Java Applets and Graphics: Structure, Execution, and Drawing Shapes, Study notes of Computer Science

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Chapter 4

Introduction to

Applets and Graphics

Topics

• Applet Structure

• Executing an Applet

• Drawing Shapes with Graphics Methods

• Using Colors

Applets

• Executed by a browser or applet viewer

• Opens a window, as well as the Java

console

• Applet viewer comes with Java Software

Development Kit (SDK)

Applet Structure

• Do not use main method

• Two methods called automatically:

1. init method

• Browser calls init when applet starts

• Use to initialize variables and objects

2. paint method

• Browser calls after init and whenever

window needs to be redrawn

• Use to draw to screen

• See Example 4.01 ShellApplet.java

Executing an Applet

• A Web page tells the browser to run the

applet

• HTML tags come in pairs, data goes

between start and end tags

start and end of HTML code start and end of header text to display in title bar start and end of page content

Tag

CODE = Classname.class CODEBASE = ‘.’ directory of class file WIDTH = nnn width of window in pixels HEIGHT = nnn height of window in pixels

Minimal HTML File

** **

HTML File for FirstApplet

****

Executing an Applet

• If HTML file is named FirstApplet.html ,

you can execute the applet using this

command:

appletviewer FirstApplet.html

• Many IDEs automatically create and launch the

HTML file for running an applet

The 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

• Provides methods to draw shapes and text

on the window

The Graphics Coordinate System Graphics Class Methods

• Methods are available for drawing lines,

rectangles, ovals, and other shapes, and for

setting the current color

• All methods have a void return type, so

method calls are standalone statements

• draw… methods draw an outlined shape

• fill… methods draw a solid shape

Displaying Text

• Example:

g.drawString( "Hello", x, y );

• See Example 4.4 DrawingTextApplet.java

drawString ( String s, int x, int y ) displays the String s. The ( x, y ) coordinate is lower-left corner of first letter.

void

Return Method name and argument list type

Drawing a Line

g.drawLine( xStart, yStart, xEnd, yEnd );

• See Example 4.5 LineDrawingApplet.java

drawLine( int xStart, int yStart, int xEnd, int yEnd ) draws a line starting at ( xStart, yStart ) and ending at ( xEnd, yEnd )

void

Return Method name and argument list type

Drawing A Rectangle

g.drawRect( x, y, width, height );

drawRect( int x, int y, int width, int height )

draws an outlined rectangle with ( x,y ) as

the upper-left corner and the width and

height specified

void

Return Method name and argument list type

Drawing A Solid Rectangle

g.fillRect( x, y, width, height );

fillRect( int x, int y, int width, int height )

draws a solid rectangle in the current

color with ( x,y ) as the upper-left corner and

the width and height specified

void

Return Method name and argument list type

Drawing An Oval

g.drawOval( x, y, width, height );

drawOval( int x, int y, int width, int height )

draws an outlined oval within an

invisible bounding rectangle.

void

Return Method name and argument list type

Drawing A Solid Oval

g.fillOval( x, y, width, height );

fillOval( int x, int y, int width, int height )

draws a solid oval in the current color

inside an invisible bounding rectangle

void

Return Method name and argument list type

Drawing Squares and Circles

• To draw a square, use drawRect or fillRect

with equal values for width and height.

• To draw a circle, use drawOval or fillOval

with equal values for width and height

• See Example 4.4 ShapeDrawingApplet.java

• When drawing a figure using Graphics

methods, specify coordinate values as

offsets from a starting (x,y) coordinate.

• This will make your figure easier to move

or resize.

• See Example 4.7 Astronaut.java

Using Color

• The Graphics context has a current

foreground color

• All drawing is done in current color; the

current color is in effect until changed

• The default color is black.

• To use color, import the Color class from

the java.awt package

Setting the Current Color

Example:

g.setColor( Color.RED );

setColor( Color c) sets the current color to the Color c

void

Return Method name and argument list value

static Color Constants

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

Custom Colors

• Colors consist of red , green , and blue

components (RGB).

• Color constructor:

• Example:

Color green = new Color( 0, 255, 0 );

• See Example 4.8 AstronautWithColor.java

Color( int rr, int gg, int bb ) creates a color consisting of the red ( rr ), green ( gg ), and blue ( bb ) values specified. rr , gg , and bb must be between 0 and 255

Homework

• Solve the following Problems:

• Number 31 Page 31

• Number 35 Page 203