FULL PRODUCT VERSION :
jdk 1.7

ADDITIONAL OS VERSION INFORMATION :
Windows Vista

A DESCRIPTION OF THE PROBLEM :
The Documentation for putIfAbsent in ConcurrentHashMap says:

/**
     * {@inheritDoc}
     *
     * @return the previous value associated with the specified key,
     * or <tt>null</tt> if there was no mapping for the key
     * @throws NullPointerException if the specified key or value is null
     */


This is ambiguous. Either the system throws a NullPointerException or returns null if key(value) is null.


REPRODUCIBILITY :
This bug can be reproduced always.