PNG read and write with Java increases file size

Question

Got a problem when reading and writing a png file. I read it with ImageIO to a byte array and then write this byte array again using ImageIO. But the file size increases significantly. How can this happen?

public BufferedImage toBufferedImage(InputStream inputstream) {
    try {
        return ImageIO.read(inputstream);
    } catch (Exception e) {
        throw new IllegalStateException("Can't convert to buffered image", e);
    }
}

public byte[] toByteArray(BufferedImage bufferedImage, String filetype) {
    ByteArrayOutputStream output = new ByteArrayOutputStream();
    try {
        ImageIO.write(bufferedImage, filetype, output);
        return output.toByteArray();
    } catch (Exception e) {
        throw new IllegalStateException(e);
    }
}

Follow up: is there any library to support compressed PNGs that is written in Java and does not need any native code?

Answer 1

This is most likely due to the compression algorithm being different between Java and whatever created the original PNG.