Displaying HTML documents and accessing the web - An Introduction to Graphical User Interfaces with Java Swing - page 260

28.

public static void main(String[] args){

29.

SimpleHTMLFrame shf = new SimpleHTMLFrame(htmlSource);

30.

shf.showIt("Simple HTML-Display");

31.

}

32.

// Later, the link listener class will be defined here.

33. }

Program SimpleHTMLFrame displays a single HTML document but it does not use
the most important HTML feature: links ( HTML hyperlinks ). When clicking on a
link, nothing happens. In the next section we see how links can be followed.

■ 22.2 Using HTML links

In order to trigger a reaction when clicking on a link we need a listener. In Java the
interface HyperlinkListener is responsible for monitoring HTML hyperlinks. It
is found in the javax.swing.event library and requires the implementation of a
single method:

void hyperlinkUpdate(HyperlinkEvent hylevt)

The listener is assigned to that component which displays the HTML document, in
our case the JEditorPanel ediPane . The listener is assigned using the following
command:

ediPane.addHyperlinkListener(HyperlinkListener hyLis)

After the listener is assigned to the panel, it is notified by the runtime system if a
link is clicked on, i.e. its hyperlinkUpdate method is called. The runtime system
also generates a HyperlinkEvent object and passes it to hyperlinkUpdate as an
argument.
The programmer has to insert the code into hyperlinkUpdate which is to be
executed as a reaction to selecting the link. In our example we want to display that
page to which the link refers. We first check whether the event type is ACTIVATED ,
i.e. whether the link has been selected. Other event types are ENTERED and EXITED .
This test is done by inspecting the hyperlink event object hylevt

if (hylevt.getEventType() == HyperlinkEvent.EventType.ACTIVATED)

Next, we extract the URL to which the link is pointing:

URL newPage = hylevt.getURL()

Then this URL is passed to the editor pane by:

ediPanel.setPage(newPage)

The new web page is displayed in the editor pane. Methods that access web
pages can throw exceptions, therefore calls of such methods have to be embedded
into try-catch blocks. In our example, we write a message to the console if an