Chapter 12: Accessing the Web

• URL (Uniform Resource Locator) class

• Applet methods– for audio clips– for images– context interface

Living on the Web

• An HTML document contains reference to an Applet, not the applet code itself

• Code retrieved by URL

• URL (Uniform Resource Locator)– essentially a web addressing system– URL: protocol://hostname/pathname/filename

URL Parts

• Protocols– http - hypertext transfer protocol– ftp - file transfer protocol– gopher - use gopher transfer mechanism– file - local file

• Host - series of domains separated by “.”– www.d.umn.edu– may have a port number: www.d.umn.edu:8080

• Path and File names tend to be somewhat system specific

URL Class

• In java.net package• Constructors:

URL(String name) - uses name to constructURL(URL base, String relative) - constructs URL

relative to baseURL is “http://www.d.umn.edu/~rmaclin/home.html”,

if relative is “index.html” then this constructs URL“http://www.d.umn.edu/~rmaclin/index.html”

URL(String protocol, String host, String file) - creates URL from combined parts

URL(String protocol, String host, int port, String file)

URL Class

• Methods:String getProtocol() - string describing protocol

String getHost() - string describing host

int getPort() - port #

String getFile() - string describing path/filename

Other Methods from Applet

URL getCodeBase() - get URL of directory containing applet’s .class file

URL getDocumentBase() - get URL of current HTML document

AudioClip getAudioClip(URL u) - retrieves AudioClip interface associated with URL

AudioClip getAudioClip(URL u, String relative)

Image getImage(URL u) - retrieves image

Image getImage(URL u, String relative)

More Applet Methods

void play(URL u) - play AudioClip directlyvoid play(URL u, String relative)

AppletContext getAppletContext() - returns context interface associated with window (more later)

void showStatus(String message) - put message on browser’s status bar

Applet Parameters

• In HTML <APPLET> tag can have params:<APPLET CODE = “MyApp.class”

WIDTH = 500

HEIGHT = 300

<PARAM name = “NumPeople” value = “5”>

<PARAM name = “Picture” value = “my.gif”>

</APPLET>

• Can access:String getParameter(String name) - returns value

of name (or null if none)

AppletContext Interface

• Interface implemented by browser, allows you to interfact with browser

• Useful methods:Applet getApplet(String name) - gives access to

other Applets running - can send messages

void showDocument(URL u) - asks browser to load this page

void showDocument(URL u, String relative)

AudioClip Interface

• Once retrieved, an audio clip can be accessed in the following ways:void play() - play the clip straight through

void loop() - play the clip over and over

void stop() - stop the clip

• Note: Java only understands clips in AU form

Images• Check out java.awt.image classes - lots of

stuff• Graphics methods for images:

boolean drawImage(Image im, int x, int y, ImageObserver ob) - can use container for ob

other args:im, x, y, int width, int height, obim, x, y, Color bgColor, obim, x, y, width, height, bgColor, obim, dx1, dy1, dx2, dy2, sx1, sy1, sx2, sy2, obd1,d2 and s1,s2 describe rectangles in the graphics

window (destination) and image (source)

Image Class

• No ctor, created with getImage() from Applet or createImage() from Container

• methods:void flush() - reload before drawing againGraphics getGraphics() - only for image from

createImage()int getHeight(ImageObserver ob) - may be -1 if

not finished loadingint getWidth(ImageObserver ob) - similar

Image Capabilities

• Animation through multiple images and threads

• Image processing (filters of various types, color processing, etc.)– check it out online