Kotlin random() always generates the same "random" numbers

Question

I have created an app which should choose an image randomly from an array of images. On my Emulator Nexus 5X Android 5.1 everything works as expected. As soon as I try the same on my real device Galaxy Note 10 Lite I always get the same "random" numbers in same order. I first need to restart my phone to generate a new list of "random" numbers which is then always the same. Example: My array contains 200 elements, I open the app on my Galaxy and it chooses the following random number for the image ids: 43, 12, 176, 33, 2, 78. Then I close the app and I open the app again, now it has the exact same "random" numbers again: 43, 12, 176, 33, 2, 78. I need to restart my phone to get new random numbers, which will stay the same until I restart my phone again. On my emulator everything works fine and I get new random numbers always when I restart the app as expected.

Here is my full code of my app without array list of images:

MainActivity.kt

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

 val imageList = arrayOf(Image(R.drawable.image1, false),
            Image(R.drawable.image2, false),
            Image(R.drawable.image3, false))

val imageViewMain = findViewById<ImageView>(R.id.imageViewMain)
    loadNextImage(imageViewMain, imageList)

    imageViewMain.setOnClickListener {
        val dialogClickListener =
            DialogInterface.OnClickListener { _, which ->
                when (which) {
                    DialogInterface.BUTTON_POSITIVE -> {
                        loadNextImage(imageViewMain, imageList)
                    }
                    DialogInterface.BUTTON_NEGATIVE -> { }
                }
            }
        val builder: AlertDialog.Builder = AlertDialog.Builder(this)
        builder.setMessage("Nächstes Bild?").setPositiveButton("Ja", dialogClickListener)
            .setNegativeButton("Nein", dialogClickListener).show()
    }
}

private fun getNextChoice(): Int {
    return (0..1).random()
}

private fun getNextImage(imageList: Array<Image>): Int {
    val listSize = imageList.size
    var imageId: Int
    do {
        imageId = (0 until listSize).random()
    } while (imageList[imageId].played)

    imageList[imageId].played = true
    return imageList[imageId].image
}

private fun loadNextImage(imageViewMain: ImageView, imageList: Array<Image>) {
    val imageQuestionmark = R.drawable.questionmark
    val nextChoice = getNextChoice()
    if (nextChoice == 0) {
        imageViewMain.load(imageQuestionmark)
    } else if (nextChoice == 1) {
        imageViewMain.load(getNextImage(imageList))
    }
    Toast.makeText(this, "Bild hat geladen", Toast.LENGTH_SHORT).show()
}
}

Image:

data class Image(
    val image: Int,
    var played: Boolean
)

EDIT: I tried what cactustictacs suggested in the comment and create a simple app, once with the kotlin random function and once with the java random function. here is the code I used:

Kotlin:

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.Button
import android.widget.Toast

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val buttonTest = findViewById<Button>(R.id.buttonTest)

        buttonTest.setOnClickListener {
            val getRandomNumber = (0..999).random()
            Toast.makeText(this, getRandomNumber.toString(), Toast.LENGTH_SHORT).show()
        }
    }
}

Java:

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.widget.Button;
import android.widget.Toast;

import java.util.Random;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Button buttonTest = (Button) findViewById(R.id.buttonTest);

        buttonTest.setOnClickListener(v -> {
            int randomNumber = new Random().nextInt(999);
            Toast.makeText(this, "" + randomNumber, Toast.LENGTH_SHORT).show();
        });
    }
}

on Kotlin I get the same behavior as with my inital problem, doesnt matter what I do with the app (I CAN EVEN UNINSTALL AND INSTALL AGAIN) I always get the same set of numbers. On Java its working as exptected, as soon as I close the app I get a new set of numbers. So the error definetly lays in kotlin.

Maybe it helps, my Android version is 12 and my phone Galaxy Note 10 Lite.

Answer 1

This is quite a terrible implementation of Kotlin defaul Random class. Java Random class tries a lot to always use a different seed on every new instance whereas Kotlin hardcoded the same seed through the whole device. How can this be a default behavior for a Random implementation. It took me quite a lot to understand it.

See Java implementation:

 public Random() {
    this(seedUniquifier() ^ System.nanoTime());
}

private static long seedUniquifier() {
    // L'Ecuyer, "Tables of Linear Congruential Generators of
    // Different Sizes and Good Lattice Structure", 1999
    for (;;) {
        long current = seedUniquifier.get();
        long next = current * 181783497276652981L;
        if (seedUniquifier.compareAndSet(current, next))
            return next;
    }
}

private static final AtomicLong seedUniquifier
    = new AtomicLong(8682522807148012L);

Aaaaaand here comes Kotlin one:

companion object Default : Random(), Serializable {
    private val defaultRandom: Random = defaultPlatformRandom()

    private object Serialized : Serializable {
        private const val serialVersionUID = 0L

        private fun readResolve(): Any = Random
    }

    private fun writeReplace(): Any = Serialized

    override fun nextBits(bitCount: Int): Int = defaultRandom.nextBits(bitCount)
    override fun nextInt(): Int = defaultRandom.nextInt()
    override fun nextInt(until: Int): Int = defaultRandom.nextInt(until)
    override fun nextInt(from: Int, until: Int): Int = defaultRandom.nextInt(from, until)

defaultRandom is a singleton always initiates with same seed...

(I took this code through the Android Studio sources...)

Note: So it was a bug on kotlin version 1.7.10 and Android api less than 33-34 something. Fixed on 1.7.20...

Answer 2

I once had this issue too. My solution is to use a seeded random object instead of calling the function Random.nextSomething(). Then I seed currentTimeInMillis as the seed.

var randomGenerator = Random(System.currentTimeMillis())
var result = randomGenerator.nextInt(30, 50)

Answer 3

It looks like you're calling loadNextImage() from activity's onCreate(). That means that unless the activity is destroyed, it'll never re-generate the random IDs that you want. What happens if you force-stop the activity, and then relaunch it? I would expect that you get a new set of IDs.

If I'm right, and you want a new set of IDs every time you open the activity, then the solution is to call loadNextImage() from onResume() (and if you don't want it generated every time the activity is resumed, you'll need to include some logic that decides when to regenerate those IDs)