'Unresolved reference: java' 在 Android 工作室

Question

作为背景，我正在开发我们教授提供的 Guess It 应用程序（入门应用程序来自 Udacity），并且我正在使用 Kotlin 创建一个 GameViewModel。我通过在 GameFragment class 的 onCreateView 上实现 ViewModelProvider 来编码 viewModel，如下所示：

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

但是当我运行应用程序时它显示 'Unresolved reference: java'。

这是我在 GameFragment 上的代码：


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()
    }
}

我的 gradle 代码（模块）：

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'
}

还有我的其他 gradle 代码（

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

IDE 提供了上下文操作，例如重命名引用、创建扩展属性'KClass.java 以及将赋值转换为赋值表达式，但我不知道是什么操作选择每一项后要做的事情。

Answer 1

我下载了项目，一切正常。请检查分支final-solution-code并与你的代码进行比较。

Answer 2

我已经通过添加以下内容解决了问题：

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

关于 gradle（模块）下的依赖项。 gradle 同步后，我转到文件 > 使 Caches/Restart 无效，关闭 Android Studio，转到 proj 文件夹并删除 .idea 文件夹，然后打开 Android再次工作室，再次打开项目。

'Unresolved reference: java' 在 Android 工作室

'Unresolved reference: java' in Android Studio

java

android

android-fragments

kotlin