Major Graphic User Interface (GUI) Objects

Class Hierarchy in Package java.awt

                        java.lang.Object                    
       -------------------------------------------------------
          /                   |                   \ 
         /                    |                    \
        /                     |                  java.awt.Graphics (abstract)
       /                      |                  java.awt.Color
java.util.EventObject   java.awt.Component       java.awt.Font
     /                   (abstract)              java.awt.Image
    /                      /     \               java.awt.BorderLayout
   /                      /       \              java.awt.MenuComponent
  /                      /         \
 /                      /           \ 
java.awt.AWTEvent    Container    Button,Canvas,Checkbox,Choice, 
                     (abstract)   Label,List,Scrollbar,TextComponent
                      /   \       Label,List,Scrollbar,   TextComponent
                     /     \                                  /  \
                    /       \                                /    \
                 Panel    Window                            /      \
                  /           \                            /        \ 
                 /             \                          /          \
                /             Frame                   TextField     TextArea
               /   
           Applete       
     (not in the package)

Contains all of the classes for creating user interfaces and for painting graphics and images.

• The abstract Graphics class is a base class for all graphics contexts that allow an application to draw. A Graphics object encapsulates state information (e.g., color, font) needed for the basic rendering operations that Java supports.
  
  
• The abstract class Component is the root of all AWT components. A user interface object such as a button or a scrollbar is called, in AWT terminology, a component. Methods of this class include paint( Graphics g), update().
  
  
• Some components fire events when a user interacts with the components. The AWTEvent class and its subclasses are used to represent the events that AWT components can fire. The Component class has various methods to add listeners for different events.
  
  
• The abstract class Container represents components that can contain components and other containers. Components added to a container are tracked in a list. It has an overloaded method add(....) to add components into the container. It also has a method paint(Graphics g) and a method update(), which override the methods in the parent class (i.e., the Component class).
  
  
• A container can also have a layout manager that controls the visual placement of components in the container. The AWT package contains several layout manager classes and an interface for building your own layout manager.
  
  
• Class Panel is the simplest derived class of Container. It provides space in which an application can attach any other component, including other panels. It does not have many methods, but it inherits all the methods from the parent classes.
  
  
• Class Window represents a top-level window with no borders and no menubar. A Window object is an independent space on the screen which can not be contained in a Panel.
  
  
• Class Frame represents a top-level window with a title and a border.
  
  

Major Classes in Package Java.awt

Java Applet

• Class Applet is a derived class of java.awt.Panel.

  java.lang.Object
    |
    |_ java.awt.Component
        |
        |_ java.awt.Container
            |
            |_ java.awt.Panel
                |
                |_ java.applet.Applet
  

• It is in package java.applet.
  
  
• You need to write your class which extends the Applet class. Your class is called an applet.
  
  
• An applet does not need a main() method.
  
  
• An applet is a small program that is intended not to be run on its own, but rather to be embedded inside another application.
  
  
• The applet framework involves two entities: the applet and the applet context.
  • An applet is an embeddable window (see the Panel class) with a few extra methods.
  • The applet context is an application that is responsible for loading and running applets. For example, the applet context could be a Web browser or an applet development environment.
  • The applet context makes a (fixed) sequence of calls to initialize, start, and stop the applet.
  
  
  
• In Applet class, all methods are realized. In your class, you do not have to override any methods of the parent classes. (But it does nothing for you in this case).
  
  
• But we have to override some of the following methods (in order for it to do something useful):
  
  • public void init():
    called (only once) after the applet is loaded. You initialize your datamembers, set the fonts etc. inside this method.
    public void start():
    called after init(). It is also called whenever the page is moved to the back and then to the front, i.e., the applet is made active. In the implementation of the Applet class, this method simply calls the paint() method.
    
    
  • public void paint(Graphics g):
    called by start(). This method is where you produce the graphic output. The argument to this method is passed from the applet context, e.g., the browsers.
    
    
  • public void stop():
    called when the page is made inactive.
  • public void destroy():
    called when the browser is exited. You need to stop all the active threads.
    
    

import java.awt.*;
import java.applet.*;


public class AppletTest extends Applet {

  int x;

  public void init() {
      x = 5;
  }

  public void paint (Graphics g) {
      g.setColor(Color.blue);
      g.drawRect(10,10,50,50);

      g.setColor(Color.red);
      g.fillRect(10,70,50,50);

      g.setColor(Color.yellow);
      g.drawRoundRect(70,10,50,50,25,25);

      g.setColor(Color.blue);
      g.fillRoundRect(70,70,50,50,25,25);

      g.setColor(Color.cyan);
      g.draw3DRect(130,10,70,50,false);

      g.setColor(Color.green);
      g.fill3DRect(130,70,70,50,true);

      g.setColor(Color.red);
      g.drawString("x="+x,100,150);
  }

}

<html>
<head>
  <title>Applet Test</title>
</head>
<body>

    <applet code="AppletTest.class" width="400" height="400"> </applet> 

</body>
</html>