Applets & Graphics
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Transcript Applets & Graphics
Applets & Graphics
Applets
• programs that run inside a browser
• Java platform-independence makes applets possible
• security restrictions:
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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
<html>
<head><title>My Applet</title></head>
<body>
Here is my first applet:
<applet code=“MyApplet.class” width=200
height=200>
MyApplet here
</applet>
</body>
</html>
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
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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
x
y
width, height
Color Class
• Constructor (values from 0 - 255)
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
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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>
<param name=“…” value=“…”>
<param name=“font” value=“monospaced”>
</applet>
• an applet can read these parameters
String s = this.getParameter (“font”);
<html>
<head><title>Sample Applet</title></head>
<body>
<applet code=SampleApplet.class width=350
height=150>
<param name=size value=24>
<param name=font value=monospaced>
</applet>
</body>
</html>
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