i am trying to generate a hex color value from an integer input, and I'm not sure I'm using the concat method correctly. when i output the string theColor, i only get "0x", any ideas?

public String generateColor(String redVal, String blueVal, 
  String greenVal, String alphaVal){
      String theColor = "0x";

      theColor.concat(alphaVal);
      theColor.concat(redVal);
      theColor.concat(greenVal);
      theColor.concat(blueVal);

      return theColor;
}