private String[] radky = new String[9]; // deklarace a inicializace pole s kurzy měn

public String ZmenKurzNakup(double castka, int mena) throws IOException
    {
            FileInputStream fstream = new FileInputStream("kurzy.txt");
            DataInputStream vstup = new DataInputStream(fstream);
            BufferedReader br = new BufferedReader(new InputStreamReader(vstup));
            BufferedWriter bw = new BufferedWriter(new FileWriter ("kurzy.txt"));
            String castkas = String.valueOf(castka);
            String radek;
            String strLine = null;
            int x = 0;
            int i = 0; 
            int a = 0;
            
            while ((radek = br.readLine()) != null)   {
            radky[x]=radek;
            x++;
            
            }
            while(i != mena+1)
            {
                strLine = br.readLine();
                i++;
                radky[i]=strLine;
            }

            while(a < 10)
            {
                bw.write(radky[i]);
                a++;
                
            }
            
            
            bw.close();
            vstup.close();
}