Java hash collision probability

I'm storing a large number of objects (with unique combinations of values stored in a byte array in the object) in a hashmap (~2.8million objects) and, when checking if I have any collision of hash code (32-bit hash), I'm very surprised to see there is none while statistically, I have nearly 100% chances of having at least one collision (cf. http://preshing.com/20110504/hash-collision-probabilities/).

I am thus wondering if my approach to detect collisions is bugged or if I'm extremely lucky...

Here is how I try to detect collisions from the 2.8million values stored in the map:

HashMap<ShowdownFreqKeysVO, Double> values;
(...fill with 2.8 mlns unique values...)
HashSet<Integer> hashes = new HashSet<>();
for (ShowdownFreqKeysVO key:values.keySet()){
    if (hashes.contains(key.hashCode())) throw new RuntimeException("Duplicate hash for:"+key);
    hashes.add(key.hashCode());
}

And here is the object's approach to create a hash value:

public class ShowdownFreqKeysVO {
    //Values for the different parameters
    public byte[] values = new byte[12];

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + Arrays.hashCode(values);
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        ShowdownFreqKeysVO other = (ShowdownFreqKeysVO) obj;
        if (!Arrays.equals(values, other.values))
            return false;
        return true;
    }
}

Any idea/hint on what I'm doing wrong would be greatly appreciated !

Thanks, Thomas