CODEWITHSUNDEEP
androidkotlin
lollerskates
lollerskates

Reputation: 1147

Why is Kotlin's mutableListOf() returning a null value

I am working through Google's Android developer codelab for the the unscramble app. I am trying to initialize my variable wordsList to be an empty mutable list of strings with:

private var wordsList: MutableList<String> = mutableListOf()

However, during my debugging process, I see that the value of wordsList is null and not an empty list like []. I tested the code in kotlin playground and it should be []. I attached a screenshot of what android studio is showing me. What am I doing wrong, why am I not seeing the expected behavior? enter image description here

Here is the gradle file for my project:

// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {
    ext.kotlin_version = "1.4.21"
    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:4.1.1'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

and the gradle file for my app


plugins {
    id 'com.android.application'
    id 'kotlin-android'
    id 'kotlin-kapt'
}

android {
    compileSdkVersion 30
    buildToolsVersion "30.0.2"

    defaultConfig {
        applicationId "com.example.android.unscramble"
        minSdkVersion 23
        targetSdkVersion 30
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
    kotlinOptions {
        jvmTarget = '1.8'
    }
    buildFeatures {
        viewBinding = true
    }
}

dependencies {

    implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
    implementation 'androidx.core:core-ktx:1.3.2'
    implementation 'androidx.appcompat:appcompat:1.2.0'
    implementation 'com.google.android.material:material:1.2.1'
    implementation 'androidx.constraintlayout:constraintlayout:2.0.4'
    // LiveData
    implementation 'androidx.lifecycle:lifecycle-livedata-ktx:2.2.0'
    // ViewModel
    implementation 'androidx.lifecycle:lifecycle-viewmodel-ktx:2.2.0'
    implementation 'androidx.fragment:fragment-ktx:1.2.5'
}

Upvotes: 3

Views: 1378

Answers (2)

Aktawa
Aktawa

Reputation: 21

I stumbled into the same problem. Putting the init block after the variable declaration solved the problem for me

private var wordsList: MutableList<String> = mutableListOf()
...
init {
    Log.d("GameFragment", "GameViewModel created!")
    getNextWord()
}

Upvotes: 0

Fedor Chebnev
Fedor Chebnev

Reputation: 86

Referring to Kotlin docs:

During an instance initialization, the initializer blocks are executed in the same order as they appear in the class body, interleaved with the property initializers

So, your breakpoint is probably hit from the upper init { } block when wordsList is not initialized yet. You may choose to initialize your variable before calling getNextWord() or just move this init { } block down so it is executed when all your properties are already initialized.

Upvotes: 7

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