Below is the code to define a String and a String? val canBeNull: String? = null
We will start with the simplest case, null safety with a local nullable variable. The same principle can be applied to any other type. But at the end of the day, Kotlin code is still compiled into bytecode and runs on the JVM.Īs an example, today we will find out how Kotlin distinguishes between the String and String? types when the JVM only knows of Ljava/lang/String. Kotlin distinguishes between references that can hold null (nullable types) and references that cannot (non-nullable types).
One of Kotlin’s improvements over Java is null safety. In Kotlin, the type system distinguishes between references that can hold null (nullable references) and those that cannot (non-null references).Note: phiên bản Tiếng Việt của bài này ở link dưới. Other issues caused by external Java code. For example, a piece of Java code might add null into a Kotlin MutableList, therefore requiring a MutableList for working with it. 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.
Null safety Nullable types and non-null types
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