2.6M/16M collisions for X,Y pair

/**
 * Calculate a hash code for a pair of coordinates
 * @return The hashcode
 * @author REghZy
 */
public static int hash(int x, int y) {
    // 15.8M/16M collisions
    //   return this.hash = (this.x * 31) ^ (68 + this.y);
    // 15.3M/16M collisions
    //   return this.hash = (this.x * 0x1f1f1f1f) ^ (this.y);
    // 8000/16M collisions
    // int hash = 'X';
    // hash = (31 * hash) + ':';
    // hash = aggregateHash(hash, Integer.toString(x));
    // hash = (31 * hash) + 'Y';
    // hash = (31 * hash) + ':';
    // hash = aggregateHash(hash, Integer.toString(y));
    // return hash;
    // return new StringBuilder(24).append("X:").append(x).append("Y:").append(y).toString().hashCode();
    // 8M/16M collisions
    //   return (x * 0x101DEAD) ^ (y);
    // 2.6M/16M collisions
    return (x * 0x101DEAD) ^ (y * 0xBEEF);
}

public static void main(String[] args) throws IOException, InterruptedException {
    int collisions = 0, totazzl = 0;
    int w = 2000, d = 2000;
    HashMap<Integer, String> aaaamap = new HashMap<Integer, String>((w*d) * 5);
    for(int x = -w; x < w; x++) {
        for (int z = -d; z < d; z++) {
            int hash = hash(x, z);
            if (aaaamap.put(hash, "") != null) {
                collisions++;
                if (collisions < 20) {
                    System.out.println(RZFormats.format("Collision: {0},{1} ({2})", x, z, hash(x, z)));
                }
            }
            totazzl++;
        }
    }
    System.out.println(collisions + " / " + totazzl + " collisions");
}