foreign key constrain

hello when im inserting data to my database this error occurs javax.servlet.ServletException: Cannot add or update a child row: a foreign key constraint fails pls advice thank you . Send instant messages to your online friends http://uk.messenger.yahoo.com -- MySQL General Mailing List

not null values

i want to make it so that i will have to fill in all the attributes in .so i put not null for all the attributes. Is this correct ?? when for example do not fill in user_name , all the other values is accepted and stored in the databaseplease advise create table user_details (user_id