ArrayList : ask user to enter name and check if the name exist in the list then show a message that sorry name already exist else store to the list.
ArrayList : ask user to enter name and check if the name exist in the list then show a message that sorry name already exist else store to the list. Create object of Customer class and do the same as above.
HashMap : Ask user to enter key, if key present then ask user to enter new key.
If the key not present in Map, then ask user to enter value and store it.
ConcurrentHashMap