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Trail: Custom Networking
Lesson: Working with URLs

Creating a URL

The easiest way to create a URL object is from a String 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: (outside of the tutorial) 
In your Java program, you can use a String containing this text to create a URL object:
URL gamelan = new URL("");
The 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 create URL 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 and MyKids.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:
You can create URL objects for these pages relative to their common base URL: like this:
URL gamelan = new URL("");
URL gamelanGames = new URL(gamelan, "");
URL gamelanNetwork = new URL(gamelan, "");
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 treats relativeURL 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 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");

Other URL Constructors

The URL class provides two additional constructors for creating a URL 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 a URL object from a protocol, host name, and filename. The following code snippet creates a URL to the file at the Gamelan site:

new URL("http", "", "/pages/");
This is equivalent to
new URL("");
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.

The final URL constructor adds the port number to the list of arguments used in the previous constructor:

URL gamelan = new URL("http", "", 80,
This creates a URL object for the following URL:
If you construct a URL object using one of these constructors, you can get a String containing the complete URL address by using the URL object's toString method or the equivalent toExternalForm method.


Each of the four URL constructors throws a MalformedURLException if the arguments to the constructor refer to a null or unknown protocol. Typically, you want to catch and handle this exception by embedding your URL constructor statements in a try/catch pair, like this:
try {
    URL myURL = new URL(. . .)
} catch (MalformedURLException e) {
    . . .
    // exception handler code here
    . . .
See Handling Errors with Exceptions (in the Custom Networking trail) for information about handling exceptions.

Note:  URLs are "write-once" objects. Once you've created a URL object, you cannot change any of its attributes (protocol, host name, filename, or port number).

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