The Ujhirdetes method h.hirfel=df.parse(sc.nextLine()); part says: Exception in thread "main" java.text.ParseException: Unparseable date: "" 
I'm might point to an empty string? 
I don't think so because the sample text looks like this: 
Bekre Pál;110;2018-10-01;42000000 
So the text always contains the date.
package vizsgamintab;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintStream;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.util.Scanner;
public class VizsgaMintaB {
static SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd");
static Scanner sc= new Scanner(System.in);
    public static void main(String[] args) throws ParseException, FileNotFoundException {
       ArrayList<Hirdetes> hirdetesek = new ArrayList<>();
       Feltolt(hirdetesek);
       Kiir(hirdetesek);
       Ujhirdetes(hirdetesek);
       Filebair(hirdetesek);
    }
    private static void Feltolt(ArrayList<Hirdetes> hirdetesek) throws ParseException {
        Hirdetes h = null;
        File f = new File ("Lakashirdetes.txt");
        try{
        Scanner scan = new Scanner(f, "iso-8859-2");
        while (scan.hasNextLine()) {
        String sor = scan.nextLine();
                String[] adatok = sor.split(";");
                if (adatok.length==3){
                h=new Hirdetes();
                h.elado= adatok[0];
                h.alapter= Integer.parseInt(adatok[1]);
                h.hirfel= df.parse(adatok[2]);}
                else if(adatok.length>3) {
                 h = new Hirdetes (adatok[0],Integer.parseInt(adatok[1]),
                         df.parse(adatok[2]),Integer.parseInt(adatok[3]));
                }
                hirdetesek.add(h);
        }}
        catch(FileNotFoundException ex){
                System.out.println("Nincs ilyen file");
                }}
        public static void Kiir(ArrayList<Hirdetes> hirdetesek){
        for ( Hirdetes h: hirdetesek){
            System.out.println(h);
        }
        }
        private static void Ujhirdetes(ArrayList<Hirdetes> hirdetesek) throws ParseException{
        Hirdetes h = new Hirdetes();
            System.out.println("Adjon meg egy új eladót: ");
            h.elado=sc.nextLine();
            System.out.println(" Adja meg a lakás alapterületét:");
            h.alapter=sc.nextInt();
            System.out.println(" Adja meg a hirdetés feltöltésének idejét:");
            h.hirfel=df.parse(sc.nextLine());
            System.out.println(" Adjon meg egy eladási árat:");
            h.ar=sc.nextInt();
            hirdetesek.add(h);
        }
        public static void Filebair(ArrayList<Hirdetes> hirdetesek) throws FileNotFoundException{
        PrintStream f2 = new PrintStream(new File ("Lakashirdetes2.txt"));
        for (Hirdetes h : hirdetesek){
        f2.println(h.toString());
        }
        }
}
    class Hirdetes {
            String elado;
            int alapter, ar ;
            Date hirfel;
            SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd");
        @Override
        public String toString(){
        return "Elado neve:" +elado + " Lakás alapterülete:" + alapter+" Hirdetésfeladás:"+df.format(hirfel)  + " Ár:" + ar; 
        }
        public Hirdetes(String elado, int alapter,Date hirfel, int ar){
            this.elado = elado;
            this.alapter = alapter;
            this.hirfel = hirfel;
            this.ar=ar;
        }
    public Hirdetes(){}
    }
 
    