Enumeration adapters in SE 8

[Andrew Thompson]

The key advance would be the last one, unfortunately the one Dan says may not make it: allowing the for loop to take an Iterator. 

I had some legacy code in the last couple of weeks where I wanted to be able to process arbitrary sequences: arrays, iterables and iterators and I ended up copying and pasting control flow code because there's no unifying interface. 

Enumerations are nice to have. Although I can use the for loop with an array, there's no way to write a generic method to handle arrays and Iterables. Arrays that implement Iterable would be more valuable if we're talking convenience. 



On Aug 24, 2011, at 6:13 PM, Collin Fagan <collin.fagan at gmail.com> wrote:

> There was a time where it was considered best practice to return an iterator
> instead of a collection.  There are now many apis and legacy code that are
> not compatible with the for each loop.
> 
> I think even if there are only a few places in the jdk where this would
> apply it would still be convenient in the wild.
> Collin
> 
> On Wed, Aug 24, 2011 at 4:58 PM, Colin Decker <cgdecker at gmail.com> wrote:
> 
>> I've come across various scenarios. One example is
>> javax.swing.tree.DefaultMutableTreeNode. It has numerous methods that
>> return
>> Enumerations, including:
>> - children
>> - depthFirstEnumeration
>> - breadthFirstEnumeration
>> - preorderEnumeration
>> - postorderEnumeration
>> - pathFromAncestorEnumeration
>> 
>> I've created a utility class with methods to return Iterables for each of
>> these types of Enumerations for convenience, but it would be much nicer to
>> just use a method reference.
>> 
>> Deque's descendingIterator() method is a more modern good candidate for
>> treating as an Iterable. See this question on StackOverflow:
>> 
>> http://stackoverflow.com/questions/3883131/idiomatic-way-to-use-for-each-loop-given-an-iterator
>> 
>> --
>> Colin
>> 
>> 
>> On Wed, Aug 24, 2011 at 5:45 PM, Ben Evans <benjamin.john.evans at gmail.com
>>> wrote:
>> 
>>> Hi Dan,
>>> 
>>> On Wed, Aug 24, 2011 at 7:10 PM, Dan Smith <daniel.smith at oracle.com>
>>> wrote:
>>> 
>>>> I was pointed to some comments on core-libs about adapting Enumerations
>>> to
>>>> for loops in SE 8.  (Sorry for the new thread -- I wasn't subscribed.)
>>> It
>>>> turns out lambdas + extension methods will make this very easy.
>>>> 
>>>> In the API:
>>>> 
>>>> interface Enumeration<E> extends Iterator<E> {
>>>> boolean hasMoreElements();
>>>> E nextElement();
>>>> boolean hasNext() default #hasMoreElements;
>>>> E next() default #nextElement;
>>>> void remove() default { throw new UnsupportedOperationException(); }
>>>> }
>>>> 
>>>> Note that Iterable is a function type -- a thunk producing an Iterator
>> --
>>>> and so we can express Iterables with lambdas and method references.
>> It's
>>>> becoming clear that a for loop should provide a target type for these
>>>> things, so that will probably be part of the SE 8 feature set.
>>>> 
>>>> With a method reference:
>>>> 
>>>> Hashtable<String,Object> h = …;
>>>> for (String s : h#keys) { … }
>>>> 
>>> 
>>> I'm struggling to see where this might be useful. This example is
>> obviously
>>> too trivial and would add nothing, but I can't see to come up with any
>> more
>>> extensive example where having a method reference in this position would
>> be
>>> actually useful.
>>> 
>>> Can someone else provide a more compelling example for a for loop taking
>> a
>>> method reference?
>>> 
>>> Thanks,
>>> 
>>> Ben
>>> 
>>> 
>> 
>> 
> 


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