I introduce your adjustments and now the method random can only have 1 params!
EDIT:
All fixed, I see that I can import whatever I want from java libraries so I imported utils.Random and initialize a new Random() var!
And now I finally use instance method 
Thank you quark for let me deep more!
Anyway I have a little question, what difference between Math and Random?
Becaue I can call Math.sqrt() without import nothing and when I call Random.nextInt() I cant!
Is this problem due to static non static class?
Reading between docs I saw that MAth is Public Final class, instead Random is just public class. IS this the main difference?