资料类别：
data class Topic( @SerializedName("id") val id: Long, @SerializedName("name") val name: String, @SerializedName("image") val image: String, @SerializedName("description") val description: String)到JSON：
val gson = Gson()val json = gson.toJson(topic)
从JSON：
val json = getJson()val topic = gson.fromJson(json, Topic::class.java)
