pass = pass+"E";break;
                    case 5 : pass = pass+"F";break;
                    case 6 : pass = pass+"G";break;
                    case 7 : pass = pass+"H";break;
                    case 8 : pass = pass+"I";break;
                    case 9 : pass = pass+"J";break;
                    case 10 : pass = pass+"K";break;
                    case 11 : pass = pass+"L";break;
                    case 12 : pass = pass+"M";break;
                    case 13 : pass = pass+"N";break;
                    case 14 : pass = pass+"O";break;
                    case 15 : pass = pass+"P";break;
                    case 16 : pass = pass+"Q";break;
                    case 17 : pass = pass+"R";break;
                    case 18 : pass = pass+"S";break;
                    case 19 : pass = pass+"T";break;
                    case 20 : pass = pass+"U";break;
                    case 21 : pass = pass+"W";break;
                    case 22 : pass = pass+"X";break;
                    case 23 : pass = pass+"Y";break;
                    case 24 : pass = pass+"Z";break;
                }
            }
            if (alterna==0)
            {
                contnum++;
                appo = rnd.nextInt(10);
                switch (appo)
                {
                    case 0 : pass = pass+"0";break;
                    case 1 : pass = pass+"1";break;
                    case 2 : pass = pass+"2";break;
                    case 3 : pass = pass+"3";break;
                    case 4 : pass = pass+"4";break;
                    case 5 : pass = pass+"5";break;
                    case 6 : pass = pass+"6";break;
                    case 7 : pass = pass+"7";break;
                    case 8 : pass = pass+"8";break;
                    case 9 : pass = pass+"9";break;
                }
            }
        }

        return pass;
    }