What is !! mean in Kotlin

Question

• I'm maintaining a microservice system was written in Kotlin and Spring-boot and at some code, I found !! syntax which i don't know what it meaning:

fun getListOrderStatus(orderStatusAgentRequestDto: OrderStatusAgentRequestDto): ResponseEntity<List<OrderStatusAgentResponseDto>>? {
        val accessTokenAgent = agentAuthService.getAccessTokenAgent()
        val requestUri = UrlBuilder(mposApiEnpoint.orderStatusListRead)
                .addParam(HttpUtils.PARAM_ACCESS_TOKEN, accessTokenAgent!!.accessToken)
                .addParam("page_size", "${orderStatusAgentRequestDto.pageSize}")
                .addParam("order_codes", if (orderStatusAgentRequestDto.orderCodes != null) orderStatusAgentRequestDto.orderCodes.joinToString(separator = ",") else "")
                .addParam("modified_time", "${orderStatusAgentRequestDto.modifiedTime}")
                .addParam("service_id", "${orderStatusAgentRequestDto.serviceId}")
                .toString()
        val restTemplate = RestTemplate()

        return restTemplate.exchange(requestUri, HttpMethod.GET, null, object : ParameterizedTypeReference<List<OrderStatusAgentResponseDto>>() {})
    }

....
....
val status = orderMappingApi.getListOrderStatus(agentRequestDto)
val orderStatus = status.body!![0]

• The status was the response of a above function. Can somebody please explain to me what !! mean and what its use in this case? Thank in advance!!!

Answer 1

!! in Kotlin is the not-null assertion operator. it converts any value to a non-null type and throws an exception if the value is null.

val len = query!!.length

if query is not null it will return its length but if query is null we'll get NPE.