As Seetharaman says, if you will not have any nested classes that refer to Lookup6 instance variables or instance methods then you can use static nested classes.
But having everything as static, you will not be able to take advantage of many aspects of OOP dont you think? In terms of design(imho)
you should account for possible future improvements to the program. If everything is static it is not very different from programs in languages that do not use OOP.