Re: HashSet keeps all nonidentical equal objects in memory

Robert Klemme <>
Wed, 20 Jul 2011 08:38:49 -0700 (PDT)
On 20 Jul., 11:43, Frederik <> wrote:

I've been doing java programming for over 10 years, but now I've
encoutered a phenomenon that I wasn't aware of at all.

Apparently you didn't - as you found out in the meantime. :-)

I had an application in which I have a HashSet<String>. I added a lot
of different String objects to this HashSet, but many of the String
objects are equal to each other. Now, after a while my application ran
out of memory, even with -Xmx1500M. This happened when there were only
about 7000 different Strings in the set! I didn't understand this,
until I started adding the "intern()" of every String object to the
set instead of the original String object. Now the program needs
virtually no memory anymore.
There is only one explanation: before I used "intern()", ALL the
different String objects, even the ones that are equal, were kept in
memory by the HashSet! No matter how strange it sounds. I was
wondering, does anybody have an explanation as to why this is the case?

No, that conclusion is not warranted by the facts. You only know that
*something* kept hold of a lot of memory (String instances). Since we
do neither know all the code nor do we know the application
architecture we can only speculate but it seems a realistic assumption
that those String instances are not only kept by the HashSet but
somewhere else.

An easy way you can create such a situation is that you are reading
from some external source (file) repeated content and create an object
which - among other things - holds the String. Now you have 1,000,000
objects holding on to 1,000,000 String instances but there are only
7,000 different character sequences. In such a situation it may be
better to have a HashMap<String,String> where you store the String
only once and reuse that first instance. Basically this is what
happened when you used String.intern() only that you do not have
control over this storage any more which - depending on application
type - can still create a serious memory leak, e.g. long running app
which over time reads multiple files with different sets of repeated

Kind regards


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