Learning Java

The URL Class

A URL is represented by an instance of the java.net.URL class. A URL object manages all the component information within a URL string and provides methods for retrieving the object it identifies. We can construct a URL object from a URL specification string or from its component parts:

try {  
    URL aDoc =
      new URL( "http://foo.bar.com/documents/homepage.html" );
    URL sameDoc =
      new URL("http","foo.bar.com","documents/homepage.html");
}   
catch ( MalformedURLException e ) { }

These two URL objects point to the same network resource, the homepage.html document on the server foo.bar.com. Whether the resource actually exists and is available isn’t known until we try to access it. At this point, the URL object just contains data about the object’s location and how to access it. No connection to the server has been made. We can examine the URL’s components with the getProtocol( ), getHost( ) , and getFile( ) methods. We can also compare it to another URL with the sameFile( ) method (which has an unfortunate name for something which may not point to a file). sameFile( ) determines whether two URLs point to the same resource. It can be fooled, but sameFile( ) does more than compare the URLs for equality; it takes into account the possibility that one server may have several names, and other factors.

When a URL is created, its specification is parsed to identify just the protocol component. If the protocol doesn’t make sense, or if Java can’t find a protocol handler for it, the URL ...