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HttpURLConnection

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  HttpURLConnection bob smith <bob@coolfone.comze.com> - 2013-02-19 13:45 -0800
    Re: HttpURLConnection Lew <lewbloch@gmail.com> - 2013-02-19 14:31 -0800
    Re: HttpURLConnection Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-02-19 17:56 -0500
      Re: HttpURLConnection bob smith <bob@coolfone.comze.com> - 2013-02-20 10:26 -0800
        Re: HttpURLConnection Arne Vajhøj <arne@vajhoej.dk> - 2013-02-22 13:28 -0500
    Re: HttpURLConnection Roedy Green <see_website@mindprod.com.invalid> - 2013-02-20 22:05 -0800

#22374 — HttpURLConnection

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?

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#22377

On Tuesday, February 19, 2013 1:45:25 PM UTC-8, bob smith wrote:
> How can people use the class HttpURLConnection when the abstract method connect()
> from URLConnection is never defined?

False.

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

True.

Look up polymorphism and widening conversions.

http://docs.oracle.com/javase/specs/jls/se7/html/jls-5.html#jls-5.1.5

Understand the difference between declared type (compile time) and run-time type.

Read the Java tutorials.

Did you know that any object of a given type is also an object of every supertype of its type?

That's how you can get 

List<Foo> foos = new ArrayList<>();

'List', being an interface, has only abstract instance methods. Yet somehow you can call 

foos.add(new Foo());

Same thing.

This is basic Java stuff. Heck, it's basic O-O stuff. Google around for some introductory texts.

-- 
Lew

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#22378

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.

-- 
Eric Sosman
esosman@comcast-dot-net.invalid

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#22395

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.
> 
> 
> 
> -- 
> 
> Eric Sosman
> 
> esosman@comcast-dot-net.invalid

Thanks.  This clears up a lot.

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#22453

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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#22403

On Tue, 19 Feb 2013 13:45:25 -0800 (PST), bob smith
<bob@coolfone.comze.com> wrote, quoted or indirectly quoted someone
who said :

>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?

If you look at what URL.openConnection gives you with getClass to will
see a variety of objects such as: 
HttpURLConnection 
HttpsURLConnectionImpl 
FileURLConnection 
JarURLConnection 

depending on which protocol you specified in the URL e.g. http:,
https:, file:, jar:

See http://mindprod.com/jgloss/http.html
-- 
Roedy Green Canadian Mind Products http://mindprod.com
The generation of random numbers is too important to be left to chance. 
~ Robert R. Coveyou (born: 1915 died: 1996-02-19 at age: 80)

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