A DESCRIPTION OF THE PROBLEM :
In Java 11 StringJoiner treated the cases of zero and one element strings such no duplicates where created.
In Java 17+ it not cares. Even if the result string is of zero length it not returns a shared String instance (unless in some constellations, if emptyValue has been set).
In the application I am currently working on, this causes myriads of duplicate strings which not occurred in Java 11.

REGRESSION : Last worked in version 11

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
In Java 17:
"new StringJoiner("").toString()" returns a new empty String instance.
"new StringJoiner("").add("foo").toString()" returns a dupe of the single input string "foo".

In Java 11:
"new StringJoiner("").toString()" returns the empty string constant.
"new StringJoiner("").add("foo").toString()" returns the same String instance "foo".

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
At least the behaviour like in Java 11 should be restored.
Perfect would be to also avoid all other cases of unnecessary duplications.

CUSTOMER SUBMITTED WORKAROUND :
Handcrafted treatment of special cases, but that's not always as simple, especially when using StringJoiner in a Collector.

FREQUENCY : always