Patterns for arrays of specific length

[Brian Goetz]

In general, there’s a duality between the ways in which we construct composites and the way we deconstruct them.  There’s an obvious duality between constructor and deconstructor patterns; between static factories and static patterns, etc.  

For structural composites, the obvious place to start is the dual of array/list/map literals.  (We’re not ready to do these just yet, but it makes sense that we consider structural literals and structural patterns at the same time.)  So for example, if an array literal looks like [ e, f, g ], an array pattern might look like [ p, q, r ].  

Given that, your example might look like:

    return resolveResults instanceof [ var e ] && e.isValid()  ? e : null

Without collection literals, we’d write methods to do what we want, so these methods have duals too.  For example:

    return resolveResults instanceof oneElementArray(var e) && e.isvalid() ? e : null




> On Mar 4, 2019, at 3:30 AM, Tagir Valeev <amaembo at gmail.com> wrote:
> 
> Hello!
> 
> In intellij IDEA code we often see snippets like this:
> 
> final ResolveResult[] resolveResults = multiResolve(false);
> return resolveResults.length == 1 && resolveResults[0].isValidResult() ?
>       resolveResults[0].getElement() : null;
> 
> I wonder if special kind of patterns to cover such case could be invented like
> 
> return multiResolve(false) instanceof ResolveResult[] {var res} &&
> res.isValidResult() ?
>          res.getElement() : null;
> 
> In essence it should be a deconstruction pattern for arrays. I don't
> remember whether it was discussed, but probably I'm missing something.
> 
> Alternatively this could be covered by utility method like
> 
> static <T> T getOnlyElement(T[] array) {
>  return array.length == 1 ? array[0] : null;
> }
> 
> return getOnlyElement(multiResolve(false)) instanceof ResolveResult
> res && res.isValidResult() ?
>          res.getElement() : null;
> 
> But this doesn't scale for arrays of bigger length.
> 
> With best regards,
> Tagir Valeev.