For background, I'm working on a Guess It App provided by our professor (the Starter App comes from Udacity), and I'm on the part of creating a GameViewModel in Kotlin. I was on the part of coding viewModel by implementing ViewModelProvider on onCreateView in the GameFragment class, like this:

viewModel = ViewModelProvider(this).get(GameViewModel::class.java)

But when I run the app it shows 'Unresolved reference: java'.

Here is my code on GameFragment:

package com.example.android.guesstheword.screens.game

import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.databinding.DataBindingUtil
import androidx.fragment.app.Fragment
import androidx.lifecycle.ViewModelProvider
import androidx.navigation.fragment.NavHostFragment.findNavController
import com.example.android.guesstheword.R
import com.example.android.guesstheword.databinding.GameFragmentBinding

/**
 * Fragment where the game is played
 */
class GameFragment : Fragment() {

    private lateinit var viewModel: GameViewModel

    // The current word
    private var word = ""

    // The current score
    private var score = 0

    // The list of words - the front of the list is the next word to guess
    private lateinit var wordList: MutableList<String>

    private lateinit var binding: GameFragmentBinding

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
                              savedInstanceState: Bundle?): View? {

        // Inflate view and obtain an instance of the binding class
        binding = DataBindingUtil.inflate(
                inflater,
                R.layout.game_fragment,
                container,
                false
        )
        
        //this line generates the error. In Android Studio, 'java' is colored red
        viewModel = ViewModelProvider(this).get(GameViewModel::class.java)

        resetList()
        nextWord()

        binding.correctButton.setOnClickListener { onCorrect() }
        binding.skipButton.setOnClickListener { onSkip() }
        updateScoreText()
        updateWordText()
        return binding.root

    }

    /**
     * Resets the list of words and randomizes the order
     */
    private fun resetList() {
        wordList = mutableListOf(
                "queen",
                "hospital",
                "basketball",
                "cat",
                "change",
                "snail",
                "soup",
                "calendar",
                "sad",
                "desk",
                "guitar",
                "home",
                "railway",
                "zebra",
                "jelly",
                "car",
                "crow",
                "trade",
                "bag",
                "roll",
                "bubble"
        )
        wordList.shuffle()
    }

    /**
     * Called when the game is finished
     */
    private fun gameFinished() {
        val action = GameFragmentDirections.actionGameToScore(score)
        findNavController(this).navigate(action)
    }

    /**
     * Moves to the next word in the list
     */
    private fun nextWord() {
        //Select and remove a word from the list
        if (wordList.isEmpty()) {
            gameFinished()
        } else {
            word = wordList.removeAt(0)
        }
        updateWordText()
        updateScoreText()
    }

    /** Methods for buttons presses **/

    private fun onSkip() {
        score--
        nextWord()
    }

    private fun onCorrect() {
        score  
        nextWord()
    }

    /** Methods for updating the UI **/

    private fun updateWordText() {
        binding.wordText.text = word

    }

    private fun updateScoreText() {
        binding.scoreText.text = score.toString()
    }
}

My gradle code (Module):

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-kapt'
apply plugin: 'kotlin-android-extensions'
apply plugin: "androidx.navigation.safeargs.kotlin"

android {
    compileSdkVersion 30
    defaultConfig {
        applicationId "com.example.android.guesstheword"
        minSdkVersion 19
        targetSdkVersion 30
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }

    buildFeatures {
        dataBinding true
    }

    compileOptions {
        sourceCompatibility = JavaVersion.VERSION_1_8
        targetCompatibility = JavaVersion.VERSION_1_8
    }

    kotlinOptions {
        jvmTarget = JavaVersion.VERSION_1_8
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation 'androidx.appcompat:appcompat:1.2.0'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    implementation 'androidx.legacy:legacy-support-v4:1.0.0'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test:runner:1.1.1'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.1.1'

    // KTX
    implementation 'androidx.core:core-ktx:1.3.1'

    // Navigation
    implementation "android.arch.navigation:navigation-fragment-ktx:1.0.0-rc02"
    implementation "android.arch.navigation:navigation-ui-ktx:1.0.0-rc02"

    // Lifecycles
    implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0'
}

And my other gradle code (

buildscript {
    ext.kotlin_version = '1.3.72'
    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:4.0.1'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        classpath "android.arch.navigation:navigation-safe-args-gradle-plugin:1.0.0-rc02"

        // 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

The IDE provided context actions, like renaming the reference, creating an extension property'KClass.java, and convert assignment to assignment expression, but I don't know what action to do after choosing each of them.

CodePudding user response：

I downloaded the project and everything looks fine. Please check the branch final-solution-code and compare it with what you have.

CodePudding user response：

I have solved the problem by adding this:

implementation("org.jetbrains.kotlin:kotlin-reflect:1.6.0")

on the dependencies under the gradle (module). Once the gradle synced, I then went to File > Invalidate Caches/Restart, closed Android Studio, went over to the proj folder and deleted the .idea folder, then opened Android Studio again, and opened the project again.