From the code:

    long a,b,c; // randomly assigned 64-bit values

As I first understood the way this blog post was written, the author was relying on the JRE to populate these uninitialized variables with random values.

Edit: that was an incorrect understanding. It's not shown in the post, but in his real code he actually does assign values to these (thanks for the clarifying replies).

More of the code for context:

    // in Java, long is 64-bit, int 32-bit

    long a,b,c; // randomly assigned 64-bit values

    int hash32(long x) {
      int low = (int)x;
      int high = (int)(x >>> 32);
      return (int)((a * low + b * high + c) >>> 32);
    }