png screenshot

Question

以下是我为了拍摄活动窗口的截图而编写的代码。但是，我不确定为什么图像周围有黑色边框。请帮忙告诉我我的代码是否有任何错误。

    Robot robot; //create robot instance
    robot = new Robot(); //gets and saves a reference to a new Robot    

    robot.delay(3000); //delay robot for 3 seconds 

    //get dimensions of the bounding rectangle of the specified window
    WinDef.HWND hwnd = User32.INSTANCE.GetForegroundWindow(); 
    RECT WindowDimensions = new RECT(); 
    //get screen coordinates of upper-left and lower-right corners of the window in dimensionsOfWindow
    User32.INSTANCE.GetWindowRect(hwnd, WindowDimensions); 
    //capture image of only active window
    BufferedImage screenFullImage = robot.createScreenCapture(WindowDimensions.toRectangle());

    //write buffered image to file
    try{ 
        //File f = new File(dir + "\\screenshot " + createTimeStampStr() + ".png");  //file path of output
        File f = new File("D:\\screenshot " + createTimeStampStr() + ".png");  //file path of output
        ImageIO.write(screenFullImage, "png", f);
    }catch(IOException e){
        System.out.println("Error: " + e);
    }