RFC: draft API for JEP 269 Convenience Collection Factories

Stuart Marks stuart.marks at oracle.com
Fri Oct 9 22:01:20 UTC 2015

On 10/8/15 7:39 PM, Paul Benedict wrote:
> I don't think the statements "Creates an unmodifiable set containing X elements"
> is always true. Since sets cannot have duplicates, it's possible passing in X
> elements gives you less than that based on equality. I think the Set docs should
> say "...X possible elements if unique". Wordsmith something better if you can,
> of course.

Fair point, I probably had the idea of throwing an exception for duplicate 
elements when I wrote the "Creates an unmodifiable set containing X elements" 
lines. If the policy were to silently ignore duplicates, applying the usual dose 
of spec weasel wording might result in something like

     "Creates an unmodifiable set from the X elements passed as arguments."

Then again, one of the advantages of rejecting duplicates is that the resulting 
collection's size equals the number of arguments passed. This also applies to 
the varargs cases; the resulting size equals the varargs array length. This not 
only provides clear semantics to the programmer, but it also lets the 
implementation preallocate internal arrays knowing that they'll have exactly the 
right size.


> Cheers,
> Paul
> On Thu, Oct 8, 2015 at 6:39 PM, Stuart Marks <stuart.marks at oracle.com
> <mailto:stuart.marks at oracle.com>> wrote:
>     Hi all,
>     Please review and comment on this draft API for JEP 269, Convenience
>     Collection Factories. For this review I'd like to focus on the API, and set
>     aside implementation issues and discussion for later.
>     JEP:
>     http://openjdk.java.net/jeps/269
>     javadoc:
>     http://cr.openjdk.java.net/~smarks/reviews/jep269/api.20151008.mod/
>     specdiff:
>     http://cr.openjdk.java.net/~smarks/reviews/jep269/api.20151008.specdiff/overview-summary.html
>     Most of the API is pretty straightforward, with fixed-arg and varargs "of()"
>     factories for List, Set, ArrayList, and HashSet; and with fixed-arg "of()"
>     factories and varargs "ofEntries()" factories for Map and HashMap.
>     There are a few issues on which I'd like to solicit discussion.
>     1. Number of fixed arg overloads.
>     I've somewhat arbitrarily provided up to 5 fixed-arg overloads for the lists
>     and sets, and up to 8 pairs for the fixed-arg map factories. The rationale
>     for 8 pairs is that there are 8 primitives, and various language processing
>     tools often have maps for the primitive types. (But such tools also often
>     need to handle the Void type, which exceeds the limit of 8. So this might
>     need to change if we want to follow this rationale.)
>     I also note that Guava's immutable factories provide 11 fixed-arg overloads
>     for list, 5 for set, and 5 pairs for map. I'd be curious as to the rationale
>     for this, and whether it also would apply to the JDK.
>     2. Other concrete collection factories.
>     I've chosen to provide factories for the concrete collections ArrayList,
>     HashSet, and HashMap, since those seem to be the most commonly used. Is
>     there a need to provide factories for other concrete collections, such as
>     LinkedHashMap?
>     3. Duplicate handling.
>     My current thinking is for the Set and Map factories to throw
>     IllegalArgumentException if a duplicate element or key is detected. The
>     current draft specification is silent on this point. It needs to be
>     specified, one way or another.
>     The rationale for throwing an exception is that if these factories are used
>     in a "literal like" fashion, then having a duplicate is almost certainly a
>     programming error. Consider this example:
>          Map<String,TypeUse> m = Map.ofEntries(
>              entry("CDATA",       CBuiltinLeafInfo.NORMALIZED_STRING),
>              entry("ENTITY",      CBuiltinLeafInfo.TOKEN),
>              entry("ENTITIES",    CBuiltinLeafInfo.STRING.makeCollection()),
>              entry("ENUMERATION", CBuiltinLeafInfo.STRING.makeCollection()),
>              entry("NMTOKEN",     CBuiltinLeafInfo.TOKEN),
>              entry("NMTOKENS",    CBuiltinLeafInfo.STRING.makeCollection()),
>              entry("ID",          CBuiltinLeafInfo.ID),
>              entry("IDREF",       CBuiltinLeafInfo.IDREF),
>              entry("IDREFS",
>                        TypeUseFactory.makeCollection(CBuiltinLeafInfo.IDREF));
>              entry("ENUMERATION", CBuiltinLeafInfo.TOKEN));
>     (derived from [1])
>     If duplicates were silently ignored, this might result in hard-to-spot errors.
>     There's also the matter of which value ends up being used in the case of
>     duplicate map keys, and whether this should be specified. A fairly obvious
>     policy would be "last one wins" but I'm reluctant to specify that, as it
>     starts to place unnecessary constraints on implementations. However, the
>     alternative of leaving it unspecified is also unpalatable.
>     I'm aware that very few programming systems with similar constructs will
>     signal an error on duplicate elements. Python, Ruby, Groovy, Scala, and Perl
>     all seem to allow duplicates in maps or equivalent, apparently with a
>     last-wins policy. (Though sometimes it's hard to tell if the policy is
>     specified.)
>     The only system I've been able to find that explicitly rejects duplicates is
>     Clojure, and this policy isn't without controversy. [2] The main rationale
>     is to prevent programming errors.
>     There is a python bug [3] where it was proposed that duplicates in a dict
>     should raise an error or warning, also in order to catch programming errors.
>     The request was rejected, not necessarily because it was a bad idea, but
>     primarily because it would be a backward incompatible change.
>     The easiest thing to do would simply to require last-wins, since "everybody
>     else is doing it" ... but that doesn't mean it's right. Since we're
>     introducing a new API here, there is no compatibility issue. Throwing an
>     exception for duplicates seems like a good way to prevent a certain class of
>     programming errors.
>     What do people think?
>     s'marks
>     [1]
>     http://hg.openjdk.java.net/jdk8/jdk8/jaxws/file/d03dd22762db/src/share/jaxws_classes/com/sun/tools/internal/xjc/reader/dtd/TDTDReader.java#l420
>     [2]
>     http://dev.clojure.org/display/design/Allow+duplicate+map+keys+and+set+elements
>     [3] https://bugs.python.org/issue16385

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