How to remove the alpha channel of a png-24 image by java？

Question

I have a png-24 images,how can I do to remove its alpha channel?Or how to convert it to png-8 format? Here is my codes,but it can't be worked.

BufferedImage src = ImageIO.read(source);
AffineTransform xform=AffineTransform.getScaleInstance(1.0, 1.0);
AffineTransformOp xformOp=new AffineTransformOp(xform,AffineTransformOp.TYPE_BILINEAR);
BufferedImage outImg = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB); 
outImg=xformOp.filter(src, outImg);
ImageIO.write(outImg, "jpg",result);

The result image is black.

Or How to convert png-24 image to jpg by java？