Re: JDK-8210280 - Unnecessary reallocation when invoking HashMap.putAll()

[Doug Lea]

On 12/14/18 1:37 AM, Michal Vala wrote:
> Thanks Martin for finding this serious issue and the testcase.
> 

Sorry that I wasn't paying attention to this and so forced Martin to
discover the hard way that because of LinkeHashMap, you can't skip
doubling steps (at least not without a lot of rework). Also, the
documentation should have mentioned this. A simpler way to reduce
overhead in the case at hand is just to loop in putMapEntries:

--- HashMap.java.~1.9.~ 2018-11-11 15:43:24.982878495 -0500
+++ HashMap.java        2018-12-16 09:05:48.924727867 -0500
@@ -502,8 +502,13 @@
                 if (t > threshold)
                     threshold = tableSizeFor(t);
             }
-            else if (s > threshold)
-                resize();
+            else {
+                // Because of LinkedHashMap constraints, we cannot
+                // expand all at once, but can reduce total resize
+                // effort by repeated doubling now vs later
+                while (table.length <  MAXIMUM_CAPACITY && s > threshold)
+                    resize();
+            }
             for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
                 K key = e.getKey();
                 V value = e.getValue();