Void safety is a static (compile-time) guarantee that a void call will never arise.
In such cases, the call above will be a void call, leading to a run-time exception, often resulting in abnormal termination of the program. At execution time, however, a reference can be void (or null). Where f denotes an operation and x denotes a reference to some object. In object-oriented languages, access to objects is achieved through references (or, equivalently, pointers). It kills one major benefit of using Kotlin.Void safety (also known as null safety) is a guarantee within an object-oriented programming language that no object references will have null or void values. It is very error-prone as plain old NPE may appear at run-time again. It is nothing more than a way to stop using all the benefits of Kotlin type system. The !! operator in Kotlin allow the programmer to tell the compiler “Ok, I know what I do, and I promise, it won’t be null”.
lateinit: In case of late-initialization (like in a test), there is a specific language keyworkd lateinit which allow to declare a non-null member without initializing it in the constructor.Īll of that makes dealing with nullability really great in Kotlin. Therefore the Kotlin compiler is able to smart-cast. Contracts: requireNotNull(x) x.lenght compiles, because requireNotNull defines a contract stating that if the function does not throw, it means the argument is not null. Smart-cast: if (x != null) x.length compiles, because the Kotlin compiler knows that it cannot be null when length is called. It can also be used to return the function or throw an exception. Elvis operator: val x: String = input ?: "default" allow to choose what to do if something return null. 
Safe call: nullable?.doSomething() will only call doSomething if nullable is not null. Kotlin also provide a set of convenient language construct in order to deal with nullability: This is very useful, because it allows the compiler to detect potential null-pointer exception at compile-time. It means that one has to use a different type if something may accept null (for instance String?) than if it cannot be null (for instance String). In Kotlin, nullability is encoded in the type system.
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