老师，下午好，再请教一个问题，我看ImageIO.java的源码，代码如下，

public static boolean write(RenderedImage im,
                                String formatName,
                                File output) throws IOException {
        if (output == null) {
            throw new IllegalArgumentException("output == null!");
        }
        ImageOutputStream stream = null;

        ImageWriter writer = getWriter(im, formatName);
        if (writer == null) {
            /* Do not make changes in the file system if we have
             * no appropriate writer.
             */
            return false;
        }

        try {
            output.delete();
            stream = createImageOutputStream(output);
        } catch (IOException e) {
            throw new IIOException("Can't create output stream!", e);
        }

        try {
            return doWrite(im, writer, stream);
        } finally {
            stream.close();
        }
    }

我查了一下， IIOException 是 IOException 的子类 ，但 IllegalArgumentException 却不是，那 throws IOException 这样写的意义是什么，IllegalArgumentException 异常不对外抛出了吗？

谢谢老师解答！