I am afraid your five lines of code fill me with dread. And not about finding things in the List.
Why are you using a method which returns a String and parsing it to a number? Why did you not return a number?
Why are you trying to put things into a List, and then check whether they have already been added?
Why are you calling that code from a Listener? Why are you not developing it as a standalone application, which you will later wrap in a GUI? That is a far better way to develop applications. Your application should have an interface so you can do something like this:-I think there is a piece of hardware getting between you and the program. Unfortunately it is the computer, and I think you will have to put it aside. Instead, use a two‑dimensional carbohydrate‑based flexible writing surface¹.
Write down how you would do it is real life, with a bag of balls. See whether you can change the bag of balls to a Java collections implementation. I presume you are familiar with the random number methods in Java? (They are not really random, but you won’t notice the difference.)