Initialize an Array class property in a constructor

Question

I'm new in kotlin and I'm tryng to understand how can I initialize this array in the correct way.

My java code:

 private Bitmap[] leftToRights;
 private Bitmap[] rightToLefts;
 private Bitmap[] topToBottoms;
 private Bitmap[] bottomToTops;


 //On constructor(colCount = 3)
 this.topToBottoms = new Bitmap[colCount]; // 3
 this.rightToLefts = new Bitmap[colCount]; // 3
 this.leftToRights = new Bitmap[colCount]; // 3
 this.bottomToTops = new Bitmap[colCount]; // 3

In Kotlin(my try):

//Declaration
private val leftToRights: Array<Bitmap>
private val rightToLefts: Array<Bitmap>
private val topToBottoms: Array<Bitmap>
private val bottomToTops: Array<Bitmap> 

//Init
Array<Bitmap>(colCount,/*Missing argument, what shall I initialize with?*/)
this.topToBottoms = Array<Bitmap>(colCount,/*Missing argument, what shall I initialize with?*)
this.rightToLefts = Array<Bitmap>(colCount,/*Missing argument, what shall I initialize with?*)
this.leftToRights = Array<Bitmap>(colCount,/*Missing argument, what shall I initialize with?*)
this.bottomToTops = Array<Bitmap>(colCount,/*Missing argument, what shall I initialize with?*)

So what is a good way to initialize these arrays same way as java does? Can someone explain how it works in java, does java initialize Bitmap array with null?

Sorry for my english, hope you can understand it. I'm open to any question about this post.

Yes, Java initializes those arrays with null.

Louis Wasserman
– Louis Wasserman

2019-05-08 21:54:07 +00:00
Commented May 8, 2019 at 21:54 — Louis Wasserman
– Louis Wasserman, Commented May 8, 2019 at 21:54

forpas · Accepted Answer · 2019-05-08 17:04:27Z

3

In Kotlin you can declare the array with lateinit:

private lateinit var leftToRights: Array<Bitmap?>

Notice that you must use Bitmap? instead of Bitmap if you want to fill the array with nulls when you initialize it and also you must use var instead of val because lateinit is allowed only on mutable properties.
Later initialize the array using the size you want with arrayOfNulls():

leftToRights = arrayOfNulls(3)

answered May 8, 2019 at 17:04

forpas

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Comments

Quinn · Accepted Answer · 2019-05-08 16:36:18Z

2

In kotlin if you want to initialize an array with nulls then you have to make it accept null as a value.

To do this you would do something like:

private val leftToRights: Array<Bitmap?> = arrayOf(null, null, null)

By specifying it's an array of Bitmap? rather than Bitmap it allows you to make an array of null's since each item is an optional bitmap.

answered May 8, 2019 at 16:36

Quinn

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Comments

Denis Dorohov · Accepted Answer · 2019-05-08 19:06:15Z

Answering your second question, yes, object fields with non-primitive types in Java will be initialised with nulls.

In addition to previous response, if you don't want to use var lateinit and you prefer code readability over performance gain from fixed-size arrays, you could use something like this:

private val leftToRights = mutableListOf<Bitmap>()
private val rightToLefts = mutableListOf<Bitmap>()
private val topToBottoms = mutableListOf<Bitmap>()
private val bottomToTops = mutableListOf<Bitmap>()

P.S. I would gladly make this a comment instead of response, but I don't have enough reputation :(

David Soroko · Accepted Answer · 2019-05-08 19:25:51Z

By your design, the values in Bitmap arrays can be null. In Kotlin this is reflected by the arrays having the typeArray<Bitmap?>. With this in mind the following Rows class will work:

class Bitmap

class Rows(val colCount: Int) {
    val leftToRights by lazy { init() }
    val rightToLefts by lazy { init() }
    val topToBottoms by lazy { init() }
    val bottomToTops by lazy { init() }

    private fun init() = Array<Bitmap?>(colCount) { null }
}

fun main() {
    val rows = Rows(3)

    with(rows) {
        leftToRights[1] = Bitmap()

        println(leftToRights[0]) // null
        println(leftToRights[1]) // Bitmap@...
        println(leftToRights[2]) // null
    }
}

This way you dont need lateinit (lazy does this for you) and you dont need to use var. Here https://kotlinlang.org/docs/reference/delegated-properties.html is an explanation on how by lazy works

birneee · Accepted Answer · 2019-05-09 09:22:13Z

0

There are multiple ways to initialize arrays in Kotlin with null:

val leftToRights: Array<Bitmap?> = arrayOf(null, null, null)
val leftToRights = Array<Bitmap?>(3){_ -> null }
val leftToRights = arrayOfNulls<Bitmap?>(3)

answered May 9, 2019 at 9:22

birneee

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