polygon should List<List<LngLatAlt>>

[targei]

According to RFC7946 definition, to present the ring (with hole), it has to two dementional array.

https://tools.ietf.org/html/rfc7946#section-3.1.6
{
"type": "Feature",
"geometry": {
"type": "Polygon",
"coordinates": [
[
[100.0, 0.0],
[101.0, 0.0],
[101.0, 1.0],
[100.0, 1.0],
[100.0, 0.0]
]
]
},
"properties": {
"prop0": "value0",
"prop1": {
"this": "that"
}
}

[Degooya]

If you look at the Geometry class, it already wraps T inside of a list

protected List<T> coordinates = new ArrayList<T>();