Answer 1

Do you mean how to call the display method?

I am not sure but I think the only way would be to create an object of Demo inside inner () method:

static void inner {

//your Demo class

Demo d = new Demo();
d.display();

inside your main method:
inner();

if you call inner() it will also call display().

Answer 2

display () is already declared in the demo class as
void display()

+ there are typo in:
Class A
Static void inner(){