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URLconnection for nonHTTP connections

I am trying to understand when to use URLConnection Class & when to use HttpURLConnection class. On doing some research, I came to know that URLConnection is used for nonHTTP connections & HttpURLConnection is used for specific HTTP connections. Can someone help me to know that what these nonHTTP connections refer to ?
PS - Please note that my qn is not regarding inheritance relationship of URLConnection Class & HttpURLConnection class.
It is regarding what do we mean by nonHTTP connections (that can be handled by URLConnection Class). Is nonHTTP connection means a datagram connection or ftp connection or something else.Refer below website:
https://www.experts-exchange.com/questions/21420009/URLConnection-and-HttpURLConnection.html

A URLConnection is an abstract class, representing any connection to an URL.
A HttpURLConnecton IS a URLConnection (subclass) and offers more methods.
That being said, you could open a URL with the HTTP protocol and assign the return value to a URLConnection, but you'd be missing the more specific methods.
You can open a URL to an ftp resource, or anything else and assign the result to a URLConnection, but it might not support the underlying protocol. For example, FTP support is very limited.
See the documentation for URL.openConnection():
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.
So, yes, use HttpURLConnection for HTTP connection and any specialized URLConnection you have, or use the raw URLConnection otherwise. That being said, it depends with which protocol you want to work - you might need a more complete client for any non-trivial protocol.

How to prevent openConnection() to actually connect

I want to use HttpURLConnection to connect to my webservice, POST an XML and get a result. I am using the following code:
URL url = new URL(urlString);
connection = (HttpURLConnection)url.openConnection();
connection.setRequestProperty("Content-Type", "text/xml; charset=utf-8");
Problem is, when I call the setRequestProperty method, it fails with an IllegalStateException saying I am "already connected". Apparently, openConnection in fact opens the connection to the URL (in my debugger, I can see the connected boolean set to true). According to the URL documentation of Oracle though, it shouldn't. The Android docs are unclear about it.
How do I prevent openConnection to connect, so I can set extra properties?
Update it looks like the connection is in some pool and doesn't get disconnected, not even after calling connection.disconnect(), or even killing the server.
I do not think this is a duplicate of this question as it gives no real answer. Also, the documentation seems to be unclear.

openConnection() does not connect, and the code you have posted does not behave as yu have described. What opens the TCP connection is any of the following:
getInputStream()
getErrorStream()
getResponseCode()
Ergo you must have called one of those before trying to set a request property.

Getting HTTP response code in Java

I need to find the HTTP response code of URLs in java. I know this can be done using URL & HTTPURLConnection API and have gone through previous questions like this
and this.
I need to do this on around 2000 links so speed is the most required attribute and among those I already have crawled 150-250 pages using crawler4j and don't know a way to get code from this library (due to which I will have to make connection on those links again with another library to find the response code).

In Crawler4J, the class WebCrawler has a method handlePageStatusCode, which is exactly what you are looking for and what you would also have found if you had looked for it. Override it and be happy.

The answer behind your first link contains everything you need:
How to get HTTP response code for a URL in Java?
URL url = new URL("http://google.com");
HttpURLConnection connection = (HttpURLConnection)url.openConnection();
connection.setRequestMethod("GET");
connection.connect();
int code = connection.getResponseCode();
The response code is the HTTP code returned by the server.

java HttpUrlConnection POST: need to ask for a response for the POST to be committed?

I'm trying to post data on a server using java HttpUrlConnection class. It seems that if I somehow read the response of the server, the post works fine, but if I don't, the information is never posted.
This kind of behaviour is not mentionned in the HttpUrlConnection doc and all examples I've seen of HttUrlConnection ask for a response from the server.
I want to know if I made a mistake in my code or if this is a normal behaviour of HttpUrlConnection, and if it's the case, can someone with a better understanding of how this class works explain to me why it's so?
Thanks a lot!
Here is the code, the commented line is the one that makes the POST either work or fail:
URL url=new URL("myUrl");
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("POST");
connection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded;charset=utf-8");
connection.setRequestProperty("Host", "myHost");
connection.setDoOutput(true);
os.write(staffToPost.getBytes("utf-8"));
os.flush();
os.close();
//System.out.println(connection.getResponseCode()+" "+connection.getResponseMessage());

The URLCOnnection class needs a complete handshake for a POST to occur.This means, once you open a connection, you have to readback the response for the POST to actually take place.

You can explicitly call URLConnection.connect() or get any information about the HTTP response such as HttpURLConnection.getResponseCode etc to trigger HTTP request.
You should go through this comprehensive post on How to use java.net.URLConnection to fire and handle HTTP requests for better understanding.

Trying to GET a Google Spreadsheet in Java is returning HTTP error 405

Been working on this all day and have gotten no where with it.
My Java code looks like this:
final URL url = new URL(String.format("https://spreadsheets.google.com/feeds/download/spreadsheets/Export?key=%s&exportFormat=tsv&gid=0", spreadsheetId));
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestProperty("Authorization", "GoogleLogin auth=" + wiseAuth.getAuthToken());
conn.setRequestProperty("GData-Version", "3.0");
conn.setRequestMethod("GET");
conn.setDoOutput(true); // trouble here, see below
conn.setInstanceFollowRedirects(true);
conn.connect();
I always get a FileNotFound error when attempting to do conn.getInputStream(). I narrowed it down to being that the response code is 405 Method Not Allowed. The exception is returning me my URL and I can access the page just fine in my browser.
It was then that I discovered that setDoOutput(true) executes a POST internally. But if I remove that line, conn.getInputStream() is null, and conn.getOutputStream() appears to return nothing--though maybe I am setting it up wrong?

I don't recommend you to do it like this, even if you get it working now you cannot ensure you will get it working in the future if Google started changing it.
Instead, consider using Google Spreadsheet API. The provided Java examples are pretty straightforward and you should able to accomplish what you want.

I would recommend using a web debugger like Fiddler to see what exactly your application is sending in the GET request and compare it to your browser. You might be missing an important header or something, and Fiddler makes it really easy to slowly strip down your browser's request to the essential elements (just drag a request to clone it, then take out headers).