How do I post some message on a friend's wall using the android facebook sdk?

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Mobile

Android

Facebook-API

hello, Im a bit new in android .I have successfully done posting in my own wall and I wish to post on my friends' wall through my android mobile app. The above example code fits best for my requirement.

BUT I do not understand where to get the userID from(that has to be passed as a string to this method). Please please guide me and provide me with something that would give me the IDs of user's fiends..

here is he link where I found this code:
http://stackoverflow.com/questions/6480582/how-can-i-post-text-on-a-friends-wall-using-the-android-facebook-sdk[^]
My requirement is exactly the same!

protected void postToWall(String userID){
try {
    if (isSession()) {
        String response = mFacebook.request((userID == null) ? "me" : userID);

        Bundle params = new Bundle();
        params.putString("message", "put message here");
        params.putString("link", "some link");
        params.putString("caption", "{*actor*} just posted this!");
        params.putString("description", "description of my link.  Click the link to find out more.");
        params.putString("name", "Name of this link!");
        params.putString("picture", "some pciture");

        response = mFacebook.request(((userID == null) ? "me" : userID) + "/feed", params, "POST");

        Log.d("Tests",response);
        if (response == null || response.equals("") ||
                response.equals("false")) {
            Log.v("Error", "Blank response");
        }
    } else {
        // no logged in, so relogin
        Log.d(TAG, "sessionNOTValid, relogin");
        mFacebook.authorize(this, PERMS, new LoginDialogListener());
    }
}catch(Exception e){
    e.printStackTrace();}