Create a class PrimeRandomGenerator that extends class RandomGenerator In JAVA

I have to create a class that extends RandomGenerator and produces a random prime number each time the user enters an interval in the main program.

I have created the constructors and the isPrime method but I can't figure out what to do next... Any ideas?

Notes:

• If the user enters only one number ( lets say: x ) then the interval will be (0,x).In other words we have to create either two methods either two constructors.
  For example:
  public int nextPrime(int n) or public int nextPrime(int low,int high)

• I am not allowed to use arrays (or other ready-made methods that make the problem way easier).

• I searched everything that involved Random Prime Numbers Generators but I couldn't find anything similar to this.