在 Kotlin 中使用 Map 过滤对象列表

Question

I am trying to create a dictionary filter that will filter the incoming list of parameters.
The problem is that when there are parameters in the list that should be filtered and removed - I get a NullPointerException.
Basic informations:
This is how filter looks:

data class Filter(
    val filters: Map<ParameterName, Set<String>>
)

data class ParameterName(
    val key: String
)

Example of filter:

Filter(filters={ParameterName(key=Manufacturer)=[brand, producer]})

The filtered list ( List<Parameter> ) has the following structure:

data class Parameter(
    val id: String?,
    val name: String?,
    val values: List<String>?
)

Example of List<Parameter>

[Parameter(id=123, name=brand, values=[Nike]), Parameter(id=345, name=color, values=[black, pink]), Parameter(id=823, name=test, values=[some, thing])]

The logic is that the filtered list is changed to Map <String, ParameterValue>. Where:
a) String (key) is the value of the filter key (The key in the filter map has the main value and any other names under it. For example - under Manufacturer we have got values "brand" or "producer")

b) The parameter value (eg Nike) is assigned to the ParameterValue object (val value: String).

What have I prepared:

        fun filterParameters(
            parametersList: List<Parameter>,
            filters: Map<ParameterName, Set<String>>
        ): Map<String, ParameterValue> {
            return parametersList.associate { param ->
                filters.filter {
                    it.value.first().equals(param.name, ignoreCase = true)
                }.keys.first().key to ParameterValue(value = param.values!!.first())
            }.toMap()
        }

Flow example:

1. Input to the method:
   a) Parameters list:

        listOf(
            Parameter(
                "123",
                "brand",
                listOf("Nike")
            ),
            Parameter(
                "345",
                "color",
                listOf("black", "pink")
            ),
            Parameter(
                "823",
                "test",
                listOf("some", "thing")
            )
        )

b) Filter:

        mapOf(
            ParameterName("Manufacturer") to setOf("brand", "producer")
        )

2)Expected output:

{Manufacturer=ParameterValue(value=Nike)}

Expected output structure:

Map<String, ParameterValue>

where

data class ParameterValue(
val parameterId: String? = null,
val value: String,
val valueId: String? = null

)

So we just fill value - String

Answer 1

data class Parameter(
  val id: String?,
  val name: String?,
  val values: List<String>?
)

val parameters = listOf(
  Parameter("123", "brand", listOf("Nike")),
  Parameter("3433", "color", listOf("red", "black")),
  Parameter("22313", "model", listOf("air max")),
  Parameter("2312", "origin", listOf("USA")),
  Parameter(null, "size", listOf("7", "7.5", "8", "9")),
  Parameter("987", null, listOf("lalala")),
  Parameter("999", "code", null),
  Parameter(null, null, null)
)

val filters = mapOf(
  "Manufacturer" to setOf("brand", "producer"),
  "Country" to setOf("origin", "from")
)

fun getValuesForParameterName(parameters: List<Parameter>, filters: Map<String, Set<String>>): Map<String, List<String>> {
  return parameters
    .filter { parameter -> parameter.name in filters.flatMap { map -> map.value } }
    .map { parameter -> filters.filter { map -> map.value.contains(parameter.name) }.keys.first() to parameter.values }
    .groupBy { (name, _) -> name }
    .map { (name, values) -> name to values.map { (_, list) -> list ?: emptyList() }.flatten() }
    .toMap()
}

val result = getValuesForParameterName(parameters, filters)
println(result)                   // Output: {Manufacturer=[Nike], Property=[USA]}
println(result["Manufacturer"])   // Output: [Nike]
println(result["Country"])        // Output: [USA]