import java.awt.Dimension;    
import java.awt.Rectangle;    
import java.awt.Robot;    
import java.awt.Toolkit;    
import java.awt.image.BufferedImage;    
import javax.imageio.ImageIO;    
import java.io.File;    
    
...    
    
public void captureScreen(String fileName) throws Exception {    
    
   Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();    
   Rectangle screenRectangle = new Rectangle(screenSize);    
   Robot robot = new Robot();    
   BufferedImage image = robot.createScreenCapture(screenRectangle);    
   ImageIO.write(image, "png", new File(fileName));    
    
}