Collections.emptyList().sort() does nothing
stuart.marks at oracle.com
Thu Nov 16 06:35:15 UTC 2017
On 11/14/17 7:52 PM, Tagir Valeev wrote:
> According to `List.sort` specification  "This list must be modifiable".
> According to `Collections.emptyList` specification  "Returns an
> empty list (immutable)."
> So I assume that `List.sort` cannot be called on
> `Collections.emptyList` result. However in fact this call is allowed
> doing nothing. Is this behavior documented somehow? Can I rely that it
> will not change in future Java updates?
> It's even more strange with `Collections.singletonList`  which
> "Returns an immutable list containing only the specified object".
> To me it seems that emptyList and (especially) singletonList
> implementation does not follow the specification. Am I missing
This is indeed an inconsistency. As Martin noted, different maintainers of the
collections framework took different approaches on how strict to be in these cases.
Note also that
is a no-op, whereas
both throw UnsupportedOperationException.
The issue is whether a collection that considers itself unmodifiable allows
mutator methods to be called, if the operation wouldn't actually make a change
(lenient); or if it unconditionally throws UOE even if the operation wouldn't
actually make a change (strict).
The Collections.unmodifiableX implementations are strict, as are the new
List.of/Set.of/Map.of implementations I added in JDK 9.
As you've noticed, certain special-case collections like Collections.emptyList()
and singletonList() are described as "immutable" (really, unmodifiable), but
they are lenient. While this isn't guaranteed by the spec, changing these at
this point would be a behavioral incompatibility, so we're unlikely ever to
My view is that it's preferable for collections implementations to be strict. If
I'm handing out an unmodifiable List, it should always be illegal for the callee
to attempt to sort it, even if it has zero or one elements.
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