JDBC-ResultSet is closed in while-Loop

The problem is that the Statement can only maintain a single group of ResultSets per executed statement. Since you share the same Statement stmt for your two methods, in alleAbrufen the Statement executes another statement, which will break the reference to the prior ResultSet.

The best solution for this case is to create a Statement per statement execution. This is, every method should contain its unique Statement and related ResultSets.

public LinkedList<Athlet> alleAbrufen () throws SQLException { LinkedList<Athlet> alleAthleten = new LinkedList<Athlet>(); String abrufenAthleten = "SELECT * FROM Athlet ORDER BY athlet_id"; //here Statement stmtAlleAbrufen = con.createStatement(); ResultSet athleten_rs = stmtAlleAbrufen.executeQuery(abrufenAthleten); while (athleten_rs.next()) { long id = athleten_rs.getInt(1); String name = athleten_rs.getString(2); LinkedList<Leistung> alleLeistungen = alleAbrufen((int) (id)); //after this line the ResultSet gets closed alleAthleten.add(new Athlet(id, name, alleLeistungen)); } return alleAthleten; } public LinkedList<Leistung> alleAbrufen(int athlet_id) throws SQLException { LinkedList<Leistung> alleLeistungen = new LinkedList<Leistung>(); //here again, but since you need to use parameters in your query //use PreparedStatement instead //note that I commented the current query //String selectLeistungen = "SELECT * FROM Leistung WHERE athlet_id="+athlet_id; //this is how a query with parameters look like String selectLeistungen = "SELECT * FROM Leistung WHERE athlet_id=?"; //the connection prepares the statement PreparedStatement pstmt = con.prepareStatement(selectLeistungen); //then we pass the parameters pstmt.setInt(1, athlet_id); ResultSet rs = pstmt.executeQuery(); while (rs.next()) { long id = rs.getInt(1); String bezeichnung = rs.getString(2); String datum = rs.getString(3); double geschwindigkeit = rs.getDouble(4); boolean selectedForSlopeFaktor = rs.getBoolean(5); int strecke_id = rs.getInt(7); long longAthlet_id = (long) athlet_id; Leistung leistung = new Leistung(strecke_id, longAthlet_id, bezeichnung, datum, geschwindigkeit); leistung.setLeistungID(id); leistung.setIsUsedForSlopeFaktor(selectedForSlopeFaktor); alleLeistungen.add(leistung); } return alleLeistungen; } 

Don't forget to close the resources, Statement and ResultSet, after using them.