java - 如何使用泛型来处理具有公共(public)父类(super class)及其集合的不同类的对象的定义操作而不进行强制转换？

Question

让我们拥有父类(super class)ProxyHost和扩展类HttpProxyHost、HttpsProxyHosts、Socks4ProxyHost、Socks5ProxyHost

让我们对这些具体类进行一些集合:

private ArrayList<HttpsProxyHost> httpsProxies = new ArrayList<>();
private ArrayList<Socks5ProxyHost> socks5Proxies = new ArrayList<>();
private ArrayList<HttpProxyHost> httpProxies = new ArrayList<>();
private ArrayList<Socks4ProxyHost> socks4Proxies = new ArrayList<>();

现在我想做的是做类似的事情(下面有两个变体，都失败了):

    private <T extends ProxyHost> void processRow(Element row, T h, ArrayList<T> httpProxies) {
        h.setIp("someIp");
        h.setOrigin("Some origin");
        h.setPort(1234);
        httpProxies.add(h);
    }

//some other part of app:
    ArrayList<? super ProxyHost> list;
            ProxyHost proxy;
            switch (type) {
            case "HTTP":
                proxy = new HttpProxyHost();
                processRow(row,proxy,httpProxies); // argument missmatch
                break;
            case "HTTPS":
                proxy = new HttpsProxyHost();
                processRow(row,proxy,httpsProxies); //argument missmatch
                break;
            case "SOCKS4":
                list = socks4Proxies; //lists type mismatch
                proxy = new Socks4ProxyHost();
                break;
            case "SOCKS5":
                list = socks5Proxies; //lists type mismatch
                proxy = new Socks5ProxyHost();
                break;
}

所以问题是，为了在添加到具体集合之前保持该代码段干燥，我想执行一些常见操作。如何使用泛型而不使用父类(super class)型和显式转换来做到这一点？有办法吗？

每次我以为我理解了通用集合，不久之后我发现我实际上不理解;(

编辑:我想出了目前能想到的最优雅的解决方案:

private <T extends ProxyHost> void processRow(Element row, T h, List<T> list) {
    h.setIp(row.child(0).text());
    h.setPort(Integer.parseInt(row.child(1).text()));
    h.setOrigin("SomeOrigin");
    list.add(h);
}
//and calls:
processRow(row, new HttpProxyHost(), httpProxies);
processRow(row, new HttpsProxyHost(), httpsProxies);
//etc works like charm now

Answer 1

为了processRow(row,proxy,httpProxies)上类，T必须解析为 HttpProxyHost ，否则参数ArrayList<T> httpProxies将与 httpProxies 不兼容类型值ArrayList<HttpProxyHost> .

但是，如果 T是 HttpProxyHost ，然后参数T h需要 HttpProxyHost ，但该值给出为 proxy类型 ProxyHost ，即使它实际上拥有 HttpProxyHost 类型的对象.

通过分配new HttpProxyHost()至ProxyHost proxy ，您失去了调用工作所需的兼容性。

您需要:

case "HTTP": {
    HttpProxyHost httpProxy = new HttpProxyHost();
    processRow(row, httpProxy, httpProxies);
    proxy = httpProxy;
    break;
}