hash value in java objects

Xin Tong xerox.time.tech at gmail.com
Fri Dec 7 21:39:21 PST 2012

On Fri, Dec 7, 2012 at 11:38 PM, Krystal Mok <rednaxelafx at gmail.com> wrote:
> Short answer is the identity hash code is calculated when:
> 1. the hashCode() method is called the first time for this object, if
> the type of this object doesn't override Object.hashCode();
> 2. System.identityHashCode() is called the first time for this object.
would not materialize the hashcode takes much code. why not materialize lazily ?

> A "0" value encoded as the hash in the mark word indicates that the
> identity hash code hasn't been calculated yet (which is the initial
> state after an object is created).
> If the type of an object overrides Object.hashCode(), then calling it
> doesn't have anything to do with the identity hash code.
> The identity hash code of a Java object is stored in the mark word of
> the object header if it has been calculated and the object is
> unlocked. Otherwise it may be stored in the displaced mark word or in
> the inflated lock.
> - Kris
> On Sat, Dec 8, 2012 at 6:31 AM, Vitaly Davidovich <vitalyd at gmail.com> wrote:
>> Hash is encoded in the mark word, so I think the answer to your question is
>> it's created when object is allocated (and thus gets a markOop).  The actual
>> hash value is decoded from the mark word at call time, but it's just a shift
>> and mask at that point.  Someone can correct me if this is wrong.
>> Thanks
>> Sent from my phone
>> On Dec 7, 2012 4:52 PM, "Xin Tong" <xerox.time.tech at gmail.com> wrote:
>>> I am wondering when the hash values in the java objects are
>>> materialized. are they materialized when the object is created ? or
>>> when hashCode on the object is called.  what about modifications to
>>> the object ?
>>> Xin

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