The following code snippet shows how to generate Gravatar URLs from email address. This utility method allows you to pass the size of your email.

public String getGravatarUrl(String email, int size) {
    if (null == email) return null;

    final String hash = MD5Util.md5Hex(email.toLowerCase());
    final String gravtar = "http://www.gravatar.com/avatar/%s?s=%s&r=g&d=404";
    final String gravatarUrl = String.format(gravtar, hash, size);

    try {
        return URLEncoder.encode(gravatarUrl, "UTF-8");
    } catch (UnsupportedEncodingException e) {
        e.printStackTrace();
    }
    return null;
}