Chapter 7. URLs and URIs
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, the format of the data being retrieved, or how to communicate with
the server; you simply tell Java the URL and it gets the data for you.
Although Java can only handle a few protocols and content types out of the
box, in later chapters you’ll learn how to write and install new content
and protocol handlers that extend Java’s capabilities to include new
protocols and new kinds of data. You’ll also learn how to open sockets and
communicate directly with different kinds of servers. But that’s later;
for now, let’s see how much can be done with a minimum of work.
The URL Class
java.net.URL class is an abstraction of a Uniform Resource Locator
such as http://www.hamsterdance.com/ or ftp://ftp.redhat.com/pub/. It extends
java.lang.Object, and it is a final class that
cannot be subclassed. Rather than relying on inheritance to configure
instances for different kinds of URLs, it uses the strategy design
pattern. Protocol handlers are the strategies, and the
URL class itself forms the context through
which the different strategies are selected:
public final class URL extends Object implements Serializable
Although storing a URL as a string would be trivial, it is helpful to think of URLs as objects with fields that include the scheme (a.k.a. the protocol), hostname, port, path, query string, and ...