URLs and URIs - Java Network Programming - page 127

throw new RuntimeException ( "shouldn't happen; all VMs recognize http" );
}
This creates a URL object that points to http://www.eff.org/blueribbon.html#intro , using
the default port for the HTTP protocol (port 80). The file specification includes a ref‐
erence to a named anchor. The code catches the exception that would be thrown if the
virtual machine did not support the HTTP protocol. However, this shouldn't happen
in practice.
For the rare occasions when the default port isn't correct, the next constructor lets you
specify the port explicitly as an int . The other arguments are the same. For example,
this code fragment creates a URL object that points to http://fourier.dur.ac.uk:8000/
~dma3mjh/jsci/ , specifying port 8000 explicitly:
try {
URL u = new URL ( "http" , "fourier.dur.ac.uk" , 8000 , "/~dma3mjh/jsci/" );
} catch ( MalformedURLException ex ) {
throw new RuntimeException ( "shouldn't happen; all VMs recognize http" );
}
Constructing relative URLs
This constructor builds an absolute URL from a relative URL and a base URL :
public URL ( URL base , String relative ) throws MalformedURLException
For instance, you may be parsing an HTML document at http://www.ibiblio.org/javafaq/
index.html and encounter a link to a file called mailinglists.html with no further quali‐
fying information. In this case, you use the URL to the document that contains the link
to provide the missing information. The constructor computes the new URL as http://
www.ibiblio.org/javafaq/mailinglists.html . For example:
try {
URL u1 = new URL ( "http://www.ibiblio.org/javafaq/index.html" );
URL u2 = new URL ( u1 , "mailinglists.html" );
} catch ( MalformedURLException ex ) {
System . err . println ( ex );
}
The filename is removed from the path of u1 and the new filename mailinglists.html is
appended to make u2 . This constructor is particularly useful when you want to loop
through a list of files that are all in the same directory. You can create a URL for the first
file and then use this initial URL to create URL objects for the other files by substituting
their filenames.
Other sources of URL objects
Besides the constructors discussed here, a number of other methods in the Java class
library return URL objects. In applets, getDocumentBase() returns the URL of the page
that contains the applet and getCodeBase() returns the URL of the applet .class file.