Applets:
- Applet allows java programmer to implement GUI programming.
- “applet” is a package in java API.
- “Applet” is a pre-defined class provides the functionality to user-defined Applet classes.
- “Applet” is a window where we can arrange all the components to implement GUI.
- Every User-defined Applet class must extends pre-defined Applet.
class MyApplet extends java.applet.Applet
{
//user-defined applet class.
}
AWT :
- Stands for Abstract Window Toolkit.
- A set of Pre-defined(Abstract) classes and interfaces to implement Applets(Window programming) available as a Toolkit.
- for example, Button, CheckBox, Paint, Graphics………
First Applet Program :
- Applets are defined to be embedded into HTML pages.
- Applets can be run by the browser interpreter along with HTML tags.
- Every browser having an interpreter to execute HTML code.
- HTML code will not be compiled, it will be executed directly by the interpreter.
- Hence it is hard to find if error has encountered.
- Intrepreter produces un-expected output page if any error in the HTML code.
- JVM cannot execute applet program, hence no need to define main() method inside the applet class.
- public void paint() method is the pre-defined method in Applet class.
- Graphics class which is the pre-defined class available in java.awt package.
import java.applet.Applet;
import java.awt.*;
public class FirstApplet extends Applet
{
public void paint(Graphics g)
{
g.drawString("Hello world", 200,200);
}
}
/*
<applet code = "FirstApplet.class" width="600" height = "600">
</applet>
*/
Running Applets from HTML pages :
<HTML>
<HEAD>
<title>My Applet Program</title>
</HEAD>
<BODY>
<applet code = "FirstApplet.class" width="600" height = "600">
</applet>
</BODY>
</HTML>
Applet Life cycle :
- Some of the pre-defined funtions of Applet executes implicitly which can describe the flow of Applet Life cycle.
- CUI based java application execution starts from main() function which is implicitly invokes my JVM.
- GUI based java application will be invoked either by “appletviewer” or “browser interpreter”.
- When applet execution starts, it will instantiated implicitly.
- At the time of instantiation, constructor must be invoked.
- Hence the only way to know whether the Applet has been instantiated or not by defining a constructor in the Applet class.
- Members participate in Applet Life cycle :
- Constructor()
- init()
- start()
- paint()
- stop()
- destory()
import java.applet.Applet;
import java.awt.*;
public class FirstApplet extends Applet
{
public FirstApplet()
{
System.out.println("Applet is instantiated....");
}
public void paint(Graphics g)
{
g.drawString("Hello world", 200,200);
}
public void init()
{
System.out.println("Applet is initialized....");
}
public void start()
{
System.out.println("Control enter into Applet....");
}
public void stop()
{
System.out.println("Control goes off the Applet...");
}
public void destroy()
{
System.out.println("Resource released.....");
}
}
/*
<applet code = "FirstApplet.class" width="600" height = "600">
</applet>
*/
Layout :
- Arrangement of Components by setting Bounds on the window.
- Java API has many pre-defined layout classes to arrange the components.
- Flow Layout
- Grid Layout
- Border Layout
- Box layout….
Note : By default Applet arranges the components using FlowLayout structure.
import java.applet.Applet;
import java.awt.*;
public class Flow extends Applet
{
public void init()
{
//this.setLayout(new FlowLayout(FlowLayout.LEFT));
for(int i=1 ; i<=5 ; i++)
{
Button b = new Button("BUTTON#"+i);
this.add(b);
}
}
}
/*
<applet code = "Flow.class" width="600" height = "600">
</applet>
*/
Event Listeners:
- An Event is an operation(action) performed on a component of Applet.
- When action performed, Component fires one event.
- Listener class can collect event information and reacts to the event according to defined logic.
- “Listener” is a pre-defined interface of all the sub Listeners which are….
- ActionListener
- TextListener
- MouseListener
- MouseMotionListener
- FocusListener
- When we create Buttons, these cannot be reacted to Events which are performed.
- ActionListener interface must be implemented to write EventListener logic for Buttons.
import java.applet.Applet;
import java.awt.*;
import java.awt.event.*;
public class ButtonListener extends Applet implements ActionListener
{
Button b1,b2;
public void init()
{
this.setLayout(null);
this.b1 = new Button("BUTTON#1");
this.b2 = new Button("BUTTON#2");
b1.setBounds(150,200,150,50);
b2.setBounds(300,200,150,50);
this.add(b1);
this.add(b2);
b1.addActionListener(this);
b2.addActionListener(this);
}
public void actionPerformed(ActionEvent ae)
{
if(ae.getSource() == b1)
System.out.println("you clicked button1");
else
System.out.println("you clicked button2");
}
}
/*
<applet code = "ButtonListener.class" width="600" height = "600">
</applet>
*/
Frames :
- Applets are designed to embed with HTML pages and these can be run by Browser Interpreter.
- Frames are designed to implements Standalong GUI java applications run by JVM directly.
- main() is allowed while implementing Frames.
import java.applet.Applet;
import java.awt.*;
import java.awt.event.*;
public class RadioButton extends Frame
{
Checkbox r1,r2;
CheckboxGroup rg;
TextField t;
public RadioButton(String name)
{
this.setTitle(name);
this.setSize(600,600);
this.setLayout(new FlowLayout());
rg = new CheckboxGroup();
r1 = new Checkbox("RED",rg,false);
r2 = new Checkbox("BLUE",rg,true);
t = new TextField("A TextField",20);
this.add(r1);
this.add(r2);
this.add(t);
}
public static void main(String args[ ])
{
RadioButton r = new RadioButton("MY Frame");
r.show();
}
}
