ArrayList(Collection<? extends E> c) thread safe?

Question

Is it thread safe to create a new instance of ArrayList via constructor ArrayList(Collection<? extends E> sourceCollection) without any additional sychronization, supposing that sourceCollection is synchronized? More specifically, can we rely in that case on the new list to contain exactly the elements that were in the collection at the time new ArrayList(sourceCollection) was invoked? And can we rely on the new list to be in a consistent state?

I'm asking this question because I've seen examples in books on concurrency of how to confine objects to local variables on a thread's stack. In these examples a method is passed a reference to a shared object, and inside the method a copy of the object is stored in a local variable -- all this without any synchronization. It is claimed that thread safety can be achieved in this way. A general example would be:

public void someMethod(Collection<String> source) {
    List<String> localList = new ArrayList<>(source);
    ...
}

Answer 1

Hint: as @John Bollinger justly mentioned, particular ArrayList implementation is not covered by language specification. So written below is true for Oracle java 8 implementation.

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Yes, it is safe if source is synchronised collection, because ArrayList constructor in this case uses toArray() method of source collection, which is synchronised as well and produce new copy of data:

public ArrayList(Collection<? extends E> c) {
    elementData = c.toArray();
    // ...
}