I can see one scenario where trimming down an array causes memory leak
and hence setting its reference to null is a good way to garbage
collect those obsolete references.
Strings are reference types, so all the rules for reference types with respect to garbage collection apply to strings. The JVM may also do some optimizations on String literals but if you're worrying about these, then you're probably thinking too hard.
When does the JVM collect unreferenced objects?
The only answer that matters is: you can't tell and it needn't ever, but if it does you can't know when that will be. You should never write Java code around deterministic garbage collection. It is unnecessary and fraught with ugliness.
Speaking generally, if you confine your reference variables (including arrays or collections of reference types) to the narrowest possible scope, then you'll already have gone a long way toward not having to worry about memory leaks. Long-lived reference types will require some care and feeding.
"Trimming" arrays (unreferencing array elements by assigning null
to them) is ONLY necessary in the special case where the array represents your own system for managing memory, eg. if you are making your own cache or queue of objects.
Because the JVM can't know that your array is "managing memory" it can't collect unused objects in it that are still referenced but are expired. In cases where an array represents your own system for managing memory, then you should assign null
to array elements whose objects have expired (eg. popped off a queue; J. Bloch, Essential Java, 2nd Ed.).