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Java Applets

In this section we will cover Applet Structure, Executing an Applet, Drawing shapes with Graphics Methods and Using Color and Font classes.


Applet Environment

An applet is a small program that is intended to be embedded inside another application such as a browser. The JApplet class must be the superclass of any applet that is to be embedded in a Web page or viewed by the Java Applet Viewer (appletviewer.exe). The JApplet class provides a standard interface between applets and their environment. JApplet hierarchy is as follows:

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java.lang.Object
  java.awt.Component
    java.awt.Container
      java.awt.Panel
        java.applet.Applet
          javax.swing.JApplet


Applets Structure

- There is no main method.
- Two methods that are called automatically- init() and paint()
- The init method initializes variables and objects; if you don't have one you will inherit one from the JApplet class.
- Use paint to draw screen
- A lot of methods exist in JApplet class so the "extends" keyword inherits everything that the class has. In the above example JApplet is parent class and shellapplet is the subclass so use the keyword "extends" to create inheritance.
- You have to import JApplet and java.awt.Graphics (abstract windowing toolkit) to get Graphics to paint.
- All applets must inherit JApplet
- Use the "super" keyword in subclass to invoke method in the superclass.
- super.paint( g ); this says uses the paint method from JApplet in my paint class and I'm not adding anything to it.
Exampl

import javax.swing.JApplet;
import java.awt.Graphics;

public class AnyApplet extends JApplet
{
        // declare variables here

      public void init( )
      {
               // data initialization goes here
      }

     public void paint( Graphics g )
    {
             super.paint( g );
    // your code goes here
   }
}


- The window gets drawn by invoking the paint class of the applet, which calls the superclass paint which draws the actual window you've given the size for.

- Other applet methods

  • repaint() – repaints when the window which the applet resides re-gains focus
     

  • stop() called when applet is no longer visible. Signature: public void stop(). Stopt is called by the browser or applet viewer to inform this applet that it should stop its execution. It is called when the Web page that contains this applet has been replaced by another page, and also just before the applet is to be destroyed. A subclass of JApplet should override this method if it has any operation that it wants to perform each time the Web page containing it is no longer visible. The implementation of stop method provided by the JApplet class does nothing.
     

  • start() called when the applet is visible. The signature for the start method is as follows: public void start(). Start is called by the browser or applet viewer to inform this applet that it should start its execution. It is called after the init method and each time the applet is revisited in a Web page. A subclass of JApplet should override this method if it has any operation that it wants to perform each time the Web page containing it is visited. The implementation of start method provided by the JApplet class does nothing.
     

  • destroy() when hosting window is closed (exit). Tha signature is as follows: public void destroy(). Destory method is called by the browser or applet viewer to inform this applet that it is being cultivated and that it should destroy any resources that it has allocated. The stop method will always be called before destroy. A subclass of JApplet should override this method if it has any operation that it wants to perform before it is destroyed. The implementation of destory method provided by the JApplet class does nothing.

     

Running an Applet

We need to use html code to run applets. The minimum html required to run applet with a browser (java host) is as follows:


<HTML>
<HEAD>
<TITLE>TitleName</TITLE>
</HEAD>
<BODY>
<APPLET>
    CODE = Classname.class
    CODEBASE = . directory of class file
    WIDTH = 50 width of window in pixels
    HEIGHT = 50 height of window in pixels

</APPLET>
</BODY>
</HTML>

Note that in the applet tag you include the. class bytecode file and not the .java.

Graphics Class

- Browser or appletviewer sends a Graphics object to the paint method.
- The Graphics object represents the applet window, current font, and current color and provides methods to draw shapes (rectangles, triangles, circles, etc.) and text on the window.

The Graphics Class Coordinate System

- Never use 0,0 because the applet itself uses it for the title bar.

Graphics class methods

- No return type so they are all void
- Draw ()… methods draw an outlined share
- Fill ()… methods draw a solid shapeDisplaying text use g.drawString( "string", x, y); Need to draw the strings.
- Draw a lin g.drawLine( xStart, yStart, xEnd, yEnd );
- How do you draw a square? You need to make multiple lines that attach at the points.
Drawing a rectangle, Use pixels for height and width and an anchoring corner.
Drawing an oval, drawn inside an invisible rectangle. Give rectangle coordinates.
Graphics methods, Use offsets to make your figure easier to move resize.


import javax.swing.JApplet;
import java.awt.Graphics;

public class ShapesApplet extends JApplet
{
  public void paint( Graphics g )
  {
    super.paint( g );
    g.drawRect( 900, 40, 30, 900 );
    g.fillRect( 200, 70, 80, 80 );
    g.fillOval( 100, 50, 40, 100 );
    g.drawOval( 100, 200, 100, 40 );
    int X = 250, Y = 225;
    int r = 25;
   g.drawOval( X - r, Y - r, r * 2, r * 2 );
 }
}


- Draw a rectangle g.drawRect( x, y, width, height );
- Draw a solid recta ngle: g.fillRect( x, y, width, height );
- Draw am oval: g.drawOval( x, y, width, height );








- Draw a filled (solid) oval: g.fillOval( x, y, width, height );


Using the Color class

  • Found in the Java.awt

  • All drawing is done in the current color

  • Default color is black

  • On the Graphics object method called setColor. This takes a Color object. Example:
    g.setColor( Color.RED );

  •  Many different constants in the color class defining many preset colors

  • Colors consist of red, green and blue components (RGB)

  • You may create custom colors in the Color class by passing in three integers for red, green and blue. Example:
    Color green = new Color(0,255,0);

  • Static color Constants defined in the Color class
    Color.BLACK                Color.GRAY
    Color.WHITE               Color.ORANGE
    Color.RED                   Color.YELLOW
    Color.GREEN               Color.PINK
    Color.BLUE                 Color.MAGENTA
    Color.CYAN
    Color.LIGHT_GRAY
    Color.DARK_GRAY

Using the Font class

  • Also found in Java.awt

  • Works just like color, in that color, in that the drawing of strings is done in the current font. Example:
        Font myFont = new Font(“TimesRoman” Font.ITALIC, 28);
        g.Font.setFont(myFont);
        Font(string fontName, int fontStyle, int fontSize)

    Example

    import java.awt.*;
    class FontsExample extends JApplet {
       public void paint(Graphics g) {
       g.setFont(new Font("TimesRoman", Font.ITALIC, 28));
       g.drawString("Some Examples of Fonts", 20, 45);
       g.setFont(new Font("Helvetica", Font.PLAIN, 12));
       g.drawString("This is an example of plain 12pt Helvetica font", 20, 70);
       g.setFont(new Font("TimesRoman", Font.PLAIN, 12));
       g.drawString("This is an example of plain 12pt TimesRoman font", 20, 90);
      }
    }

     

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