blocks|key|2632|text|它是一个类，扩展了返回的URLConnection。这是一种隐藏实现细节的方法，因为可以根据输入参数返回不同的URLConnection实现。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2633|entityMap^0|C|D|1J|D|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

It's a class extending <code>URLConnection</code> that is being returned. It's a way of hiding implementation detail as different implementation of <code>URLConnection</code> can be returned depending on the input parameters.

blocks|key|2735|text|openConnection返回一些扩展URLConnection的具体类。该方法的签名被定义为返回URLConnection，因为使用它的代码不应该依赖于任何特定的实现。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2736|entityMap^0|0|E|K|D|1E|D|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|O|8|@]|D|@]|E|$]]]|G|$]]

<code>openConnection</code> returns some concrete class which extends<code>URLConnection</code>. The method's signature is defined as returning <code>URLConnection</code> since your code using it should not rely on any specific implementation.

blocks|key|2788|text|多态概念将在这里应用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2789|对象的类型将是URLConnection(父类\接口)，并且对象将属于其子类(扩展或实现(在接口的情况下))。|2790|下面的例子可能会让你明白这一点。|2791|abstract+class+URLConnection+{
++//+some+methods.+
}|code-block|syntax|javascript|2792|假设您有自己的类。|2793|class+ConnectionClass+extends+URLConnection+{
++//+some+methods+
}+

class+Main+{
++URLConnection+connect+=+null

++public+URLConnection+getUrl()+{
++++return+connect+=+new+ConnectionClass();
++}
}|2794|你将返回类的object，它给出了URLConnection类的openConnection()的实现。同样的概念也适用于接口情况。|offset|length|style|CODE|2795|entityMap^0|0|0|0|0|0|0|H|D|W|G|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|X|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|Y|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|Z|8|@]|9|@]|A|$I|J]]|$1|K|3|L|5|6|7|10|8|@]|9|@]|A|$]]|$1|M|3|N|5|H|7|11|8|@]|9|@]|A|$I|J]]|$1|O|3|P|5|6|7|12|8|@$Q|13|R|14|S|T]|$Q|15|R|16|S|T]]|9|@]|A|$]]|$1|U|3|-4|5|6|7|17|8|@]|9|@]|A|$]]]|V|$]]

Polymorphism concept will apply here. 

The type of objects will be the URLConnection(Parent class\interface) and and object will be of its child class(extends or implements( in case of interface)). 

Following example might make this thing clear to you.

<pre><code>abstract class URLConnection {
 // some methods. 
}
</code></pre>

Let's suppose you have your own class. 

<pre><code>class ConnectionClass extends URLConnection {
 // some methods 
} 

class Main {
 URLConnection connect = null

 public URLConnection getUrl() {
 return connect = new ConnectionClass();
 }
}
</code></pre>

You will return object of class, who gives implementation of <code>openConnection()</code> of <code>URLConnection</code> class. Same concepts apply in interface case.

blocks|key|2821|text|如果所有的子类都是抽象的，那么这是因为您只能通过在工厂中创建类来使用这些类。如果仔细检查Using+java.net.URLConnection+to+fire+and+handle+HTTP+requests中的代码，就会发现URLConnection对象从未实例化过。相反，我们通过调用openConnection来获得它，如下所示：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2822|URLConnection+connection+=+new+URL(url+%2B+"?"+%2B+query).openConnection();|code-block|syntax|javascript|2823|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/2793150/using-java-net-urlconnection-to-fire-and-handle-http-requests^0|36|D|41|E|18|1P|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@$9|X|A|Y|1|Z]]|E|$]]|$1|F|3|G|5|H|7|10|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|11|8|@]|D|@]|E|$]]]|L|$M|$5|N|O|P|E|$Q|R]]]]

When all subclasses are abstract, then it's because you can only use the classes by creating it in a factory. If you check carefully the code in <a href="https://stackoverflow.com/questions/2793150/using-java-net-urlconnection-to-fire-and-handle-http-requests">Using java.net.URLConnection to fire and handle HTTP requests</a> , you will see that <code>URLConnection</code> object is never instanced. Instead, we get it by calling <code>openConnection</code> like this:

<pre><code>URLConnection connection = new URL(url + "?" + query).openConnection();
</code></pre>

Relatively new Java programmer and I've been taught that you can't create an instance of an abstract class. I've also done a little research and I learned that in most cases when it appears an abstract class is being created, it is actually an anonymous subclass. But here is the issue I ran into:

Java's <code>URL</code> class has a method <code>openConnection</code> that returns a <code>URLConnection</code>. <code>URLConnection</code> is an abstract class, and the Java documentation also lists all of its subclasses as abstract..... so I'm really lost. What's being returned?

Instantiate Java Abstract class?

Java

相对较新的Java程序员和我被教导不能创建抽象类的实例。我也做了一些研究，我了解到在大多数情况下，当一个抽象类被创建时，它实际上是一个匿名子类。但这是我遇到的问题：Java的URL类有一个返回URLConnection的方法openConnection。URLConnection是一个抽象类，Java文档还将它的所有子...

问实例化Java抽象类？
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