Null Safety

See https://kotlinlang.org/docs/reference/null-safety.html

?.let

To perform a certain operation only for non-null values, you can use the safe call operator together with let:

case1:

//Java:
    private void updateView(ArticlesUiModel uiModel) {
        if (uiModel == null) {
            return;
        }
        // update view
    }

//Kotlin:
    private fun updateView(uiModel: ArticlesUiModel?) {
        uiModel?.let {
            // update view
        }
    }

case2:

Only safe(?.) or non-null asserted(!!.) calls are allowed on a nullable receiver of type Boolean?

return if (element != null && !element.isJsonNull) element.asString else null

null cannot be a value of non-null type

override fun deserialize(json: JsonElement, typeOfT: Type, context: JsonDeserializationContext): Article? {
    if (TextUtils.isEmpty(title) && TextUtils.isEmpty(description) && TextUtils.isEmpty(imageHref)) {
        return null // 如果需要返回null，返回类型必须后缀 ?
    }
    return Article(title, description, imageHref)
}