Question

This code is from the book Effective Java

Object[] snapshot = list.toArray();// Locks list internally

I am mainly interested in the comment here . Does it make the list unmodifiable ? What does it mean to say that a list is locked internally ? How long is this lock kept ? Is there a better alternative to convert a List to an array ?

Answer 1

I would imagine that it means the list doesn’t maintain a reference to the returned array, meaning that the array can be modified without affecting the original list from where it came. Likewise, any modifications to the list won’t be reflected in the array.

This is important in terms of thread safety, because it means you can iterate on the contents of the list from a thread-safe perspective, without worrying about another thread altering the sttae of the list in the meantime. In this sense the state of the list is “locked” in the returned array, no matter what changes are made to the list afterwards – you can see it as taking a snapshot.

toArray(); doesn’t alter the state of the list – so it doesn’t make it unmodifiable or anything like that.