Filter nested list with kotlin

Question

Ques: I want to filter list within a list. All of my data models are immutable.

My JSON structure looks like this

{
  "root": [
    {
      "id": 2,
      "val": 1231.12,
      "fruit": [
        {
          "id": 2,
          "name": "apple"
        }
      ]
    },
    {
      "id": 3,
      "val": 1231.12,
      "fruit": [
        {
          "id": 2,
          "name": "apple"
        },
        {
          "id": 3,
          "name": "orange"
        }
      ]
    }
  ],
  "fruits": [
    {
      "id": 1,
      "name": "apple"
    },
    {
      "id": 2,
      "name": "guava"
    },
    {
      "id": 3,
      "name": "banana"
    }
  ]
}

Problem Statement - Basically, I want to create a list of all items of root where fruit name is apple. Currently, my naive solution looks like this. This involves creating a temporary mutuable list and then add specific items to it.

Below solution works fine but is there any other better way to achieve the same.

val tempList = arrayListOf<RootItem>()

root?.forEach { item -> 
    item.fruit.filter {
        // filter condition
        it.id != null && it.name == "apple"
    }
    testList.add(item)
}

Answer 1

A combination of filter and any will do the work:

val result = root?.filter { item -> 
    item.fruits.any { it.id != null && it.name == "apple" } 
}

BTW: Your solution will not work. The filter function does return a new list, that you are not using. And you always add the item to the list, not only if the predicate returns true.