boolean flag=false;

        do{
            System.out.println("Introduza o Username:");
            String user=Ler.umaString();
            System.out.println("Introduza a Password");
            String passwd = Ler.umaString();

            stmt = con.createStatement();
            rs = stmt.executeQuery("SELECT * FROM Colaboradores WHERE Username='" +user+ "' AND Password="+passwd);
            colaboradorID=rs.getInt("Id");

            if(!rs.next())
            {
                System.out.println("Username e/ou password incorretos, tente novamente!\n");
            }
            else
            {
                flag=true;
            }
        }while (flag==false);