::javaClass

::class.java

// Will default to Foo's companion class if there is one
// otherwise it complains
// "Classifier 'Foo' does not have a companion object and thus must be initialized here"
Foo

// KClass<Foo>
Foo::class

// java.lang.Class<Foo>
Foo::class.java

// don't use this.
// yields some kinda generic `Class<T>` that isn't useful for comparison
Foo::javaClass

// yields the same thing as Foo::class.java, use this on instances
// to match against the class type if you need to
foo.javaClass
// e.g.
when (foo.javaClass) {
  Foo::class.java -> { ... }
}

javaClass