Can you declare more than one single main method in java program why?

It doesn't really make sense. The JVM needs to know where to start running the program; that's what the "main" method is for. A class - one that can be run directly - needs to have a single entry point, to avoid ambiguity. This main method can then call any number of other methods.

I assume you can call a second method "main", but using a different signature. However, to avoid confusion, I would recommend you don't do this.