在Java中创建抽象类URLConnection或HttpURLConnection的对象

Question

I saw the following code in a tutorial:

URLConnection connection = new URL("http://example.com").openConnection();

How is that possible? The API says that URLConnection (and also the Subclass HttpURLConnection) is an abstract class. But abstract classes cannot be instantiated. I also saw other tutorials with (for example) this code:

URL url = new URL( "http://java-tutor.com/index.html" );
URLConnection con = url.openConnection();

Why is there no 'new'? For me that's very strange, so can someone explain this to me.

Answer 1

openConnection();

creates an instance of implementation of URLConnection class (sun.net.www.protocol.http.HttpURLConnection @Credit goes to Sotirios).

Based on javadoc :

If for the URL's protocol (such as HTTP or JAR), there exists a public, specialized URLConnection subclass belonging to one of the following packages or one of their subpackages: java.lang, java.io, java.util, java.net, the connection returned will be of that subclass. For example, for HTTP an HttpURLConnection will be returned , and for JAR a JarURLConnection will be returned.

Answer 2

First of all, you don't instantiate URLConnection but URL at:

URLConnection connection = new URL("http://example.com").openConnection();

URLConnection is the variable type (which can be abstract or even an interface ), URL is the specific class you instantiate.

About the no new word, openConnection() returns URLConnection object so you don't need to create one yourself.

Answer 3

URL is a factory that creates a URLConnection based (if memory serves me correctly) by a scheme. You can have HttpsURLConnection or HttpURLConnection .

Answer 4

URLConnection is indeed an abstract class. It cannot be instantiated.

So as not to be useless, however, for every abstract class in a class structure there must be a subclass (maybe several levels removed) that is concrete, ie that can be instantiated.

URL.openConnection() is returning one of those subclasses to you. You could verify this with a debugger, if you like. URLConnection is the type of the reference you call "con," not the type of the object referred to by that reference.