I have a listof() variable with a Coordinate(latitude,longitude) data class, for example this is how I initialise it:

var coordinate = listof<Coordinate>()

I would like to add latitude and longitude for each marker of a polygon by retrieving them from Firebase Database, for example:

val databaseReference = FirebaseDatabase.getInstance().getReference("polygon").child("coordinate")

databaseReference.addListenerForSingleValueEvent(object: ValueEventListener {
    override fun onDataChange(snapshot: DataSnapshot) {
                if (snapshot.exists()) {
                    val numberOfCoor = snapshot.childrenCount.toInt()
                                        var i = 1

                                        for (n in i until numberOfCoor   1) {
                                            val latitude = snapshot.child("coordinate$i")
                                                .child("latitude").getValue().toString().toDouble()
                                            val longitude = snapshot.child("coordinate$i")
                                                .child("longitude").getValue().toString().toDouble()
                                            coordinate.plus(Coordinate(longitude, latitude))
                             }
               }
override fun onCancelled(error: DatabaseError) {

  }
})

So from the code above, can be seen that I add each latitude and longitude by using coordinate.plus(Coordinate(longitude, latitude))

But when I download this GeoJSON file, the coordinate has no latitude and longitude.

So how to add item to a listof() function in Kotlin?

Thank you.

CodePudding user response：

listOf returns an immutable List<out Coordinate>

To be able to add items to the list, you can instantiate it with mutableListOf<Coordinate>

CodePudding user response：

The .plus method does not modify the original list. Alternatively you can use .add.

It is good practice to use mutablelist. You can add and remove elements.

var coordinate = mutableListOf<Coordinate>()
// ...
override fun onDataChange(snapshot: DataSnapshot) {
  if (snapshot.exists()) {
    val numberOfCoor = snapshot.childrenCount.toInt()
    var i = 1

    for (n in i until numberOfCoor   1) {
      val latitude = snapshot.child("coordinate$i")
        .child("latitude").getValue().toString().toDouble()
      val longitude = snapshot.child("coordinate$i")
        .child("longitude").getValue().toString().toDouble()
      coordinate.add(Coordinate(longitude, latitude))
    }
  }
}