Java Reference
In-Depth Information
CHAPTER 5
URLs and URIs
In the last chapter, you learned how to address hosts on the Internet via host names and
IP addresses. In this chapter, we increase the granularity by addressing resources, any
number of which may reside on any given host.
HTML is a hypertext markup language because it includes a way to specify links to other
documents identified by URLs. A URL unambiguously identifies the location of a re‐
source on the Internet. A URL is the most common type of URI, or Uniform Resource
Identifier. A URI can identify a resource by its network location, as in a URL, or by its
name, number, or other characteristics.
The URL class is the simplest way for a Java program to locate and retrieve data from the
network. You do not need to worry about the details of the protocol being used, or how
to communicate with the server; you simply tell Java the URL and it gets the data for
you.
URIs
A Uniform Resource Identifier (URI) is a string of characters in a particular syntax that
identifies a resource. The resource identified may be a file on a server; but it may also
be an email address, a news message, a book, a person's name, an Internet host, the
current stock price of Oracle, or something else.
A resource is a thing that is identified by a URI. A URI is a string that identifies a resource.
Yes, it is exactly that circular. Don't spend too much time worrying about what a resource
is or isn't, because you'll never see one anyway. All you ever receive from a server is a
representation of a resource which comes in the form of bytes. However a single resource
may have different representations. For instance, https://www.un.org/en/documents/
udhr/ identifies the Universal Declaration of Human Rights; but there are representa‐
tions of the declaration in plain text, XML, PDF, and other formats. There are also
representations of this resource in English, French, Arabic, and many other languages.
 
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