Consequently, any variable cannot contain the null value unless you explicitly say it can. This is why a few programming languages introduced the possibility to indicate that a variable is nullable, or in other words, that it can accept the null value. This leads to several issues that may be difficult to address while programming. One of the most common problems coming with programming languages is that accessing a variable with a null value causes a null reference exception at runtime. A complete guide to null safety in Kotlin Spreading knowledge through writing is my mission. We have a class called Message.Antonello Zanini Follow I'm a software engineer, but I prefer to call myself a technology bishop. Digging into the source code of Kotlin's compiler, one can find various flavors of the nullability annotation that aims to achieve this. The way to leverage the existing null safety mechanism is by representing platform types as an actual Kotlin type (nullable or non-nullable). When we try to invoke Java code from Kotlin, null checks are relaxed due to platform types. The following mnemonic notation can be used to denote them:

Hence, there is no syntax in the language to represent them. E.g.įrom the Kotlin compiler perspective, it is a type that can be nullable and non-nullable. if we try to declare a variable of a platform type, we will receive a compilation error. These types cannot be mentioned explicitly in the program, i.e.

Java's declarations are treated as platform ( flexible) types in Kotlin. In this post, we will focus on how to take advantage of null safety when invoking Java code.

Using an uninitialized this in a constructor (data inconsistency).

Invoking external Java code, which in turn can throw NPEs.

Nevertheless, we can still encounter NPEs by: Which means we can deal with null at compile time rather than bumping into a Null Pointer Exception. The JVM already provides a safety net in the form of bytecode verification, buffer overflow, type safety, etc., but Kotlin took this to a step further and baked null safety right into the type system.

Kotlin is null