public function updateHyperlink($hyperlinkIndex, $url, $screenTip, $displayText)
    {
        //check whether worksheet name is set or not
        if ($this->worksheetName == '')
            throw new Exception('Worksheet name not specified');
	
        if ($hyperlinkIndex == '')
            throw new Exception('Hyperlink Index not specified');
	
        if ($url == '')
            throw new Exception('URL not specified');
	
        if ($screenTip == '')
            throw new Exception('Screen Tip not specified');
	
        if ($displayText == '')
            throw new Exception('Display Text not specified');
			
	$data = array('address' => $url, 'ScreenTip' => $screenTip, 'TextToDisplay' => $displayText);
	$json_data = json_encode($data);	
						
        $strURI = Product::$baseProductUri . '/cells/' . $this->getFileName() .
            '/worksheets/' . $this->worksheetName . '/hyperlinks/' . $hyperlinkIndex;
        $signedURI = Utils::sign($strURI);
        $response = Utils::processCommand($signedURI, 'POST', 'json', $json_data);
        $json = json_decode($response);
        if ($json->Code == 200)
            return true;
        else
            return false;
    }