How do I create a Unit test in Java?

I have a java class that takes an array of numbers and an array of corresponding probabilities and acts as a random number generator that returns one of the numbers from the array according to the corresponding probabilities defined in the other other. How do I write a minimal but effective set of unit tests?

class RandomGen{

    private int[] randomNums;
    private float[] probabilities;
    private float[] total_prob;

    RandomGen(int[] randomNums, float[] probabilites) throws Exception{
        if (this.set_arrays(randomNums, probabilites));
        else {
            throw new Exception("Invalid array format");
        }
    }

    private boolean set_arrays(int[] randomNums, float[] probabilities){
        if (check_arrays(randomNums, probabilities)) {
            this.randomNums = randomNums;
            this.probabilities = probabilities;
            this.total_prob = new float[this.probabilities.length];

            total_prob[0] = probabilities[0];
            for (int i = 1; i < probabilities.length; i++){
                total_prob[i] = total_prob[i-1] + probabilities[i];
            }
            return true;
        }
        return false;
    }

    public int[] return_randomNums(){
        return this.randomNums;
    }

    public float[] return_probabilites(){
        return this.probabilities;
    }

    private boolean check_arrays(int[] randomNums, float[] probabilities){
        float total_prob = 0;
        for (int i = 0; i < probabilities.length; i++){
            if (probabilities[i] < 0) return false;
            total_prob += probabilities[i];
        }
        if (randomNums.length != probabilities.length || total_prob != 1) return false;
        return true;
    }

    public int nextNum(){
        float random = (float) Math.random();
        if (random <= total_prob[0]){
            return randomNums[0];
        }
        for (int i=1; i < total_prob.length; i++){
            if (total_prob[i-1] < random  && random <= total_prob[i]) return randomNums[i];
        }
        return 0;
    }
}