Random letter on Button Click?

I have a problem:
I was coding in Android Studio and to get used to it I wanted to make a simple app , that when a button is pressed, it showed a random letter on screen, with the button everything is fine, but I can't code the random letter to appear since I cant use

function 

and Android Studio doesen't even accept

var

could someone give me the code I need for the random letter to appear? What I have is

Btn.setOnClickListener(new View.OnClickListener(){
@Override
public void onClick(View view) {
   //here i'd need to enter the function
}