Bug 6321873 explains that the hash code returned by System.identityHashCode(Object) is not guaranteed to be unique (it’s a hash code after all). That being stated, there are situations in which ...

Java’s equals() and hashcode() are two methods ... We use the hashcode() method to optimize performance when comparing objects. Executing hashcode() returns a unique ID for each object in ...

and influenced the developers of Perl and CRuby to "change their hash functions to include randomization." But Klink and Wälde showed that "PHP 5, Java, ASP.NET as well as V8 are fully vulnerable ...