I want to refer to the classes defined by User.kt and ActionView in MainActivity.kt.

unresolved reference

In MainActivity, you cannot reference classes in other files.

I want to refer to User class of User.kt and ActionView class of ActionView.kt.

MainActivity

package sample.qiitaclient
import android.os.Bundle
import android.support.design.widget.FloatingActionButton
import android.support.design.widget.Snackbar
import android.support.v7.app.AppCompatActivity
import android.support.v7.widget.Toolbar
import android.view.View
import android.view.Menu
import android.view.MenuItem
import sample.qiitaclient.model.Article
class MainActivity: AppCompatActivity () {
    override fun onCreate (savedInstanceState: Bundle?) {
        super.onCreate (savedInstanceState)
        setContentView (R.layout.activity_main)
        val toolbar = findViewById (R.id.toolbar) as Toolbar
        setSupportActionBar (toolbar)
        val fab = findViewById (R.id.fab) as FloatingActionButton
        fab.setOnClickListener {view->
            Snackbar.make (view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction ("Action", null) .show ()
        }
        val articleView =  ArticleView  (applicationContext)  unresolved reference 
        articleView.setArticle (Article (id = "123",
                title = "Introduction to Kotlin",
                url = "http://www.example.com/articles/123",
                user =  User  (id = "456", name = "taro", profileImageUrl = "")))  unresolved reference 
        setContentView (articleView)
    }
}

java/sampleqiitaclient/model/User.kt

package sample.qiitaclient.model

data class User (val id: String,
                val name: String,
                val profileImageUrl: String)

java/sampleqiitaclient/view/ActionView.kt

package sample.qiitaclient.view


import android.content.Context
import android.graphics.Color
import android.util.AttributeSet
import android.view.LayoutInflater
import android.widget.FrameLayout
import android.widget.ImageView
import android.widget.TextView
import sample.qiitaclient.R
import sample.qiitaclient.model.Article
class ArticleView: FrameLayout {
    constructor (context: Context?): super (context)
    constructor (context: Context ?,
                attrs: AttributeSet?): super (context, attrs)
    constructor (context: Context ?,
                attrs: AttributeSet ?,
                defStyleAttr: Int): super (context, attrs, defStyleAttr)
    constructor (context: Context ?,
                attrs: AttributeSet ?,
                defStyleAttr: Int,
                defStyleRes: Int): super (context, attrs, defStyleAttr, defStyleRes)
    var profileImageView: ImageView? = null
    var titleTextView: TextView? = null
    var userNameTextView: TextView? = null
    init {
        LayoutInflater.from (context) .inflate (R.layout.view_article, this)profileImageView = findViewById (R.id.profile_image_view) as ImageView
        titleTextView = findViewById (R.id.title_text_view) as TextView
        userNameTextView = findViewById (R.id.user_name_text_view) as TextView
    }
    fun setArticle (article: Article) {
        titleTextView? .text = article.title
        userNameTextView? .text = article.user.name
        profileImageView? .setBackgroundColor (Color.RED)
    }
}

When I checked on the net, there seems to be a problem with the initial setting of kotlin, so
m (_ _) m

build.gradle (Project)

// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {
    ext.kotlin_version = '1.1.2-4'
    repositories {
        jcenter ()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:2.3.1'
        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 {
        jcenter ()
    }
}
task clean (type: Delete) {
    delete rootProject.buildDir
}

build.gradle (Module)

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android-extensions'
android {
    compileSdkVersion 25
    buildToolsVersion "25.0.3"
    defaultConfig {
        applicationId "sample.qiitaclient"
        minSdkVersion 15
        targetSdkVersion 25
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile ('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}
dependencies {
    compile fileTree (dir: 'libs', include: ['* .jar'])
    androidTestCompile ('com.android.support.test.espresso: espresso-core: 2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile 'com.android.support:appcompat-v7:25.3.1'
    compile 'com.android.support.constraint: constraint-layout: 1.0.2'
    compile 'com.android.support:design:25.3.1'
    testCompile 'junit: junit: 4.12'
    compile "org.jetbrains.kotlin: kotlin-stdlib-jre7: $kotlin_version"
}
repositories {
    mavenCentral ()
}

AndroidStudio 2.3.1
Kotlin 1.1.2-2