Java Network Programming, Second Edition

Chapter 7. Retrieving Data with URLs

The simplest way for a Java program to locate and retrieve data from the network is to use the URL class. 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 handle only 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 you can do with a minimum of work.

The URL Class

The java.net.URL class is an abstraction of a Uniform Resource Locator like 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 protocol, hostname, port, path, query string, and ...