object := `
{
  "@context": "https://www.w3.org/ns/activitystreams",
  "summary": "Martin's recent activities",
  "type": "Collection",
  "totalItems": 1,
  "items" : [
    {
      "type": "Add",
      "published": "2011-02-10T15:04:55Z",
      "generator": "http://example.org/activities-app",
      "nameMap": {
        "en": "Martin added a new image to his album.",
        "ga": "Martin phost le fisean nua a albam."
      },
      "actor": {
        "type": "Person",
        "id": "http://www.test.example/martin",
        "name": "Martin Smith",
        "url": "http://example.org/martin",
        "image": {
          "type": "Link",
          "href": "http://example.org/martin/image",
          "mediaType": "image/jpeg",
          "width": 250,
          "height": 250
        }
      },
      "object" : {
        "name": "My fluffy cat",
        "type": "Image",
        "id": "http://example.org/album/máiréad.jpg",
        "preview": {
          "type": "Link",
          "href": "http://example.org/album/máiréad.jpg",
          "mediaType": "image/jpeg"
        },
        "url": [
          {
            "type": "Link",
            "href": "http://example.org/album/máiréad.jpg",
            "mediaType": "image/jpeg"
          },
          {
            "type": "Link",
            "href": "http://example.org/album/máiréad.png",
            "mediaType": "image/png"
          }
        ]
      },
      "target": {
        "type": "Collection",
        "id": "http://example.org/album/",
        "nameMap": {
          "en": "Martin's Photo Album",
          "ga": "Grianghraif Mairtin"
        },
        "image": {
          "type": "Link",
          "href": "http://example.org/album/thumbnail.jpg",
          "mediaType": "image/jpeg"
        }
      }
    }
  ]
}
`

// Unmarshal from AS 2.0 JSON
var exobj astreams.Collection
err := json.Unmarshal([]byte(object), &exobj)
if err != nil {
	log.Println(err)
}

// Marshal to AS 2.0 JSON
jsB, err := json.Marshal(&exobj) // note the use of a pointer here; astreams MarshalJSON methods are defined on pointer receivers
if err != nil {
	log.Println(err)
}