Question about calling class

I was looking an class example code, and I got some problem with it.

1. What does it happen when I do:

classdef ClassA
  methods
     function obj = ClassA(input)
     function set_obj(obj)
     function [x y z] = unpack(obj)
  end
end

If I call ClassA in another file:

    Center = ClassA

What will return while I do this action? In the code, I didn't return anything in ClassA.

2. What does function [x y z] mean? Does it return the 1*3 matrix while calling ClassA?

3. Does all the function in ClassA run automatically? I mean function 'obj', 'set_obj', '[x y z]' start running when I call ClassA

I am not used to use class application, thanks for help!