RFR(m): 8177290 add copy factory methods for unmodifiable List, Set, Map

[Stuart Marks]

On 11/22/17 8:45 AM, Remi Forax wrote:
> I think i prefer toImmutableList() than toUnmodifiableList() because the List
> is truly immutable and not an unmodifiable proxy in front of a mutable List
> (like Collections.unmodifiableList() does).

Immutability is like wine.

If you put a spoonful of wine in a barrel full of sewage, you get sewage. If you 
put a spoonful of sewage in a barrel full of wine, you get sewage. 
(Schopenhauer's Law of Entropy)

Similarly, if you add a little immutability to something mutable, you get 
mutability. And if you add a little mutability to something immutable, you get 
mutability.

To get the desired properties of immutability (e.g., thread-safety, or the 
ability to hand around references safely without making defensive copies), it's 
insufficient for a collection to be "immutable"; you also have to make some 
statements about the contents. It's thus more precise to say that a collection 
is unmodifiable, and to provide a clear definition of an unmodifiable collection.

Although unmodifiable collections have been around since the collections 
framework was introduced, oddly enough the concept has never had a good 
definition. The current proposal includes definitions for unmodifiability, 
unmodifiable collections, and unmodifiable views, and it clearly specifies which 
APIs return unmodifiable views and which return unmodifiable collections.

s'marks