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JAVA Programing
What is an applet in Java?
An applet in Java is a small program that is designed to run within a web browser. It is written in Java programming language and is designed to be embedded in an HTML page. Applets can be used to add interactive features to a web page, such as animations, games, or data entry forms.
Applets run within a Java Virtual Machine (JVM), which is a software platform that allows Java programs to run on a variety of operating systems. When an applet is loaded into a web page, the browser downloads the Java code and runs it within the JVM.
Applets were once popular for creating interactive content on the web, but their usage has declined in recent years due to security concerns and the rise of other web technologies such as JavaScript and HTML5. However, applets can still be used in certain contexts, such as in enterprise applications or in educational settings.
How does an applet differ from a standalone Java application?
An applet in Java differs from a standalone Java application in several ways:
Execution environment: An applet runs within a web browser, while a standalone Java application runs on a computer as a standalone program.
User interface: An applet typically has a graphical user interface (GUI) that is designed to integrate with a web page, while a standalone Java application has its own standalone GUI.
Security: Because applets run within a web browser, they are subject to certain security restrictions that do not apply to standalone Java applications. For example, applets may be limited in their ability to access local resources on the user’s computer, such as files or network resources.
Deployment: Applets are typically deployed over the internet, and are loaded dynamically into a web page when the user requests them. Standalone Java applications, on the other hand, are typically distributed as executable files that must be downloaded and installed on the user’s computer.
Size and complexity: Applets are generally smaller and less complex than standalone Java applications, since they are designed to run within the constraints of a web browser environment.
Overall, while both applets and standalone Java applications can be used to create Java programs, they are designed for different use cases and have different capabilities and limitations.
How do you create an applet in Java?
To create an applet in Java, you can follow these steps:
Create a new Java source code file and save it with a .java extension.
Define the applet class by extending the java.applet.Applet class. This class provides the basic functionality required for an applet.
Override the init() method to perform any initialization required by your applet. This method is called when the applet is first loaded.
Override the paint() method to define the applet’s graphical user interface. This method is called whenever the applet needs to be redrawn.
Optionally, you can override other methods such as start(), stop(), and destroy() to perform additional actions at different stages of the applet’s lifecycle.
Here is an example of a basic applet program in Java:
import java.applet.Applet;
import java.awt.Graphics;
public class MyApplet extends Applet {
public void init() {
// Initialization code goes here
}
public void paint(Graphics g) {
// GUI code goes here
}
}
Once you have written your applet program, you need to compile it using a Java compiler. After the code has been compiled, you can embed the applet in an HTML page using the <applet> tag.
<applet code="MyApplet.class" width="200" height="200">
Your browser does not support the <applet> tag.
</applet>
When the HTML page is loaded in a web browser, the applet will be loaded and displayed within the page.
What is the lifecycle of an applet?
The lifecycle of an applet in Java consists of four main stages:
Initialization: When the applet is first loaded into a web page, the init() method is called. This method is used to initialize the applet and set up any necessary resources.
Start: After the applet has been initialized, the start() method is called. This method is used to start any threads or other background activities that the applet requires.
Stop: When the user navigates away from the web page containing the applet, the stop() method is called. This method is used to stop any background activities that were started in the start() method.
Destruction: When the applet is no longer needed (for example, when the user closes the web page), the destroy() method is called. This method is used to release any resources that were acquired by the applet during its lifecycle.
In addition to these main stages, an applet may also be paused and resumed using the methods pause() and resume(). These methods are called when the applet is hidden or shown, respectively, and can be used to save and restore the applet’s state.
It’s worth noting that the exact order and timing of these methods may vary depending on the browser and operating system being used. Applets may also be subject to various security restrictions, which can affect their behavior and lifecycle.
What are the methods used in the lifecycle of an applet?
The lifecycle of an applet in Java involves several methods, which are called at different stages of the applet’s lifecycle. Here are the main methods used in the lifecycle of an applet:
init(): This method is called when the applet is first loaded into the web page. It is used to initialize the applet and set up any resources that the applet will need.start(): This method is called after theinit()method has completed. It is used to start any background activities that the applet requires, such as animations or network connections.stop(): This method is called when the user navigates away from the web page or otherwise hides the applet. It is used to stop any background activities that were started in thestart()method.destroy(): This method is called when the applet is no longer needed, such as when the user closes the web page containing the applet. It is used to release any resources that were acquired by the applet during its lifecycle.
In addition to these main methods, there are two other methods that can be used to pause and resume the applet:
pause(): This method is called when the applet is hidden, such as when the user minimizes the web browser. It is used to save the applet’s state and stop any ongoing activities.resume(): This method is called when the applet is shown again, such as when the user restores the web browser. It is used to restore the applet’s state and resume any activities that were stopped in thepause()method.
By implementing these methods in your applet code, you can control the applet’s behavior and manage its resources throughout its lifecycle.
How do you pass parameters to an applet?
You can pass parameters to an applet in Java by using the <param> tag in the HTML code that embeds the applet. Here’s an example of how to do it:
In your Java applet code, define variables to hold the parameter values. For example:
public class MyJavaApplet extends Applet {
private String myParameter;
...
}
In your HTML code, use the <applet> tag to embed the applet, and use the <param> tag to pass the parameter values. For example:
<applet code="MyJavaApplet.class" width="300" height="200">
<param name="myParameter" value="Hello, world!">
</applet>
In this example, the <param> tag specifies the name of the parameter (“myParameter”) and its value (“Hello, world!”). The applet can then access the parameter value using the getParameter() method, like this:
public void init() {
myParameter = getParameter("myParameter");
...
}
In this way, you can pass any number of parameters to your applet using the <param> tag in your HTML code.
How do you communicate between an applet and a web page?
To communicate between an applet and a web page in Java, you can use JavaScript and the Java-Script interface. Here’s an example of how to do it:
In your Java applet code, define a method that will receive data from the web page. For example:
public class MyJavaApplet extends Applet {
...
public void setData(String data) {
// Do something with the data
...
}
...
}
In your web page code, use JavaScript to call the applet’s setData() method and pass it the data you want to send. For example:
<script>
var applet = document.getElementById("myJavaApplet");
applet.setData("Hello, world!");
</script>
In this example, the JavaScript code uses the document.getElementById() method to get a reference to the applet (which has an ID of “myJavaApplet”), and then calls the setData() method to send it some data.
To allow the applet to call JavaScript methods on the web page, define a JavaScript method in your web page code that the applet can call. For example:
<script>
function showAlert(message) {
alert(message);
}
</script>
In this example, the showAlert() method simply displays an alert dialog box with the message that is passed to it.
In your Java applet code, use the Java-Script interface to call the JavaScript method on the web page. For example:
public class MyJavaApplet extends Applet {
...
public void showAlert(String message) {
JSObject window = JSObject.getWindow(this);
window.eval("showAlert('" + message + "')");
}
...
}
In this example, the showAlert() method uses the JSObject class to get a reference to the web page’s window object, and then calls the eval() method to execute the showAlert() method on the web page, passing it the message that was passed to the applet.
By using this technique, you can easily communicate between your Java applet and your web page, allowing them to share data and interact with each other as needed.
Questions on Chapter 21