Introduction

URL stands for uniform resource locator. URLs are used on the internet to define files and the protocols with which they should be processed. Here's an example of a URL:

http://www.cs.cornell.edu/Courses/cs211/2001fa/index.html

1. An identification of a service or protocol (http);
2. A domain name or host (www.cs.cornell.edu), which is associated with a computer that is attached to the internet; and
3. A path on that computer (Courses/cs211/2001fa/index.html).

<protocol>://<authority><path>?<query>#<fragment>

Protocols

Here are the protocols one usually sees:

1. http: This stands for HyperText Transport Protocol, which is the most used protocol for accessing files over the internet. Generally, the files are html files, which are explained a bit on lesson page, but they could be any files, including text files, .jpg files, and .gif files. A domain name must be given.
2. file: This is used for a file that is on the local computer. The domain name is generally not present.
3. ftp: This stands for File Transfer Protocol, a protocol that provides for files and directories of files to be transferred from one computer to another.
4. mailto: Used to bring up a window that one can use to send mail. In this case, the // and domain name are omitted, and instead of a path one has an email address, e.g. consider this URL:   mailto:gries@cs.uga.edu

The appearance of "//" in the URL signals the beginning of a domain name, which is a name that has been registered as being assigned to or associated with a particular computer. Here are examples of domain names:

1. www.cs.cornell.edu
2. www.cs.toronto.edu
3. www.datadesk.com

Ports

A URL can optionally specify a "port", which is the port number to which the TCP connection is made on the remote host machine. If the port is not specified, the default port for the protocol is used instead. For example, the default port for http is 80. An alternative port could be specified as:

http://www.ncsa.uiuc.edu:8080/demoweb/url-primer.html

Absolute paths

 The path part of a URL is a path on the computer to the file that the URL describes. For example,

/~gries/Logic/Introduction.html

The character / is used to separate entities on the path, regardless of the operating system that is running on the computer --Unix-like, Windows, or Macintosh.

If the file name is missing at the end (so that the last entity is a folder), then a default file is chosen, usually index.html or index.htm. This default depends on (and can be changed) the computer on which the file resides. On some computers, we have seen the following defaults: default.html, default.htm, home.html, and home.htm.

Any path of any file or folder on your hard drive can be used in a URL. The form of the beginning of such a path depends on whether a Unix-like, Windows, or Macintosh operating system is being used. You can check this out on your own computer by loading any html file that is on your hard drive into a browser like Netscape Communicator or Internet Explorer and looking at the URL that is displayed. Here are examples for the three kinds of systems:

1. Unix: /home/profs/gries/public_html/index.html
2. Windows: /C:/MyDocuments/test.html
3. Mac: /ProgramLive/Course/IC/web/ICweb.htm

Within an html file, one can have a relative URL, as in href="people/faculty/faculty.htm".

The protocol and host are assumed to be the same as that of the current html file, and the path is assumed to be relative to the folder in which the html file appears. One can use .. in the path to move up in the path of folders, as in all operating systems. For example, if the current folder is /~gries/Logic, then relative path ../NoLogic/test.html refers to the file /~gries/NoLogic/test.html.
java.net

Fragments

The following URL has the "fragment" #chap1:

        http://java.sun.com/index.html#chapter1

The fragment is often the name of a "target" within the file given by the URL, but there are other uses for it. Technically, the fragment is not part of the URL. For assignments in CS211 this Fall, targets should be discarded if present.

Class URL

Package java.net contains class URL, which is used for dealing with URLs in Java. Below, we provide terse specs for a few of the most
useful methods in class URL. We don't discuss the authority, query, and fragment parts of URLs. We also make
the following comments.

First, if you have a variable of class URL, you can get a BufferedReader to it using the following procedure --this procedure
returns null if the file URL is not for an http file or a file file (a file on a local computer). Notice how an OpenStream is created for it
(returns one byte at a time), then an InputStreamReader  (which turns bytes into characters) from that, and finally the BufferedReader
(which had a method to read one line at a time). This is quite similar to dealing with input files on your computer.

     /**  = a reader for URL url (which must not be null). If the protocol
           is not http or file, null is returned. */
    private BufferedReader getReader() {
        if (!protocol.equals("http") && !protocol.equals("file")) {
           return null;
        }

        try {
            InputStream is= url.openStream();
            InputStreamReader   isr= new InputStreamReader(is);
           returnnew BufferedReader(isr);
        } catch (IOException e) {
           returnnull;
        }
    }

Next, when you have an absolute Url  as a String s, use "new URL(s)" to create an instance of URL for it. But if you
already have a URL c that is a directory, say, and you want to create a URL for a file s that is within the directory, use
"new URL(c,s)".

Finally, give a URL u, use the various getter methods shown below to get components of it, usually as Strings.

public class URL {

    // Constructor:  a URL object constructed its representation s. A call URL("...")
    // is equivalent to a call URL(null, "...").
    public URL(String s) throws MalformedURLException

    /* Constructor:  a URL object constructed from URL c and String s.
       If c's path component is empty and the scheme, authority, and query components are
       undefined, then the new URL is a reference to the current document. Otherwise, any
       fragment and query parts present in the s are used in the new URL.

       If s contains a protocol component is and it does not match the protocol in c, then
       the new URL is created as an absolute URL based on the s alone. Otherwise, the
       protocol component is inherited from the context URL.

        If the s's path component begins with a slash character "/" then the path is treated as
        absolute, and s's path replaces the c's path. Otherwise s's path is treated as a relative
        path and is appended to c's path. The path is canonicalized through the removal of
        directory changes made by occurences of ".." and ".".
    */
    public URL(URL c, String s) throws MalformedURLException

    // = the file name for this URL
    public String getFile()

    // = the host, or domain name, for this URL
    public String getHost()

    // = the path for this URL
    public String getPath()

    // = the port for this URL (-1 if the port is not set)
    public int getPort()

    // = the protocol for this URL
    public String getProtocol()

    // = a newly opened connection to this URL, as an InputStream. Shorthand for
    //          openConnection().getInputStream()
    public final InputStream openStream() throws IOException

    // =  a representation of this URL
    public String toString()
}