some IKM java questions

Q:
Base aa = new Sub(); //both Base and Sub defines STATIC method m1()
aa.m1();
(Sub)aa.m1(); // which one?

Q: Deep copy  a java array?
- clone()?
- serialization?

Q: a base class method without access modifier is callable by subclass?

I think java default method access level is "package, not subclass". In contrast, c# (and c++) default is private -- http://msdn.microsoft.com/en-us/library/ms173121.aspx.

Q: if interface1 declares method2 returning Set, can an implementing class's method return SortedSet?