Question

I have an application which displays a collection of objects in rows, one object = one row. The objects are stored in a HashMap. The order of the rows does not affect the functionality of the application (that is why a HashMap was used instead of a sortable collection).

However I have noticed that the same application runs differently when run using two different versions of the Java Virtual Machine. The application is compiled using JDK 5, and can be run using either Java 5 or Java 6 runtimes, without any functional difference.

The object in question overrides java.lang.Object#hashCode() and obviously care has been taken to follow the contract specified in the Java API. This is evidenced by the fact that they always appear in the same order in every run of the application (in the same Java runtime).

For curiosity’s sake, why does the choice of Java runtime affect the order?

Answer 1

The implementation details of HashMap can and do change. Most likely this package private method did (this is from JDK 1.6.0_16):

/**
 * Applies a supplemental hash function to a given hashCode, which
 * defends against poor quality hash functions.  This is critical
 * because HashMap uses power-of-two length hash tables, that
 * otherwise encounter collisions for hashCodes that do not differ
 * in lower bits. Note: Null keys always map to hash 0, thus index 0.
 */
static int hash(int h) {
    // This function ensures that hashCodes that differ only by
    // constant multiples at each bit position have a bounded
    // number of collisions (approximately 8 at default load factor).
    h ^= (h >>> 20) ^ (h >>> 12);
    return h ^ (h >>> 7) ^ (h >>> 4);
}

For reference, the analogue in JDK 1.5.0_06 is:

/**
 * Returns a hash value for the specified object.  In addition to 
 * the object's own hashCode, this method applies a "supplemental
 * hash function," which defends against poor quality hash functions.
 * This is critical because HashMap uses power-of two length 
 * hash tables.<p>
 *
 * The shift distances in this function were chosen as the result
 * of an automated search over the entire four-dimensional search space.
 */
static int hash(Object x) {
    int h = x.hashCode();

    h += ~(h << 9);
    h ^=  (h >>> 14);
    h +=  (h << 4);
    h ^=  (h >>> 10);
    return h;
}