To compile a single Kotlin file:

$ kotlinc [filename].kt

To compile and create a runnable .jar file:

$ kotlinc [filename].kt -include-runtime -d [executablename].jar

To run the application:

$ java -jar [executablename].jar

By default, all member functions to a class are final (aka they cannot be overriden). To allow a function to be overriden, you must declare it open.

In a derived class, to override a function, you must use the override keyword.

Furthermore, override will allow that function to be overriden by a further subclass (aka it becomes open by default). To prevent this, you must specify final with override. An example:

open class Base {
    // Can be overriden
    open fun foo () {}

    // Cannot be overriden
    fun bar () {}
}

class Derived() : Base() {
    // Specifies this is an overriden function
    // that can be overriden by any subclass
    override fun foo () {}
}

class AnotherDerived() : Base() {
    // Specifies this is an overriden function
    // But cannot be overriden by any subclass
    final override fun foo() {}
}