FULL PRODUCT VERSION :
java version "1.8.0_40"
Java(TM) SE Runtime Environment (build 1.8.0_40-b25)
Java HotSpot(TM) 64-Bit Server VM (build 25.40-b25, mixed mode)


ADDITIONAL OS VERSION INFORMATION :
inux xxx 3.2.0-4-amd64 #1 SMP Debian 3.2.65-1+deb7u2 x86_64 GNU/Linux


A DESCRIPTION OF THE PROBLEM :
Under certain circumstances, new values cannot be put to a HashMap due to a ClassCastException. This apparently happens when the magical binsize is reached and the map internally should convert to trees (treeify). Here is the cast trace:

Caused by: java.lang.ClassCastException: java.util.HashMap$Node cannot be cast to java.util.HashMap$TreeNode
        at java.util.HashMap$TreeNode.moveRootToFront(HashMap.java:1819) ~[?:1.8.0_40]
        at java.util.HashMap$TreeNode.treeify(HashMap.java:1936) ~[?:1.8.0_40]
        at java.util.HashMap.treeifyBin(HashMap.java:771) ~[?:1.8.0_40]
        at java.util.HashMap.putVal(HashMap.java:643) ~[?:1.8.0_40]
        at java.util.HashMap.put(HashMap.java:611) ~[?:1.8.0_40]

Indeed, if you look to the code that the Node<K,V>[] tab cannot contain elements that could be cast to TreeNode.

REGRESSION. Last worked in version 6u45

ADDITIONAL REGRESSION INFORMATION:
A quick look to the code will reveal this issue.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Just put values to a hashmap.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
That this code is fixed.

ERROR MESSAGES/STACK TRACES THAT OCCUR :
Caused by: java.lang.ClassCastException: java.util.HashMap$Node cannot be cast to java.util.HashMap$TreeNode
        at java.util.HashMap$TreeNode.moveRootToFront(HashMap.java:1819) ~[?:1.8.0_40]
        at java.util.HashMap$TreeNode.treeify(HashMap.java:1936) ~[?:1.8.0_40]
        at java.util.HashMap.treeifyBin(HashMap.java:771) ~[?:1.8.0_40]
        at java.util.HashMap.putVal(HashMap.java:643) ~[?:1.8.0_40]
        at java.util.HashMap.put(HashMap.java:611) ~[?:1.8.0_40]

REPRODUCIBILITY :
This bug can be reproduced often.

CUSTOMER SUBMITTED WORKAROUND :
In this specific case we use a TreeMap as a workaround.