resultSet not open, works in one case another case not

Why if i query the database from the db class using queryFromDb() i get "ResultSet not open" when it try to do rs.next() for the second time and instead if i try to use queryFromMain everything works?

public class Db {

private String protocol = "jdbc:derby:";
private ResultSet resultSet = null;
private Connection connection = null;
private Statement statement;

public Db(){

    try{
        Properties props = new Properties(); 
        props.put("user", "user");
        props.put("password", "password");

        String dbName = "database";
        connection = DriverManager.getConnection(protocol + dbName  , props);

    }
    catch(SQLException sqle){
        printSQLException(sqle);
    }
}

public ResultSet returnValue(String query){

    try{
        statement = connection.createStatement();

        resultSet = statement.executeQuery(query);  
    }
    catch(SQLException sqle){
        printSQLException(sqle);
    }

    return resultSet;   
}


public void queryFromDb(){
         try {
            statement = connection.createStatement();
            ResultSet rs = statement.executeQuery("SELECT * FROM clsbck ORDER BY id");  
            while(rs.next()){
                System.out.println(rs.getString(2));
                String str = "INSERT INTO cls rck VALUES 2";
                [...]           
                statement.execute(str); 
            }
         } catch (SQLException e) {
            printSQLException(e);
         }  
    }

   }
}

public class Main {
    private static Db db;

    public static void main(String[] args){
        db = new Db();
    }

    public static void queryFromMain(){
        ResultSet rs = db.returnValue("SELECT * FROM clsbck ORDER BY id");
        try {   
           while(rs.next()){
               String str = "INSERT INTO cls rck VALUES 2";     
               [...]
               db.addValue(str);
           }
        } catch (SQLException e) {
            printSQLException(e);
        }   
    }
}