Beginning Java 2- JDK 1.3 Version
Images and Animation
HTML for Applets
In this section we will show HTML tags and attributes in upper case to highlight
them, although they are not case sensitive. The basic HTML tags you need for
embedding an applet in a Web page are as follows:
<APPLET CODE = "AppletName.class"
WIDTH = "Applet_width_in_pixels"
HEIGHT = "Applet_height_in_pixels" >
Optional alternate text displayed if the
APPLET tag is not supported
</APPLET>
The optional default text after the APPLET tag is useful to signal
when a browser does not support the APPLET tag – when the applet
will not be executed. Other than that, the HTML tags above are the minimum
required to include an applet in a page. The value for the CODE
attribute in the APPLET tag identifies the .class file
for the applet. This presumes the applet is in the same directory as the page
containing the applet. If this is not the case you must not put the path as part
of the CODE value. Instead, you should add a CODEBASE
attribute that has a URL as a value that identifies the source for the applet –
the URL can be absolute (which would start with "http://") or it
can be relative to the directory containing the Web page. While the
WIDTH and HEIGHT attributes are mandatory, they only
determine the initial space allocated to the applet. The applet can adjust these
itself, as we shall see in this chapter. The double quotes around the parameter
values can be omitted if the parameter value does not contain spaces or other
characters that might cause confusion.
You can also specify parameter values to be read by your applet by placing
<PARAM> tags between the <APPLET> and
</APPLET> entries, one for each parameter. For example:
<APPLET CODE=MyApplet.class WIDTH=200 HEIGHT=100>
<PARAM NAME="frameRate" VALUE="10">
<PARAM NAME="description" VALUE="Some text">
</APPLET>
These tags will execute the MyApplet applet from the directory
containing the HTML file, and make a parameter with the name
frameRate available with the value "10", plus a second
parameter, description, with the value "Some text".
These values can be retrieved programmatically from within the applet – usually
from within the init() method – by calling the
getParameter() method with a String defining the
parameter name for which the value is requested as the argument. If it is
available, the parameter value is returned as a String, which you
can then convert to numeric form where necessary. For example, to retrieve the
value of the parameter with the name, frameRate, we could write:
// Default frame rate
int frameRate = 5;
// Get the value of the parameter frameRate
String value = getParameter("frameRate");
if(value != null)
frameRate = Integer.parseInt(value);
It is important to verify that the String returned by the
getParameter() method is not null. It will be if there
is no PARAM tag specifying the value. Note that the parameter name
is not case sensitive, so "FRAMERATE" would specify the same
parameter as "framerate".
When your applet expects to read parameter values, it is a good idea to override
the default getParameterInfo() method in your applet class to
identify their names, their types and their description in an array of strings.
The version of the method that your applet class will inherit from the
Applet class returns null. This method can be used by
the applet context (the Web page) to find out about your applet. For the applet
MyApplet, that expects values for the parameters
frameRate and description, you would implement the
getParamterInfo() method as:
public String[][] getParameterInfo()
{
String[][] pInfo = {
{ "frameRate", "integer", "The frames per second" },
{ "description", "String", "Description of the animation"}
};
return pInfo;
}
There must be three elements in each row of the array providing information
about a particular parameter. The three elements in a row describe the parameter
name, the type of value, and the purpose of the parameter respectively.
Remember to use the OBJECT and EMBED tags as shown in
Chapter 1 to add Java 2 support to browsers other than
appletviewer.
If you are using this book in non-linear fashion you will find more on deploying
applets at the end of Chapter 12 where I also briefly discuss the security
issues relating to applets. This is an extensive topic and a thorough discussion
of it is beyond the scope of this book. For the most up-to-date documentation
regarding the new security architecture in Java 2, refer to the Web pages at:
http://java.sun.com/security/
Let's now turn to the fundamentals of dealing with images, and then look at how
we can implement an applet from the ground up – this time to handle an image.
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