error: cannot find symbol class for add kotlin class into java class in android studio 3.0 stable

To work with Kotlin files you need to add Kotlin to your project.

project/build.gradle

buildscript {
    ext.kotlinVersion = '1.1.51'
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.0.0'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion"
    }
}

project/module/build.gradle

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlinVersion"
}

What's next

Since you're using support library 25.3.1 and Android plugin 3.0.0 your next question will probably be this: style attribute '@android:attr/windowEnterAnimation' not found.