19.5 Java4640: the url class and the urlencoder class

Table of contents

• Preface
  • Viewing tip
    • Figures
    • Listings
• General background information
• Discussion and sample code
  • The program Java4640a
  • The program Java4640d
• Run the programs
• What's next?
• Miscellaneous
• Complete program listings

Preface

This module is one in a collection of modules designed for teaching INEW 2338 Advanced Java (Web) at Austin Community College in Austin, TX. More specifically, it is one in asub-collection of modules designed for teaching network programming in that course. The purpose of this module is to introduce the student to the URL class and the URLEncoder class.

Viewing tip

I recommend that you open another copy of this module in a separate browser window and use the following links to easily find and view the Figuresand Listings while you are reading about them.

Figures

• Figure 1 . General syntax of a URL.
• Figure 2 . Screen output from the program named Java4640a.
• Figure 3 . Program output for string-parameter constructor.
• Figure 4 . Building an absolute URL.
• Figure 5 . An encoded string.
• Figure 6 . Encoding rules.
• Figure 7 . Program output.

Listings

• Listing 1 . The method named display.
• Listing 2 . Beginning of the program named Java4640a.
• Listing 3 . Building an absolute URL.
• Listing 4 . The URLEncoder.encode method.
• Listing 5 . Beginning of the program named Java4640d.
• Listing 6 . Open a connection to the URL.
• Listing 7 . Read and display the data.
• Listing 8 . The program named Java4640a.
• Listing 9 . The program named Java4640d.

General background information

What is a URL?

URL is an acronym for Uniform Resource Locator. It is also the name of a class in Java, which is the primary topic for this module.

A URL is a pointer to a particular resource at a particular location on the Internet. As you learned in an earlier module, a URL specifies the following :

• protocol used to access the server (such as http),
• name of the server,
• port on the server (optional)
• path and name of a specific file on the server (sometimes optional)
• anchor or reference within the file (optional)

Sometimes the name of the file can be omitted, in which case an HTTP server may append the file name index.html to the specified path and try to load that file. For example, we will write a simple HTTP server in afuture module that will attempt to deliver a file named index.html if the name of the file is omitted from the URL.

In addition to specifying the name of the file of interest, it is also sometimes possible to specify an anchor or reference that has beenestablished inside the file. An example of how to take advantage of this capability was provided in an earlier module.

General syntax of a URL

The general syntax of a URL is shown in Figure 1 .

The port number is optional, and is not normally required if you are accessing a server that provides the required service on a standard port.

Two ways to do network programming

Java provides at least two different ways to do network programming. The two ways are associated with socket classes and URL classes. Thesocket classes will be the topic of future modules. This module is concerned primarily with the URL class.