If you see it, that means that this type came from Java and it is so-called of unknown nullability. You can only sometimes find these type in error messages. That means you can't declare a variable of such type in the Kotlin language. You can sometimes see the annotation type followed by the exclamation mark. It's neither nullable nor a non-nullable type. It even has a special name, it's called a platform type. Such unannotated Java type will be visible from Kotlin as a regular Java type and it will behave like in Java. An interesting question is, what happens if you use unannotated Java types? These are probably the majority of Java types, let's discuss that. If you use the same annotations to annotate your Java code, then annotated Java types will be visible from Kotlin as nullable and non-nullable Kotlin types accordingly. Java types are not divided into nullable and non-nullable types, so what happens when you call Java methods from Kotlin? Kotlin nullable types under the hood are implemented using annotations. In this video you'll learn how to mix Java and Kotlin code in terms of nullability. We'll discuss: basic syntax, nullability, functional programming with Kotlin, object-oriented programming with Kotlin, the power of the Kotlin standard library, and Java interoperability. Note that this course won't cover the programming fundamentals. The course is based on your Java experience it shows the similarities between the two languages and focuses on what's going to be different. We'll have a basic overview of the language, as well as a discussion of many corner cases, especially concerning Java interoperability. This course aims to share with you the power and the beauty of Kotlin. Kotlin code might be compiled not only to JVM bytecode but to JavaScript and Native code as well, but this course is focused on Kotlin/JVM. It’s a pragmatic language with a very low learning curve, and can be quickly grasped by Java developers. Kotlin is 100% compatible with all existing Java frameworks, and has good tooling support. It can be used almost everywhere Java is used today: for server-side development, Android apps, and much more. Kotlin is concise, safe, pragmatic, and focused on interoperability with Java code. The Kotlin programming language is a modern language that gives you more power for your everyday tasks.
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