Android : how can i set a mp3 file in my root folder for specific contact using eclipse java

i want to set a mp3 sound file as a ringtone one of my contact's call receiving. i studied about this problem in stack and many other websites. but still i can not solve this problem. please help me with code. and i used codes that there are update method or insert method that insert or update ringtone for contact. last thing that i must say is that i have list of my contacts name in a list view. and i want when clicking in list view items change ringtone happen. thank you for your help.