How to use the random method with a parameter not with a fixed integer?

I know that the random() method is used with a fixed integer that I should determine. However my need is to use the same function with a parameter, I mean an integer that I haven't now but it will be determined during the runtime. here's my code:

button1.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            Random r1 = new Random();
            num1 = r1.nextInt(z);

            if (num1 < z) {             
                 //do something

            } else {

                //do other thing
            }

        }
    });

z is the parameter it determined during the runtime.