why they designed main should only have String array?
Because if both "public static void main(String args)" and "public static void main(int args)" were permitted, you would get all kinds of strange issues. Suppose you write a program that has both of the methods:
Which one of the two main methods should be called when your program starts? Before you say "if I enter only numbers on the command line, then I want the second one, otherwise the first one":
If the String as the int version would be allowed, then there would also be no reason to not have a version of main() that takes a char, a long etc. and the question about which one should be called when will be even more complicated.
In other words, to keep Java simple there is just one version of the main() method that can be used as the entry point for the program, and that's the one that takes a String.
Just my personal opinion, and others may differ, but ...
I think, when learning a new language, it pays not to ask too many "why does the language [not] have feature X?". The answers will often be difficult to understand, as a newcomer to the language. Instead, get some familiarity with the language, and the ways of thinking that are associated with it. Often, features or omissions that seem ridiculous initially turn out not to be bothersome at all, once you are thinking the right way for the language.
Once familiar with the language, it is then interesting to ask these types of question. Sometimes, there is a plain good reason for the feature or omission (as in your question). Sometimes, it is a result of a balance between competing demands (e.g. lack of multiple inheritance). Sometimes, the feature or omission is just bone-headed (Cloneable, finalize(), Calendar...).
Finally, I'll pseudo-quote a strap-line from another Rancher. "There are two types of language: the ones no-one likes and the ones no-one uses". Mind you, that was being used to defend C++, which (no troll intended) is just plain rubbish.
Betty Rubble? Well, I would go with Betty... but I'd be thinking of Wilma.
Hey, I'm supposed to be the guide! Wait up! No fair! You have the tiny ad!