Java hash collision probability

Question

I store a large number of objects (with unique combinations of values ​​stored in a byte array in the object) in hashmap (~ 2.8 million objects), and when checking if I have a hash clash (32-bit hash), I am very surprised seeing that there are no statistics, I have almost a 100% chance of having at least one collision (see http://preshing.com/20110504/hash-collision-probabilities/ ).

I'm curious to ask if my approach to detecting collisions is being listened to or if I’m very lucky ...

This is how I try to detect collisions of 2.8 million values ​​stored on a 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 approach for creating 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 that I'm doing wrong will be greatly appreciated!

Thank you Thomas

Answer 1

Arrays.hashCode,

public static int hashCode(int a[]) {
    if (a == null)
        return 0;

    int result = 1;
    for (int element : a)
        result = 31 * result + element;

    return result;
}

, 31, 31, ( ).

, , 31^11 , Java, . 31 " ", , .

. , 2 0 5. "hashCode" 0 37, 38 " ". , 5 , .

val hashes = for {
  i <- 0 to 4
  j <- 0 to 4
} yield (i * 31 + j) % 38

println(hashes.size) // prints 25
println(hashes.toSet.size) // prints 25

, , : 16 x 16 y, . , .

Answer 2

, , , , - - .

: , (, , ). , hashCode.