java为什么我的ArrayList不会出现在Servlet上。有什么不对劲或者我错过了什么？

Question

我在ArrayList上做了反复试验。试着去理解出了什么问题，但我一点运气都没有。我不知道我错过了什么。当您单击HTML页面上的按钮时，将显示arraylist。它假设显示3组数据

增强网络。爪哇

public static ArrayList<EnhancedNetwork> buildAllNetworks() throws SQLException, ClassNotFoundException{
    String query = "SELECT * FROM networkvariant WHERE N_COUNT = " + "'" +  "'";         
    ResultSet R = DBConnect.executeQuery("imperial_defense", query);
    
    
     ArrayList<EnhancedNetwork> nets = new ArrayList<>();
        EnhancedNetwork net= null;
        
        int counts;
        String nameid;
        int band;
        String stats;
        
        
        while(R.next()){
            counts = R.getInt("N_COUNT");
            nameid = R.getString("N_NAMETYPE");
            band = R.getInt("N_BANDWIDTH");
            stats = R.getString("N_STATUS");
            
            //Constructor Parameters
            net = new EnhancedNetwork(counts, nameid, band, stats);
           nets.add(net);
        }
    
    return nets;

}
public static String displayThreeNetworks() throws SQLException, ClassNotFoundException{
    
    ArrayList<EnhancedNetwork> nets = EnhancedNetwork.buildAllNetworks();
        
        for(EnhancedNetwork s : nets){
            System.out.println(s.toString());
        }
    
    
    return nets.toString();
    
}

@Override
public String toString() {
    return "netCount=" + netCount 
            + ", netNameID=" + netNameID 
            + ", netBandwidth=" + netBandwidth 
            + ", netStatus=" + netStatus;
}

索引。HTML

<div>
        <br>
    <form action="EnhancedNetworkBuild" method="get">
        <label for="N_COUNT">Network Enhance ID Number</label><br><br>
        <input type="submit" id="N_COUNT" name="N_COUNT" value="Display Network Build"> 
    </form>
    </div>

增强网络建设。servlet

@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
     response.setContentType("text/html;charset=UTF-8");
    PrintWriter out = response.getWriter();
    String net = getInitParameter("N_COUNT");
    try {
         String nets = EnhancedNetwork.displayThreeNetworks();
        out.println(nets);
    } catch (SQLException ex) {
        Logger.getLogger(EnhancedNetworkBuild.class.getName()).log(Level.SEVERE, null, ex);
        out.println("The database has failed");
    } catch (ClassNotFoundException ex) {
        Logger.getLogger(EnhancedNetworkBuild.class.getName()).log(Level.SEVERE, null, ex);
    }
}

非常感谢您的帮助