This effectively means it is a mandate to have an interface (Functional Interface) with only one abstract method (Functional Descriptor) to create a lambda expression. In Kotlin, mutableListOf () method is used to instantiate MutableList Interface. Java 8 introduced lambdas, which are built on top of a functional interface and a functional descriptor, meaning every lambda expression will map to the signature of an abstract method defined in that functional interface. In this codelab, you will use the Kotlin Playground to become familiar with lists in Kotlin and create a program for ordering different variations of noodle soup.

To keep things simple, department was passed in to the constructor even though this is an optional attribute. Learning how to create and use lists is an important programming concept to add to your toolbox, and it will enable you to create more sophisticated apps. Handling NULLs at compile time should result in cleaner code by removing unnecessary NULL checks in the form of an if statement, Objects.requireNonNull, Preconditions.checkNotNull, any other form

It promotes composability as in the departmentName() method.

The Employee class has an optional department, which is good enough to conclude that every employee may not be assigned a department Optional will not protect the code from NPE, but Optional has its advantages:

Kotlin immutable list