code: void convertToGrayscale(final BufferedImage image){
      for(int i=0; i<image.getWidth(); i++){
          for(int j=0; j<image.getHeight(); j++){
              int color = image.getRGB(i,j);
  
              int alpha = (color >> 24) & 255;
              int red = (color >> 16) & 255;
              int green = (color >> 8) & 255;
              int blue = (color) & 255;
  
              final int lum = (int)(0.2126 * red + 0.7152 * green + 0.0722 * blue);
  
              alpha = (alpha << 24);
              red = (lum << 16);
              green = (lum << 8);
              blue = lum;
  
              color = alpha + red + green + blue;
  
              image.setRGB(i,j,color);
          }
      }
  }
  