If by "unique" you mean that any distinct String has a distinct hashCode(), it
is not guaranteed.

Have you read the Javadocs for hashCode()?  The API docs are very often a
perfect first place to find such answers.
<http://java.sun.com/javase/6/docs/api/java/lang/Object.html#hashCode()>

You can examine the particular algorithm for String at
<http://java.sun.com/javase/6/docs/api/java/lang/String.html#hashCode()>

Even without Javadocs it is easy to prove that the hash codes for Strings
cannot be unique for all possible String values.  There are only 2**32 - 1
possible hashCode() results.  Any String longer than two characters has a
larger value set than that.  The sum of the value set counts for Strings of
any length is larger still.

Gotta love those Javadocs.

No.  Consider that hasCode returns an int.  An int is a 32-bit value, so  
there are only 2^32 (approx. 4.3 billion) possible values.  There number  
of possible Strings is, for all practical purposes, unlimited.  So it  
would not be possible to write a hashCode implementation that generates a  
different value for each different String.

Dan.

If by "unique" you mean that any distinct String has a distinct hashCode(), it
is not guaranteed.

Have you read the Javadocs for hashCode()?  The API docs are very often a
perfect first place to find such answers.

<http://java.sun.com/javase/6/docs/api/java/lang/Object.html#hashCode()>

(emph. orig.)

You can examine the particular algorithm for String at
<http://java.sun.com/javase/6/docs/api/java/lang/String.html#hashCode()>

Even without Javadocs it is easy to prove that the hash codes for Strings
cannot be unique for all possible String values.  There are only 2**32
possible hashCode() results.  Any String longer than two characters has a
larger value set than that.  The sum of the value set counts for Strings of
any length is larger still.

Gotta love those Javadocs.

Hash values are by definition not guaranteed to be unique.

http://en.wikipedia.org/wiki/Hash_function#Properties

Regards

    robert

The String returned by Integer.toString(int) for all possible values of
the argument have unique values. However there are String values that
cannot be returned by Integer.toString(int). As hashCode returns an int,
clearly there has to be some clashes somewhere.

Tom Hawtin

Bruintje Beer schrieb:

Hi

For two references pointing to an "equal()" String, the hash value must
be equal.
But be aware, that the old JVM (1.0.2) used a different algorithm for
"hashCode()" than the new (1.6).
I have forgotten, in which version it realy changed; but if you want to
have a unique hash-value that is unique accross different
Java-platforms, copy the source of a "good" hash code (eg VM 1.6.0) into
your hash-sensitive code.

phi