How to determine if URL is an image?

I would like to check if URL given by user is image (jpg, png, gif). First idea: check only file extension in URL. Second idea: get this resource from server (by http get) and load into some Java picture library to get info if is it image (seriously disadvantage: slow). Or maybe yet another solution?

Can a URL be an image?

A URL is a web address that specifies location. Therefore, an image URL is a web address that specifies the location of an image.

Does every image have a URL?

Every image needs a URL because every image is a unique image. mysite.com/img/potato.png and mysite.com/img/apple.png are 2 separate images on my website for example.

You should use HTTP HEAD, not a full GET. This should include the Content-Type as known by the server. You could of course test the extension first, and only do the expensive/slow HTTP roundtrip if it's inconclusive.