Working with URLs |
The easiest way to create aURL
object is from aString
that represents the human-readable form of the URL address. This is typically the form that another person will use for a URL. For example, the URL for the Gamelan site, which is a directory of Java resources, takes the following form:In your Java program, you can use ahttp://www.gamelan.com/String
containing this text to create aURL
object:TheURL gamelan = new URL("http://www.gamelan.com/");URL
object created above represents an absolute URL. An absolute URL contains all of the information necessary to reach the resource in question. You can also createURL
objects from a relative URL address.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 asJoesHomePage.html
. The links toPicturesOfMe.html
andMyKids.html
fromJoesHomePage.html
could be specified just as filenames, like this:These URL addresses are relative URLs. That is, the URLs are specified relative to the file in which they are contained--<a href="PicturesOfMe.html">Pictures of Me</a> <a href="MyKids.html">Pictures of My Kids</a>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:You can createhttp://www.gamelan.com/pages/Gamelan.game.html http://www.gamelan.com/pages/Gamelan.net.htmlURL
objects for these pages relative to their common base URL:http://www.gamelan.com/pages/
like this:This code snippet uses theURL gamelan = new URL("http://www.gamelan.com/pages/"); URL gamelanGames = new URL(gamelan, "Gamelan.game.html"); URL gamelanNetwork = new URL(gamelan, "Gamelan.net.html");URL
constructor that lets you create aURL
object from anotherURL
object (the base) and a relative URL specification. The general form of this constructor is:The first argument is aURL(URL baseURL, String relativeURL)URL
object that specifies the base of the newURL
. The second argument is aString
that specifies the rest of the resource name relative to the base. IfbaseURL
is null, then this constructor treatsrelativeURL
like an absolute URL specification. Conversely, ifrelativeURL
is an absolute URL specification, then the constructor ignoresbaseURL
.This constructor is also useful for creating
URL
objects for named anchors (also called references) within a file. For example, suppose theGamelan.network.html
file has a named anchor calledBOTTOM
at the bottom of the file. You can use the relative URL constructor to create aURL
object for it like this:URL gamelanNetworkBottom = new URL(gamelanNetwork, "#BOTTOM");Other URL Constructors
TheURL
class provides two additional constructors for creating aURL
object. These constructors are useful when you are working with URLs, such as HTTP URLs, that have host name, filename, port number, and reference components in the resource name portion of the URL. These two constructors are useful when you do not have a String containing the complete URL specification, but you do know various components of the URL.For example, suppose you design a network browsing panel similar to a file browsing panel that allows users to choose the protocol, host name, port number, and filename. You can construct a
URL
from the panel's components. The first constructor creates aURL
object from a protocol, host name, and filename. The following code snippet creates aURL
to theGamelan.net.html
file at the Gamelan site:This is equivalent tonew URL("http", "www.gamelan.com", "/pages/Gamelan.net.html");The first argument is the protocol, the second is the host name, and the last is the pathname of the file. Note that the filename contains a forward slash at the beginning. This indicates that the filename is specified from the root of the host.new URL("http://www.gamelan.com/pages/Gamelan.net.html");The final
URL
constructor adds the port number to the list of arguments used in the previous constructor:This creates aURL gamelan = new URL("http", "www.gamelan.com", 80, "pages/Gamelan.network.html");URL
object for the following URL:If you construct ahttp://www.gamelan.com:80/pages/Gamelan.network.htmlURL
object using one of these constructors, you can get aString
containing the complete URL address by using theURL
object'stoString
method or the equivalenttoExternalForm
method.MalformedURLException
Each of the fourURL
constructors throws aMalformedURLException
if the arguments to the constructor refer to anull
or unknown protocol. Typically, you want to catch and handle this exception by embedding your URL constructor statements in atry
/catch
pair, like this:See Handling Errors with Exceptions for information about handling exceptions.try { URL myURL = new URL(. . .) } catch (MalformedURLException e) { . . . // exception handler code here . . . }
Note:URL
s are "write-once" objects. Once you've created aURL
object, you cannot change any of its attributes (protocol, host name, filename, or port number).
Working with URLs |