HotSpot Java764ビットバージョンで以下を実行する場合。
int countTopBit = 0, countLowestBit = 0;
for (int i = 0; i < 100000000; i++) {
int h = new Object().hashCode();
if (h < 0)
countTopBit++;
if ((h & 1) == 1)
countLowestBit++;
}
System.out.println("The count of negative hashCodes was " + countTopBit + ", the count of odd hashCodes was " + countLowestBit);
あなたは次のような結果を得ることができます
The count of negative hashCodes was 0, the count of odd hashCodes was 49994232
これはObject.hashCode()
本当に31ビットしかないのか、なぜそうなるのか疑問に思いました。
トップビットが使用されていないわけではありません。HashMapのソースから
257 /**
258 * Applies a supplemental hash function to a given hashCode, which
259 * defends against poor quality hash functions. This is critical
260 * because HashMap uses power-of-two length hash tables, that
261 * otherwise encounter collisions for hashCodes that do not differ
262 * in lower bits. Note: Null keys always map to hash 0, thus index 0.
263 */
264 static int hash(int h) {
265 // This function ensures that hashCodes that differ only by
266 // constant multiples at each bit position have a bounded
267 // number of collisions (approximately 8 at default load factor).
268 h ^= (h >>> 20) ^ (h >>> 12);
269 return h ^ (h >>> 7) ^ (h >>> 4);
270 }