java抓屏代码

一个网上找下来的 抓屏代码只需呀输入要存放的路径和名字 就可以执行，但不能为空。

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));  
     
}  
...