Two line solution that passes all TC's

o = new Inner().new Private();
System.out.println(num + " is " + ((Solution.Inner.Private)o).powerof2(num));

First of all refer this document to understand the inner class creation - http://tutorials.jenkov.com/java/nested-classes.html

Now what I did is:

o = new Object();
Solution.Inner instance = new Solution.Inner();
Inner.Private inner = instance.new Private();
o = (Object) inner;      
System.out.println(num + " is " + inner.powerof2(num)) ;

Why is there security exception in this piece of code? I mean I have solved the actual problem, but I do not get the relevance of going through permission checking for exit of VM etc.

weird one line solution is Java

System.out.println(num + " is " + ((Solution.Inner.Private)(o = (Object) new Solution.Inner().new Private())).powerof2(num));

I solved this using reflection, as that was in the import statements, so I assumed that was what was being tested. Otherwise, it can be done in one line in Java, passes all tests (and admittedly it's much the same as SeanRMunoz's solution):

System.out.println(String.format("%d is %s", num, new Inner().new Private().powerof2(num)));

That seems too easy for a medium challenge though.