首页 \ 问答 \ 为什么Java方法参数可以显式协变？(Why can Java method parameters be explicitly covariant?)

为什么Java方法参数可以显式协变？(Why can Java method parameters be explicitly covariant?)

 在Java中，您可以使用extends关键字声明给定的类型参数是协变的。 协方差和逆变确实让我感到困惑，但我想我已经对它们有了一般的认识。 但是，在我的测试中，似乎Java类型参数本质上是协变的。 那么我们为什么要明确说明呢？  
 例如，我构建了一个如下所示的快速示例：  
public class Main<E> {

    protected E value;

    public Main(E value) {
        this.value = value;
    }

    public <T extends E> void setValue(T value) {
        this.value = value;
    }

    public E getValue() {
        return value;
    }

    public static void main(String[] args) {
        Main<Number> example = new Main<Number>(0L);
        System.out.println(example.getValue().getClass().getCanonicalName());
        example.setValue(32.0);
        System.out.println(example.getValue().getClass().getCanonicalName());
    }
}
 
 使用的类型是Number，所有盒装类型的超类。 在构造函数中，它接受一个声明为E类型的参数（在本例中为Number）。 另一方面，在setValue方法中，它接受一个声明为扩展类型E的参数。两个println调用都返回正确的值（首先是java.lang.Long ，然后是java.lang.Double ）。  
 我看到它的方式，删除泛型，你仍然可以传递子类。 如果声明的类没有类型参数，并且方法/构造函数接受/返回Numbers，它仍然可以正常工作。  
 那么在这种情况下extends关键字的目的是什么？ 

In Java, you can use the extends keyword to declare that a given type parameter is covariant. Covariance and contravariance do confuse me a bit, but I think I've gotten a general sense of them. However, in my tests, it seems that Java type parameters are inherently covariant. So why can we explicitly state it? 
For example, I built a quick sample that looks like this: 
public class Main<E> {

    protected E value;

    public Main(E value) {
        this.value = value;
    }

    public <T extends E> void setValue(T value) {
        this.value = value;
    }

    public E getValue() {
        return value;
    }

    public static void main(String[] args) {
        Main<Number> example = new Main<Number>(0L);
        System.out.println(example.getValue().getClass().getCanonicalName());
        example.setValue(32.0);
        System.out.println(example.getValue().getClass().getCanonicalName());
    }
}
 
The type used is Number, the superclass of all the boxed types. In the constructor, it takes a parameter which is declared of type E (in this case Number). In the setValue method, on the other hand, it takes a parameter which is declared to be anything extending type E. Both println calls return the proper value (first java.lang.Long, then java.lang.Double). 
The way I see it, removing generics, you can still pass subclasses. If the class was declared with no type parameters and the methods/constructor took/returned Numbers, it'd still work properly. 
So what is the purpose of the extends keyword in this case?

原文：https://stackoverflow.com/questions/25717291

更新时间：2022-05-22 16:05

最满意答案

 回答这个问题：  
 
  我们进行了一些测试，我们发现当我们在IIS中添加绑定时，它不会更新我们服务的配置文件。 这意味着我们需要在两个不同的地方保持配置。 这不是逻辑的吧？  
 
 当您使用IIS托管您的服务时，您必须配置您的App.config文件或web.config文件以允许IIS公开一些绑定，因此在您的配置文件中，您将所有绑定到您的wcf服务。 Http，net.tcp等...  
 在你的绑定中你不会指定地址，因为你将直接在IIS中指定这些地址。  
 在IIS中，您必须允许在您的网站的高级设置中可用的绑定。 之后，您将为您的网站“Web服务”设置新的绑定，并添加您要收听的每个绑定，并指定该地址。  
 您将直接在IIS中指定地址。  
 有一个例子  
 您的配置文件：  
<services>
    <service name="ServiceName">                    
        <endpoint address=""
            binding="basicHttpBinding"
            bindingConfiguration="httpMode"
            contract="IContract" />                 
        <endpoint address=""
            binding="netTcpBinding"
            contract="IContract" />
        <endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange" />
    </service>
</services>
 
 在你的IIS中，你会放弃你的设置  
 
  http，net.tcp在启用协议  
 
 之后，您将绑定到IIS中。 把你的绑定为http normaly并添加一个新绑定的net.tcp，在绑定配置中放入端口和虚拟目录  
 
  8001：*  
 
 此设置允许所有连接到任何虚拟目录的8001端口。  
 您还必须在服务器上安装“WCF激活（Http激活和非Http激活）”功能。 

To answer at those question : 
 
 We ran some tests and we found out that when we're adding bindings in IIS, it doesn't update config file of our service. That means that we would need to maintain the configuration in two different places. It's not logic, right ? 
 
When you use IIS to host your service, you must configure your App.config file or web.config file to allow IIS to expose some binding, so in your configuration file, you will put all your binding you allow to your wcf service. Http, net.tcp etc... 
In your binding you will not specified address, because you will specified those address in IIS directly. 
In IIS you must allow the binding available in the advanced settings of your web site. After that you will set new binding for your web site "web service" and add every bindings you want listen, and specify the address. 
You will specify the address directly in IIS. 
There's an example. 
Your configuration file: 
<services>
    <service name="ServiceName">                    
        <endpoint address=""
            binding="basicHttpBinding"
            bindingConfiguration="httpMode"
            contract="IContract" />                 
        <endpoint address=""
            binding="netTcpBinding"
            contract="IContract" />
        <endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange" />
    </service>
</services>
 
In your IIS advenced setting your will put  
 
 http,net.tcp in Enabled Protocols 
 
After that you will go in your binding into IIS. Put your binding for http normaly and add a new binding net.tcp, in the binding configuration put the port and virtual directory like  
 
 8001:* 
 
This setting allow all connection into the 8001 port for any virtual directory. 
You also must to have the feature "WCF Activation, (Http activation and Non-Http Activation)" installed on your server.

为什么Java方法参数可以显式协变？(Why can Java method parameters be explicitly covariant?)

最满意答案

相关问答

在IIS Express中托管WCF服务的问题(Problem hosting WCF service in IIS Express)[2023-06-21]

IIS WCF服务托管与Windows服务(IIS WCF service hosting vs Windows Service)[2021-08-14]

在Windows服务或IIS中托管WCF服务时COM异常0x80040154(COM exception 0x80040154 when hosting WCF service in Windows Service or IIS)[2022-03-22]

IIS 6上的WCF服务托管(WCF service hosting on IIS 6)[2022-01-06]

在IIS上托管WCF Web服务时显式调用服务构造函数(Explicitly calling a service constructor when hosting a WCF web service on IIS)[2023-03-19]

IIS托管WCF服务错误(IIS hosting WCF service error)[2022-06-06]

在IIS 7上托管WCF服务时发出问题(Issue while hosting WCF service on IIS 7)[2023-07-18]

有关将WCF托管为Windows服务的问题(Questions about hosting WCF as Windows Service)[2022-03-10]

为什么要在Windows服务中托管wcf服务？(Why Would You Host a wcf service in a Windows service?)[2023-06-16]

WCF托管在Windows服务+网页中？(WCF hosted in Windows service + web page?)[2022-03-25]

相关文章

最新问答