Difference between URI and URL

hi,
i'm having trouble understanding the difference between a URI and URL.
The javadoc explains that
"A URI is a uniform resource identifier while a URL is a uniform
resource locator. Hence every URL is a URI, abstractly speaking, but not every URI is a URL. This is because there is another subcategory of URIs, uniform resource names (URNs), which name resources but do not specify how to locate them. The mailto, news, and isbn URIs shown above are examples of URNs.
The conceptual distinction between URIs and URLs is reflected in the differences between this class and the URL class.
in what way does a URL 'locate' a resource rather than name a resource?
eg.
URI example: mailto:[email protected]
URL example: http://java.sun.com/j2se/1.3/
I don't see why the email address does not 'locate' the particular email alias?
thanks,
asjf

Look at the class java.net.URL.
It has a number of methods, such as 'getContent', 'openConnection', 'openStream'. It is what differs URL from URI and answers your question about "I don't see why the email address does not 'locate' the particular email alias?".
With URL you can read the contents of the resource, pointed by URL, e.g. read a file from HTTP or FTP server. An URI does not imply that it refers to such a downloadable resource. It just names something, that somebody may know how to access.
Also consider the following examples:
- an URL of a document, if followed by a '#' symbol and by an anchor name ('fragment identifier') is sometimes referred as an URI, not URL, because it does locate a fragment in the file, not a downloadable file.
- a query ('?' followed by something, up to '#') is a part of the URL, because it is required to locate the file.
The syntax of an URI is described in RFC 2396, http://www.ietf.org/rfc/rfc2396.txt.

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What is an URL
An Uniform Resource Locator (URL) is the term used to identify an Internet resource, and can be specified in a single line of text.
What is an URN
An Uniform Resource Name (URN) is the term used to identify an Internet resource, without the use of a scheme, and can be specified in a single line of text.
What is an URI
An Uniform Resource Identifier (URI) is used by a browser to identify a single document, and it too can be specified in a single line of text
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telnet 23
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ISBN: 1-930110-59-6
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Difference between URI and URL

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