The output of the program is null. The reason is that when Integer i; is declared the reference variable i is initialized to null since it is an instance variable. Now the obvious question is that the program should throw a NullPointerException but this not the case.
System.out.println(i); invokes the below overloaded version of println of the class java.io.PrintStream.
Kumarjit: Thanks for your answer. Please UseCodeTags when you post source code - as you can see, the code will be formatted in a much more readable way when you use them.
Anchu: As you can see, you could have checked this out yourself by writing a small program. Try writing lots of little programs to test out features of Java - trying things out yourself is one of the best ways to learn.