How can I create null class object in Kotlin?

Question

In java, we can declare a null class object like this

VideoRecord videoRecord;

After that, we can check if it is null or not

if (videoRecord == null){
    videoRecord = new VideoRecord();
}

I am new to Kotlin and I would like to do the same thing in Kotlin. I have tried this code below.

lateinit var videoRecord1:VideoRecord

if (videoRecord1 == null){
    videoRecord1 = VideoRecord()
}

But it gives me a warning like the condition videoRecord1 == null is always false. How can I make the same thing in Kotlin?

You have to use the ? in order to explicitly make a variable nullable, like var videoRecord1: VideoRecord?. Currently, the videoRecord1 cannot be null, which makes the warning correct. — deHaar
– deHaar, Commented Oct 13, 2020 at 11:06
If it is null I am going to create an object and I will run a different method after I create an object from class I will run another method @Lino — salih suat kükrer
– salih suat kükrer, Commented Oct 13, 2020 at 11:10

andika_kurniawan · Accepted Answer · 2020-10-13 11:07:32Z

4

var videoRecord: VideoRecord? = null

you can initial its variable like this

nullable reference : https://kotlinlang.org/docs/reference/null-safety.html

answered Oct 13, 2020 at 11:07

andika_kurniawan

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Lino · Accepted Answer · 2020-10-13 11:26:14Z

Depending on what you're trying to achieve, e.g. lazy initialization then this may be more suitable for you than the answer given by andika_kurniawan:

val videoRecord: VideoRecord by lazy { VideoRecord() }

This will lazily initialize videoRecord the first time it is accessed.

The advantage of this way is that you don't have to check for null when accessing videoRecord, because it cannot be null. This simplifies the usage of that variable significantly. Additionally you can mark videoRecord as val, meaning it is final, so it cannot be overwritten.

The example shown by @andika_kurniawan:

var videoRecord: VideoRecord? = null

Has some caveats and disadvantages:

You always have to check that videoRecord is not null (and it gets tedious), see this example:
```
if (videoRecord != null) {
    videoRecord.callAMethod()
}
```
The above will not work, because videoRecord defined as nullable, so you need to use the !! operator to tell kotlin that you're sure that the variable is not null:
```
if (videoRecord != null) {
    videoRecord!!.callAMethod()
}
```
You can of course use other idioms, like the null-safe operator ?.
```
videoRecord?.callAMethod()
```
Or the already mentioned !! operator (which throws an exception when it is null) but this time without a null check:
```
videoRecord!!.callAMethod()
```
You may also use local variables, which simplify the usage (a little bit):
```
val videoRecord = videoRecord
if (videoRecord != null) {
    videoRecord.callAMethod()
}
```
The variable is not final, so nothing stops you from doing this somewhere in your code:
```
videoRecord = null
```
You have to initialize it somewhere, so if you have multiple methods accessing videoRecord you first have to initialize it if it hasn't already been, introducing unnecessary redundancy.

Priyanka · Accepted Answer · 2020-10-13 11:25:24Z

1

In kotlin, declaration of variable is different from java as kotlin is null safe language. You have to declare variable nullable. Only then its value can be null.

You can declare it as below

var videoRecord: VideoRecord? = null

To access nullable values you have to use !! or ? with variable names.

answered Oct 13, 2020 at 11:25

Priyanka

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