Getting a new random number every time [Dice Simulator]

Hey guys I'm quite new to Java, and just started meddling with classes. I'm currently trying to simulate a dice on a basic game I'm creating. The dice seems to work, but I can only call it effectively once, as the number always remains the same after that. Is there an easy solution for the dice I've created to be difference every time I call it?

public class Dice {
  Random randomNum = new Random();

  public int diceRoll() {
    int diceResult = 0;
    int result = randomNum.nextInt(20) + 1;
    diceResult = diceResult + result;
    return diceResult;
  }
}

Appreciate any help in the slightest, thank you.