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JAVA Programming
Can you explain the use of the showDocument method in the AppletContext class in Java Applets and give an example of its usage?
The showDocument method is used to display a new web page in the user’s web browser. This method is part of the AppletContext interface, which is used by an applet to communicate with its environment. The showDocument method takes a single parameter, which is a URL object that specifies the address of the web page to display.
Here is an example of using the showDocument method in a Java Applet:
import java.applet.Applet;
import java.awt.Button;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.net.MalformedURLException;
import java.net.URL;
public class MyFirstApplet extends Applet implements ActionListener {
private Button button;
public void init() {
button = new Button("Go to Google");
button.addActionListener(this);
add(button);
}
public void actionPerformed(ActionEvent e) {
String url = "https://www.google.com/";
try {
AppletContext context = getAppletContext();
URL urlObj = new URL(url);
context.showDocument(urlObj);
} catch (MalformedURLException ex) {
ex.printStackTrace();
}
}
}
In this example, the applet creates a button that, when clicked, navigates to the Google website using the showDocument method. The getAppletContext method is used to obtain a reference to the AppletContext object, which is then used to call the showDocument method with the URL of the Google website.
When the user clicks the button, the actionPerformed method is called, which constructs a URL object from the String URL of the Google website. This URL object is then passed as a parameter to the showDocument method of the AppletContext object, which displays the Google website in the user’s web browser.
How do you pass data between Applets on the same page, and what is the purpose of the getAppletContext method in an Applet?
In order to pass data between Applets on the same page, the AppletContext class can be used. The AppletContext allows Applets on the same page to communicate with each other by passing data through the getApplet() method.
The getApplet() method returns an Applet object based on its name. Once the Applet object is returned, you can call any public methods of that Applet to pass data between the Applets.
Here is an example:
// In the first Applet
public void sendDataToOtherApplet() {
Applet otherApplet = getAppletContext().getApplet("secondApplet");
if (otherApplet != null && otherApplet instanceof SecondApplet) {
((SecondApplet) otherApplet).receiveDataFromOtherApplet(data);
}
}
// In the second Applet
public void receiveDataFromOtherApplet(Object data) {
// process the data received from the first Applet
}
In this example, the sendDataToOtherApplet() method in the first Applet uses the getApplet() method to get a reference to the second Applet by its name, “secondApplet”. If the Applet is found and is an instance of the SecondApplet class, it calls the receiveDataFromOtherApplet() method in the second Applet to pass the data.
The getAppletContext() method is used to obtain a reference to the AppletContext object associated with the current Applet. The AppletContext provides a way to access and control other Applets on the same page.
Can you give an example of using the AppletStub interface in Java Applets for providing runtime information to an Applet?
Here’s an example of using the AppletStub interface in Java Applets:
import java.applet.*;
import java.awt.*;
import java.net.*;
public class MyApplet extends Applet implements AppletStub {
private String parameter1;
private String parameter2;
public void init() {
// Get the value of parameter1 from the HTML page
parameter1 = getParameter("parameter1");
// Get the value of parameter2 from the HTML page
parameter2 = getParameter("parameter2");
// Set the layout manager for the applet
setLayout(new BorderLayout());
// Add components to the applet
add(new Label("Parameter 1: " + parameter1), BorderLayout.NORTH);
add(new Label("Parameter 2: " + parameter2), BorderLayout.CENTER);
}
public URL getDocumentBase() {
// Return the URL of the HTML page containing the applet
return getCodeBase();
}
public String getParameter(String name) {
// Return the value of the specified parameter from the HTML page
String value = super.getParameter(name);
if (value == null) {
value = "";
}
return value;
}
public boolean isActive() {
// Return true if the applet is currently active, false otherwise
return true;
}
}
In this example, we implement the AppletStub interface in the MyApplet class. The init method of the MyApplet class retrieves the values of parameter1 and parameter2 from the HTML page using the getParameter method. The getDocumentBase method returns the URL of the HTML page containing the applet, and the isActive method returns true since the applet is always active.
By implementing the AppletStub interface, the MyApplet class can provide additional runtime information to the applet, such as the URL of the HTML page and the values of any parameters passed to the applet.
What is the purpose of the AppletInitialPermission interface in Java Applets and when is it used?
The AppletInitialPermission interface is used in Java Applets to grant permissions to an Applet at runtime. It is a security feature in Java that restricts Applets from performing certain actions that can be potentially harmful, such as accessing the user’s local file system, network resources, and so on.
When an Applet is loaded from a web page, the Java Virtual Machine (JVM) checks whether the Applet has been granted the necessary permissions to perform certain actions. If the Applet does not have the required permissions, it will be prevented from performing those actions. In some cases, the user may be prompted to grant the required permissions to the Applet.
The AppletInitialPermission interface is used to grant permissions to an Applet at the time of loading. It is implemented by the Applet’s container and is used to provide information to the JVM about the permissions that the Applet requires. The Applet container can implement the AppletInitialPermission interface and override the getPermissions() method to grant permissions to the Applet. The getPermissions() method returns a PermissionCollection object that contains the permissions that the Applet requires.
Here is an example of implementing the AppletInitialPermission interface in a Java Applet:
import java.applet.Applet;
import java.security.PermissionCollection;
import java.security.Permissions;
import java.security.Policy;
public class MyCustomApplet extends Applet implements AppletInitialPermission {
public void init() {
// Initialize the Applet
}
public PermissionCollection getPermissions() {
Permissions permissions = new Permissions();
permissions.add(new java.net.SocketPermission("localhost:8080", "connect"));
permissions.add(new java.io.FilePermission("/tmp/*", "read"));
return permissions;
}
}
In this example, the getPermissions() method is overridden to grant the Applet permissions to access the localhost server on port 8080 and to read files from the /tmp directory.
Questions on Chapter 15