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Nullability issues with generic types being used for Java interoperation. Usage of the !! operator that is described below.ĭata inconsistency with regard to initialization, such as when:Īn uninitialized this available in a constructor is passed and used somewhere (a "leaking this").Ī superclass constructor calls an open member whose implementation in the derived class uses an uninitialized state.Īttempts to access a member of a null reference of a platform type The only possible causes of an NPE in Kotlin are:Īn explicit call to throw NullPointerException(). In Java this would be the equivalent of a NullPointerException, or an NPE for short. One of the most common pitfalls in many programming languages, including Java, is that accessing a member of a null reference will result in a null reference exception. Kotlin's type system is aimed at eliminating the danger of null references, also known as The Billion Dollar Mistake. This ensures that a NullPointerException doesn’t occur.Null safety Nullable types and non-null types

If you want a variable to accept nulls, you have to mark the type with a ?: val x: Int? = null Compile-time checkīut, from that point on, the compiler will force you to check the null before doing anything with the variable. Therefore, this won’t compile: val x: Int = null Remember that in Kotlin all types are objects. That is, no variable, by default, can be set to null. Nulls in Kotlin don’t exist until you say otherwise. Thanks to Kotlin, you’ll work in a much safer environment (even with Java libraries), which will minimize these problems. If you look at your bug manager, I’m sure that +90% of the errors you see are NullPointerException. If you want to start today, I recommend you take a look at my free training, where you will have an hour and a half of content to know what are your next steps to become an expert in Kotlin. Nulls are one of the most error-prone points you get when you are coding with Java. Tony Hoare, the creator of the idea of nulls, calls itself “the billion dollar mistake”. It had taken me too long to write an article to one of the most important parts of Kotlin: the treatment of nullity.