Creating a URL Relative to Another

A relative URL contains only enough information to reach the resource relative to (or in the context of) another URL.

Relative URL specifications are often used within HTML files. For example, suppose you write an HTML file called JoesHomePage.html. Within this page, are links to other pages, PicturesOfMe.html andMyKids.html, that are on the same machine and in the same directory as JoesHomePage.html. The links to PicturesOfMe.html and MyKids.html from JoesHomePage.html could be specified just as filenames, like this:

<a href="PicturesOfMe.html">Pictures of Me</a>
<a href="MyKids.html">Pictures of My Kids</a>

These URL addresses are relative URLs. That is, the URLs are specified relative to the file in which they are contained--JoesHomePage.html.

In your Java programs, you can create a URL object from a relative URL specification. For example, suppose you know two URLs at the Gamelan site:

http://www.gamelan.com/pages/Gamelan.game.html
http://www.gamelan.com/pages/Gamelan.net.html

You can create URL objects for these pages relative to their common base URL: http://www.gamelan.com/pages/ like this:

URL gamelan = new URL("http://www.gamelan.com/pages/");
URL gamelanGames = new URL(gamelan, "Gamelan.game.html");
URL gamelanNetwork = new URL(gamelan, "Gamelan.net.html");

This code snippet uses the URL constructor that lets you create a URL object from another URL object (the base) and a relative URL specification. The general form of this constructor is:

URL(URL baseURL, String relativeURL)

The first argument is a URL object that specifies the base of the new URL. The second argument is a String that specifies the rest of the resource name relative to the base. If baseURL is null, then this constructor treatsrelativeURL like an absolute URL specification. Conversely, if relativeURL is an absolute URL specification, then the constructor ignores baseURL.

This constructor is also useful for creating URL objects for named anchors (also called references) within a file. For example, suppose the Gamelan.network.html file has a named anchor called BOTTOM at the bottom of the file. You can use the relative URL constructor to create a URL object for it like this:

URL gamelanNetworkBottom = new URL(gamelanNetwork, "#BOTTOM");

http://download.oracle.com/docs/cd/E17409_01/javase/tutorial/networking/urls/creatingUrls.html