val intent = Intent(this,HelloActivity::class.java) 
startActivity(intent)

Why do we need to add .java in the end? Isn't HelloActivity a .kt file? 
    ts because you are calling Java Code from the Kotlin as Intent is a Java
    Class in Android which accepts .class reference as mentioned below. 
    Intent(Context packageContext, Class cls) Kotlin class reference is not 
    the same as a Java class reference. To obtain a Java class reference, use 
    the .java property on a KClass instance as SampleActivity::class returns 
    KClass which is Kotlin class not a java class. So you cannot pass Kotlin
    class reference to Java (Intent in your case) and so you have to pass Java 
    class reference