StackOverflowError on HashMap.hashCode() due to self reference

[Louis Wasserman]

The Javadoc of Map already specifies:

> Note: great care must be exercised if mutable objects are used as map
keys. The behavior of a map is not specified if the value of an object is
changed in a manner that affects equals comparisons while the object is a
key in the map. *A special case of this prohibition is that it is not
permissible for a map to contain itself as a key. While it is permissible
for a map to contain itself as a value, extreme caution is advised:
the **equals and hashCode methods
are no longer well defined on such a map.*

(emphasis added)

On Fri, Jul 17, 2015 at 12:11 PM Krystal Mok <rednaxelafx at gmail.com> wrote:

> Hi Vitaly,
>
> I can buy that argument. It wasn't hard for me to recognize it when I saw
> it in real world.
>
> But apparently whoever asked me about it couldn't wrap his head around it
> to understand why...
>
> Maybe it's a case for a Pitfalls book instead of for JavaDocs.
>
> Thanks,
> Kris
>
> On Friday, July 17, 2015, Vitaly Davidovich <vitalyd at gmail.com
> <javascript:_e(%7B%7D,'cvml','vitalyd at gmail.com');>> wrote:
>
> > IMHO, this doesn't warrant any special wording -- if someone hits this, I
> > think it's fairly evident what the issue is.  You can get the same
> problem
> > with adding an ArrayList to itself and calling hashCode().
> >
> > On Fri, Jul 17, 2015 at 2:45 PM, Krystal Mok <rednaxelafx at gmail.com>
> > wrote:
> >
> >> Hi everyone,
> >>
> >> I'm sure somebody has seen this stack overflow before. My question is:
> is
> >> it expected (which should be documented to warn people not to do this),
> or
> >> is it considered a bug (which should be fixed)?
> >>
> >> Example:
> >> https://gist.github.com/rednaxelafx/930f8979473185cfc0a0
> >>
> >> import java.util.*;
> >>
> >> public class HashMapStackOverflow {
> >>   public static void main(String[] args) throws Exception {
> >>     HashMap<String, Object> map = new HashMap<>();
> >>     map.put("self", map);
> >>     System.out.println(map.hashCode());
> >>   }
> >> }
> >>
> >> $ ~/sdk/jdk1.8.0/Contents/Home/bin/java HashMapStackOverflow
> >> Exception in thread "main" java.lang.StackOverflowError
> >> at java.util.AbstractMap.hashCode(AbstractMap.java:505)
> >> at java.util.Objects.hashCode(Objects.java:98)
> >> at java.util.HashMap$Node.hashCode(HashMap.java:296)
> >> at java.util.AbstractMap.hashCode(AbstractMap.java:507)
> >> at java.util.Objects.hashCode(Objects.java:98)
> >> at java.util.HashMap$Node.hashCode(HashMap.java:296)
> >> at java.util.AbstractMap.hashCode(AbstractMap.java:507)
> >> at java.util.Objects.hashCode(Objects.java:98)
> >> at java.util.HashMap$Node.hashCode(HashMap.java:296)
> >> at java.util.AbstractMap.hashCode(AbstractMap.java:507)
> >> ...
> >>
> >> This will cause a stack overflow because HashMap.hashCode() is inherited
> >> from AbstractMap, which sums the hash code of each entry, while one of
> the
> >> entries is itself so it goes recursive.
> >>
> >> Thanks,
> >> Kris
> >>
> >
> >
>