Fwd: Handling optional, non-present lists: JSON contains empty lists by default

[Steffen Dettmer]

Hi,

in one schema we have multiple optional lists, as artificial example:

"Type": {
"properties": {
"AList": {
"type": "array",
"items": { ... }
},
"BList": {
"type": "array",
"items": { ... }
},
required: []
}
}

They are optional, normally not present in the JSON on wire. If
they are not present, in JSON it looks like:

"Type": { }

GSON-annotated generated class contains:

class Type {
@SerializedName("AList")
@Expose
private List<AList> aList = new ArrayList<AList>();

@SerializedName("BList")
@Expose
private List<BList> bList = new ArrayList<BList>();
}

and happily parses this, but the resulting Type instance then has
two zero-length lists:

Type instance = gson.fromJson(json);
Assert.assertNotNull(instance.getAList());
Assert.assertEquals(0, instance.getAList().size());

instead of being null, when encoding / serializing this leads to:

"Type": { "AList": [], "BList": [] }

which is not the same as the input was.

I hope I explained well what my issue / question is about.

We can easily fix this (e.g. instance.setAList(null)), but I
wonder why this behavior is correct and expected or if we do
anything wrong.

Am I generally correct that I expect:

Type instance = new Type();
String json = gson.toJson(instance);

to produce:

"Type": { }

or is including the two empty lists actually correct / required
by some standard?

Technically it could be fixed by initializing the members to null
instead of new ArrayList<BList>() I think. Is this true?

Any hints or thoughts?

Best regards,
Steffen

[Joe Littlejohn]

Hi Steffen

You can use initializeCollections = false to turn off these empty lists.

Cheers

Joe

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[Steffen Dettmer]

* On Mon, Jun 25, 2018 at 4:06 PM, Joe Littlejohn

<joelit...@gmail.com> wrote:
> You can use initializeCollections = false to turn off these empty lists.

Ohh. So easy it is.
Sorry that I didn't read the documentation carefully enough, and
thanks so much again for your help!

Steffen