In this case i dont think there is any specific benefit of using interface.
Interfaces are useful when the child classes have same methods which have different implementation.
More than that...

why dont you use some switch statement to see whether the char entered is number or what and the convert it to the appropriate string value?
you can loop through the string using for

what error message is coming?

1. How can I make the program print a blank space in between the ****RESIDENTIAL ....***** and Max number of customers....?

At present the output should be like this...

you have defined the class name as projectta but elsewhere you have used it as Projectta.
java is case sensitive..if i remember right..
change either one and the error should go away
meaning use...