You forget to start the service:
webcam.start(); into setup()

Thanks..
this will only turn  on my webcam..
but the issue is that  i want whenever the code is run it will automatically open the webcam and capture an image..
the below program does only opening the webcam..

1. import processing.video.*;
   Capture webcam;
   
   void setup() {
     size(640,480);
     webcam = new Capture(this,640,480); 
     String[] devices = Capture.list();
     println(devices);
     webcam.start();
    
   }
   
   void draw() {
     if (webcam.available() == true) {
       webcam.read();
       image(webcam ,0,0);
     }
     saveFrame("line.bmp");
   }
   

does work for me…maybe put saveFrame("line.bmp"); in the same bracket than image…?    
 if (webcam.available() == true) {
    webcam.read();
    image(webcam, 0, 0);
    saveFrame("line.bmp");
  }
but your code does work for me_processing 2b08, Mac 10.6

thanx for reply..
i am using windows platform
Is this issue?
Does is cause the same problem on LINUX OS?

does it write the .bmp for you, and it's still blank ? Or it doesn't write the file ?

Just to throw it out there, the code also works on my machine (windows 7 with processing 2b08).