Room + cannot find implementation DB + DB_Impl does not exist

If you use Kotlin, then you have to use kapt instead of annotationProcessor, and must also apply kotlin-kapt plugin.

apply plugin: 'kotlin-android'
apply plugin: 'kotlin-kapt'

dependencies {
    implementation "androidx.lifecycle:lifecycle-extensions:2.0.0-alpha1"
    kapt "androidx.lifecycle:lifecycle-compiler:2.0.0-alpha1"
    implementation "androidx.room:room-runtime:2.0.0-alpha1"
    kapt "androidx.room:room-compiler:2.0.0-alpha1"

For Java

in app build.gradle

use

implementation 'android.arch.persistence.room:runtime:1.1.1'
annotationProcessor 'android.arch.persistence.room:compiler:1.1.1'