Add ArrayNode JsonNode.asArray() helper method

[cowtowncoder]

(note: part of https://github.com/FasterXML/jackson-future-ideas/wiki/JSTEP-3 changes)

(inspired by FasterXML/jackson-dataformat-xml#695)

With some JSON structures (as well as other formats, most notably XML), there is sometimes need to process/express things such that single-values that are not express as (JSON) Arrays would be handled same as 1-element arrays. Example shown with XML is that following documents:

    <list>
        <element>
            <a>value 1a</a>
            <b>value 1b</b>
        </element>
        <element>
            <a>value 2a</a>
            <b>value 2b</b>
        </element>
    </list>

    <list>
        <element>
            <a>value a</a>
            <b>value b</b>
        </element>
    </list>

become different JsonNodes structurally: basically equivalent of JSON like so:

{ "list":
    "element": [
        { ... },
       { .... }
   ]
}

{ "list":
     "element": {
          "a": "value a",
          "b": "value b"
     }
}

So to help handling with such case -- or, possibly, modifying such content, we could add new method:

ArrayNode asArray();

which will return:

• this for ArrayNode
• new ArrayNode() (empty ArrayNode) for MissingNode ("missing" node)
• new ArrayNode().add(this) for all other JsonNode implementations

to allow unifying processing of otherwise structurally mismatching JsonNode trees.

[yawkat]

I don't think we should encourage this kind of code by adding API to it. Wrapping all node types except for arrays in a single-element array is a bit weird.

[JooHyukKim]

@yawkat Because I think suggestion from xml-module kinda makes sense, makes me more curious why this would be weird or a bad idea? 👀

[cowtowncoder]

It seems weird from plain JSON perspective, but as @JooHyukKim pointed it makes more sense for XML handling; or more generally for "single Object as-is, two or more as Array" usage patterns which are pretty common (esp. with scripting languages).

While I personally also dislike such polymorphism (if something allows multiple values I feel it ought to be Array even if it contains just 1 element) I think there's something to be said for supporting actual use cases.