applets csc 171 fall 2004 lecture 6. applets graphical java programs run inside web browser...
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APPLETSAPPLETS
CSC 171 FALL 2004
LECTURE 6
APPLETSAPPLETS
Graphical Java programsRun inside web browser Platform-neutral Easy deployment--loads when needed Secure
AppletsAppletsApplets are Java prgrams that can be
embedded in Hypertext Markup Language (HTML) documents
The browser that executes an applet is generically knows as the applet container
So, often we use appletviewer
How Applets WorkHow Applets Work
HTMLHTML
Text and tags:Java is an <i>object-oriented</i> programming language
Browser renders the tags:Java is an object-oriented programming language
Bulleted list (like this one) is defined by tags<ul><li>. . .</li><li>. . .</li><li>. . .</li></ul>
Use <> for <> symbols
Simple HTMLSimple HTML
<html>
<head>
<title> Ted Pawlicki's CSC 171 HTML</title>
</head>
<body>
Hello Ted!
</applet>
</html>
Applet MethodsApplet Methods
Applets have no “main()”Rather, it has key methods that deal with
the unique situation of applets
Key Applet MethodsKey Applet Methods
initstartpaintstopdestroy
Key Applet MethodsKey Applet Methods
public void init()public void start()public void paint(Graphics g)public void stop()public void destroy()
public void init()public void init()
Called once by appletviewer or browserCalled when an applet is loadedPerforms initialization
– Instance variables– GUI components– Loading sounds & images (multimedia)– Creation of threads (animation)
public void start()public void start()
Called after the init method completesCalled every time the user of the browser
returns to the HTML pagePerforms tasks that must be repeated when
the applet is revisited– Restarting an animation
public void paint(Graphics g)public void paint(Graphics g)
Called after init() method completes and start() has started
“Draws stuff” on the appletThe system hands the applet a Graphics
object to draw stuff onAutomatically recalled whenever the applet
needs to be repainted– uncovering a covered window
public void stop()public void stop()
Called when the applet should stop executing– When the user leaves the HTML page
Perform tasks necessary to suspend applet– “pausing” animations
public void destroy()public void destroy()
Called when the applet is removed from memory
Performs tasks to de-allocate resources
repaint()repaint() Paint is normally called by the applet container We sometimes change the applet’s appearance
based on user input We might like to call paint() directly However, in order to call paint, we have to pass it
a Graphics object But the container owns the Graphics object repaint() invokes update() which invokes paint
with the appropriate graphics object
Some HTMLSome HTML
<html>
<head>
<title> Ted Pawlicki's CSC 171 First Applet</title>
</head>
<body>
<applet code=“FirstApplet.class" width = 256 height = 550 >
</applet>
</html>
Some JavaSome Javaimport java.awt.*;import java.applet.Applet;public class FirstApplet extends Applet {
public void paint(Graphics g) {g.drawString(“Hello Ted!", 20, 40);
}}
Cooler GraphicsCooler Graphics
class MyApplet extends Applet {public void paint(Graphics g){ Graphics2D g2 = (Graphics2D)g; // add drawing operations. . .}
}
Graphical ShapesGraphical Shapes
Shape classes Ellipse2D.Double, Line2D.Double, etc.
import java.awt.geom.Ellipse2D; // no .Double
Must construct and draw the shape Ellipse2D.Double easterEgg = new Ellipse2D.Double(5, 10, 15, 20);g2.draw(easterEgg)
EllipseEllipse
LinesLines
Line2D.Double segment = new Line2D.Double(x1, x2, y1, y2);
More object-oriented to use Point2D.Double for the end points:Point2D.Double from = new Point2D.Double(x1, y1);Point2D.Double to = new Point2D.Double(x2, y2);Line2D.Double segment = new Line2D.Double(from, to);
Draw thick lines:g2.setStroke(new BasicStroke(4.0F)); // 4 pixels
ColorsColors Specify red, green, blue between 0.0F and 1.0F
Color magenta = new Color(1.0F, 0.0F, 1.0F)
Standard colorsColor.blackColor.yellowColor.pink. . .
Set color in graphics context:g2.setColor(Color.pink);
Then draw or fill shapesg2.fill(easterEgg);
FONTSFONTS
Specify text and base point:g2.drawString("Applet", 50, 100);
Font object has – face name (Serif, SansSerif, Monospaced, ...) – style (Font.PLAIN, Font.BOLD, Font.ITALIC ) – point size (12 point = normal size)
g2.setFont(new Font("Serif", Font.BOLD, 36));
BaselineBaseline
““Local” or “Relative”Local” or “Relative” Coordinate System Coordinate System
The whole object has a locationThe whole object has a location
public Car(double x, double y){
xLeft = x;
yTop = y;
}
The object is drawn “relative” to The object is drawn “relative” to the whole object’s locationthe whole object’s location
public void draw(Graphics2D g2) {
Rectangle2D.Double body = new Rectangle2D.Double(xLeft, yTop+10, 60, 10); …Ellipse2D.Double rearTire = new Ellipse2D.Double(xLeft + 40, yTop + 20, 10, 10); …..