Re: HttpURLConnection

From:
=?ISO-8859-1?Q?Arne_Vajh=F8j?= <arne@vajhoej.dk>
Newsgroups:
comp.lang.java.programmer
Date:
Fri, 22 Feb 2013 13:28:46 -0500
Message-ID:
<5127b8df$0$294$14726298@news.sunsite.dk>
On 2/20/2013 1:26 PM, bob smith wrote:

On Tuesday, February 19, 2013 4:56:31 PM UTC-6, Eric Sosman wrote:

On 2/19/2013 4:45 PM, bob smith wrote:

How can people use the class HttpURLConnection when the abstract method connect() from URLConnection is never defined?

I thought you can't use a class till all the abstract blanks are filled in?


      The openConnection() method of the URL class returns a
URLConnection object. I haven't used it myself, but from the
documentation it appears that for an HTTP URL the URLConnection
will in fact be an HttpURLConnection.

      Both of those classes are abstract, and as such they cannot
be instantiated. The object actually returned will be an instance
of some concrete subclass, possibly anonymous. The inheritance
tree would look something like

    java.lang.Object
        java.net.URLConnection
            java.net.HttpURLConnection
                (maybe a few more levels here)
                    some.concrete.class.Thing

      If you're curious, you can do getClass() on the object you
get back from openConnection(), and print its class name or do
other snoopy things. But to use it, you're just fine treating
it as an HttpURLConnection -- because it "is an" HttpURLConnection,
in exactly the same way that an Integer "is a" Number.


Thanks. This clears up a lot.


If you want to see everything:

import java.io.IOException;
import java.lang.reflect.Modifier;
import java.net.URL;
import java.net.URLConnection;

public class WhatConnection {
    private static final String INDENT = " ";
    private static String getType(Class<?> clz) {
        if(clz.isInterface()) {
            return "interface";
        } else if(clz.isEnum()) {
            return "enum";
        } else {
            if((clz.getModifiers() & Modifier.ABSTRACT) != 0) {
                return "abstract class";
            } else {
                return "class";
            }
        }
    }
    public static void dumpClassInfo(String indent, Class<?> clz) {
        System.out.println(indent + getType(clz) + " " + clz.getName());
        if(clz.getSuperclass() != null) dumpClassInfo(indent + INDENT,
clz.getSuperclass());
        for(Class<?> intf : clz.getInterfaces()) {
            dumpClassInfo(indent + INDENT, intf);
        }
    }
    public static void test(String urlstr) throws IOException {
        System.out.println(urlstr + ":");
        URL url = new URL(urlstr);
        URLConnection con = url.openConnection();
        dumpClassInfo(INDENT, con.getClass());
    }
    public static void main(String[] args) throws Exception {
        test("http://www.oracle.com/");
        test("ftp://ftp.oracle.com/");
    }
}

Arne

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