Two objects created at the same time with the same hashcode

Question

We have this object with the following constructor:
2010-06-24 00:10:31,260 [LoadBalancerClientSubscriber(3)(pid:24312)] INFO  com.intel.swiss.sws.netstar.application.caching.framework.data
set.synchronizer.DatasetSynchronizer - Initializing dataset synchronizer for: [/nfs/iil/iec/sws/work/damar/ds_cama/tmp/ds_126631277304794
/d81], i am com.intel.swiss.sws.netstar.application.caching.framework.dataset.synchronizer.DatasetSynchronizer@2ed3cae0
2010-06-24 00:10:31,260 [LoadBalancerClientSubscriber(5)(pid:24315)] INFO  com.intel.swiss.sws.netstar.application.caching.framework.data
set.synchronizer.DatasetSynchronizer - Initializing dataset synchronizer for: [/nfs/iil/iec/sws/work/damar/ds_cama/tmp/ds_126631277304794
/d31], i am com.intel.swiss.sws.netstar.application.caching.framework.dataset.synchronizer.DatasetSynchronizer@2ed3cae0Note that two objects are created by different threads with exactly the same hash code. Any idea if/how this is possible?

Answer 1

isocdev_mb wrote:
The last part definitely suggests already that relying on uniqueness is incorrect. Hash codes are very often equal on distinct objects, viz. new String("java").hashCode == new String("java").hashCode(). Use a class level counter as suggested earlier.For that case we would of course expect the hashCodes to be equal, since the objects are equal. But even in the case of non-equal objects that don't override hashCode, you can still get the same value. Or, for that matter, non-equal objects that do override it. There are 2^32 possible hashCode values. There are 2^64 possible Long values. That means that there are 2^32 Longs that have a hashCode of 1, 2^32 Longs that have a hashCode of 2, etc.
And for non-equal objects...
package scratch;
import java.util.Set;
import java.util.Map;
import java.util.HashMap;
public class HashCodeIsNotUnique {
  public static void main(String[] args) throws Exception {
    Map<Integer, Integer> hashCodeCounts = new HashMap<Integer, Integer>();
    int numObjects = 10000;
    for (int i = 0; i < numObjects; i++) {
      Object obj = new Object();
      int hashCode = obj.hashCode();
      if (!hashCodeCounts.containsKey(hashCode)) {
        hashCodeCounts.put(hashCode, 0);
      hashCodeCounts.put(hashCode, hashCodeCounts.get(hashCode) + 1);
    for (Map.Entry<Integer, Integer> entry : hashCodeCounts.entrySet()) {
      int key = entry.getKey();
      int value = entry.getValue();
      if (value > 1) {
        System.out.println(key + " occurred " + value + " times");
9578500 occurred 2 times
14850080 occurred 2 times