This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
2.you have to get all the customers from the List. how do you do? you need an iterator
then you need to check any more Customer object is in list or not . how do you do this using hasNext() method of that iterator.
3. if any object is there you need to get one by one by using Iterator#next()
4. now you have a Customer object in your hand. you know how to get the properties for the object? using getters?
You will have to look at the Customer class and see what methods it has for getting that data out. Does it have getID() and getUserName() for example? This is something we can't answer without having access to the Customer class.