Assuming you are happy with obscure rather than secure, there a number of mechanisms you could use, but obfuscaters like proguard are not going to be able to help you.
To achieve this you will need to do encoding or encryption of the string yourself, the approach you use depends on what you are trying to defend against, if it you are just trying to hide from obvious inspection, than encoding may be sufficient (see android.util.Base64, http://developer.android.com/reference/android/util/Base64.html). Note that encoding is in NO WAY SECURE and all it will to is remove the obvious reference to your site.
If you are trying to defend against something more, then you could move to actually encrypting the string, to do this you would use a symmetric cipher like AES via javax.crypto.Cipher, http://www.androidsnippets.org/snippets/39/index.html provides a decent usage example. Again this is more annoying then secure to would be hackers, as you will need to store the key somewhere in your jar thus negating any cryptographic security.
To make this clearer, the basic steps would be:
- Manually create an encrypt your string using a known key.
- Convert your code to use a decrypted version of this string, example:
Before:
public class Foo {
private String mySecret = "http://example.com";
...
}
Becomes:
public class Foo {
private String encrypted = "<manually created encrypted string>";
private String key = "<key used for encryption";
private String mySecret = MyDecryptUtil.decrypt(encrypted, key);
...
}
A (good) alternative to all of this is considering using a third party drm solution such as the licensing server google provides http://android-developers.blogspot.com/2010/07/licensing-service-for-android.html. This may be more secure than something you roll your self, but is subject to very similar limitations to what I described above.