This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Yes! Calling a static from a non-static method is allowed but not the reverse. When a member is static it is like it is common to all instances of that class. So it should be accessible by non static methods. But a non static member, belongs to one specific instance(object) of the class and hence cannot be directly invoked without creating an instance. Does that make sense?
Yes. You can access the static variables from nonstatic methods. The difference between static and non-static variables is that the static variables are common to the class. ie, the same copy of the static data will be available for all the instances of that class.
in the reverse case, for accessing non-static data from a static method, we should use some object. that is also the same reason... non static data are object specific. ie, they are available within a class instance only.