IdentityHashCode in HashMap's bucket

问题

In the implementation details of HashMap, I can read:

When using comparators on insertion, to keep a
 * total ordering (or as close as is required here) across
 * rebalancings, we compare classes and identityHashCodes as
 * tie-breakers.

If I have constant hashCode and fine equals and my class doesn't implement Comparable how exactly it will break the ties and how the tree will be constructed?

I mean - bucket will transform to a tree and will use System.identityHashCode to break a tie. Then I will try to call containsKey method with a different instance (which will have the same hashCode and a.equals(b) == true) it will have different identityHashCode so is it possible that tree will be traversed by the wrong node (left instead right) and it will not find a key?

Am I missing something or this is normal behavior?

回答1:

The bucket will use identityHashCode during insertion, but lookup uses only hash codes and compare() calls (if available). This means it sometimes needs to scan both subtrees of a node.

The lookup logic looks line this

do {
  if (... keys are equal or can be compared ...) {
    // Go left, right or return the current node
    ...
  } else if ((q = pr.find(h, k, kc)) != null)
    // Search the right subtree recursively
    return q;
  else
   // Go to the left subtree
   p = pl;
} while (p != null);

See http://hg.openjdk.java.net/jdk10/jdk10/jdk/file/ffa11326afd5/src/java.base/share/classes/java/util/HashMap.java#l1901 and note that tieBreakOrder() (the method responsible for comparing identityHashCodes is not invoked anywhere in find().

回答2:

The motivation for the identity hash code base tie breaking is explained right before the cited part:

HashMap.java, line 212:

* When bin lists are treeified, split, or untreeified, we keep 
* them in the same relative access/traversal order (i.e., field 
* Node.next) to better preserve locality, and to slightly 
* simplify handling of splits and traversals that invoke 
* iterator.remove. When using comparators on insertion, to keep a 
* total ordering (or as close as is required here) across 
* rebalancings, we compare classes and identityHashCodes as 
* tie-breakers.

So, ordering by identity hash code provides a stable ordering to help implementing splitting and the Iterator.remove() operation (which must support continuing the traversal consistently).

As explained in this answer, it is not used for lookup operations, as you already said in your question, two equal objects may have different identity codes. So for unequal objects having the same hash code and not implementing Comparable, there is no way around traversing all of them and probing via equals.

回答3:

No, you are moving entries to left or right indeed based on System::identityHashCode but in that bucket there are entries still that have the same hashCode (well not the same, only the portion that matters).

So when you search for something it sometimes has to look at both left and right, there is no way around it, as simple as that.

来源：https://stackoverflow.com/questions/53658426/identityhashcode-in-hashmaps-bucket

标签

java

java-8

hashmap