RFR: 8167005: Comment on the need for an empty constructor in ArrayList$Itr

[Claes Redestad]

Hi,

as suggested by Stuart Marks in the review thread for JDK-8166840, we
should add a comment as to why an empty constructor is important in the
private inner Itr class, to avoid it being accidentally "cleaned up".

Webrev: http://cr.openjdk.java.net/~redestad/8167005/webrev.01/
Bug: https://bugs.openjdk.java.net/browse/JDK-8167005

Thanks!

/Claes