Applets & Graphics

Applets programs that run inside a browser Java platform-independence makes applets possible security restrictions: –cannot read or write to local disk –cannot print –can only access the server that delivered them –signed applets compatibility issues –JDK built-in to browser (still at JDK1.1) –Java plug-in may be installed into browser –Java must be enabled in browser download speed

HTML A page-definition language for browsers applets must be invoked via an HTML page My Applet Here is my first applet: <applet code=“MyApplet.class” width=200 height=200> MyApplet here

Creating an Applet An applet must extend the Applet class public class MyApplet extends Applet this allows it to inherit the hooks which enable it to run in a browser applets inherit a method: public void paint(Graphics g) from a superclass; a browser will call an applet’s paint() method in this case, the inherited method does nothing; but we may override it to draw a picture in our applet the browser creates and supplies a Graphics object the Graphics object has methods for drawing

Some Methods of the Graphics Class –void drawString (String s, int x, int y) –void drawRect (int x, int y, int width, int height) – void drawOval (int x, int y, int width, int height) –void drawLine (int x1, int y1, int x2, int y2) –void fillRect (int x, int y, int width, int height) – void fillOval (int x, int y, int width, int height) –void drawImage(Image img, int x, int y, ImageObserver observer) –void setColor (Color c) –void setFont (Font f)

Coordinate System Graphics uses an integer coordinate system with origin at top left corner of the applet 0,0 width, height x y

Color Class Constructor (values from ) public Color (int red, int green, int blue) Examples Color red = new Color (255, 0, 0); Color purple = new Color (255, 0, 255); Color whatever = new Color (25, 128, 200); Predefined colors Color.lightGray Color.magenta Color.white Color.blue, etc.

Font Class Constructor public Font (String name, int style, int size) Names –“TimesRoman” –“ Arial ” –“ Courier ” –“ Impact ” Logical names –“serif” // TimesRoman –“ sanserif ” // Helvetica –“ monospaced ” // Courier

Font Class (continued) Styles –Font.BOLD –Font.ITALIC –Font.PLAIN –Font.BOLD+Font.ITALIC Sizes (in points, 1/72 inch) –8 // pretty small –12 // readable –20 // large

Font Class (continued) Examples –Font normal = new Font (“serif”, Font.PLAIN, 12); –Font emphatic = new Font (“Helvetica”, Font.BOLD+ Font.ITALIC, 24); –Font program = new Font (“monospaced”, Font.PLAIN, 12);

import java.awt.*; import java.applet.*; public class SampleApplet extends Applet { public void paint(Graphics g) { Font f1 = new Font (“serif”, Font.PLAIN, 14); g.setFont (f1); g.drawString (“Let’s try some things”, 20, 20); g.setColor (new Color (100, 200, 100)); g.drawRect (5, 40, 50, 30); g.setColor (Color.green); g.fillRect (30, 60, 30, 50); } }

Other Methods of Applet paint(Graphics g) is called whenever the applet needs repainting the Applet class has other methods that may be overridden: –public void init() // called once,when the applet is loaded –public void start() // called whenever the applet’s page is viewed –public void stop() // called whenever the applet’s page is exited –public void destroy() // called when the browser exits

Applet Parameters You may wish to change the behavior of your applet without modifying and recompiling it parameters may be specified in the HTML applet start and end tags <applet code = “MyApplet.class” width = 200 height = 200> an applet can read these parameters String s = this.getParameter (“font”);

Sample Applet <applet code=SampleApplet.class width=350 height=150>

import java.awt.*; import java.applet.*; public class SampleApplet extends Applet { private String fontName; private int fontSize; public void init() { fontName = this.getParameter (“font”); String s = this.getParameter (“size”); fontSize = Integer.parseInt (s); } public void paint(Graphics g) { Font f1 = new Font (fontName, Font.PLAIN, fontSize); g.setFont (f1); g.drawString (“Let’s try some things”, 20, 20);... } }

Applet Size in JDK1.1 Dimension size = this.getSize(); int width = size.width; int height = size.height; in JDK1.2 int width = this.getWidth(); int height = this.getHeight(); these methods are inherited from the Component class