If your Java code doesn't have these annotations, then Kotlin will treat Java types as platform types. Kotlin can recognize such annotations when you're calling Java code from Kotlin code and will treat types according to their annotations. For example, you can use the JetBrains annotations and (from the package) or annotations from Eclipse ( ). Such annotations aren't part of the standard library, but you can add them separately. In Java, you can use annotations showing whether a variable can or cannot be null. In Java, if you don't write null checks, methods may throw a NullPointerException:įun stringLength(a: String?): Int = a?.length ?: 0 We say 'almost' because, even though intrinsic checks are generated, their overhead is minimal. That means there's almost no runtime overhead for working with nullable types in Kotlin. All checks are performed at compile time. At runtime, objects of nullable types and objects of non-nullable types are treated the same: A nullable type isn't a wrapper for a non-nullable type. Kotlin prohibits such calls at compile time and thereby prevents lots of possible exceptions. If a variable can be null, it's not safe to call a method on the variable because this can cause a NullPointerException.

It is a way to indicate which variables can possibly hold a null value.

The most important difference between Kotlin's and Java's type systems is Kotlin's explicit support for nullable types. The second part, starting from Checking the result of a function call, examines several specific cases to explain certain differences.

The first part of this guide covers the most important difference – support for nullable types in Kotlin and how Kotlin processes types from Java code. It will help you migrate from Java to Kotlin and write your code in authentic Kotlin style. This guide covers differences between Java's and Kotlin's approaches to handling possibly nullable variables. There are many ways to write code in order to minimize the probability of receiving null pointer exceptions. When a variable contains null, an attempt to dereference the variable leads to a NullPointerException. Nullability is the ability of a variable to hold a null value.

Kotlin null safety