CODEWITHSUNDEEP
kotlin
Zorgan
Zorgan

Reputation: 9123

Why does lateinit work on Object type but not primitive type?

This answer says that Object types use the null value on lateinits of Object type. Can someone elaborate on this? What do they mean by "uses the null value"?

Furthermore it says "For primitive types, there is no such value" - why is there no

Upvotes: 2

Views: 89

Answers (1)

hotkey
hotkey

Reputation: 147971

On the JVM, which was historically the first platform targeted by Kotlin, there is a distinction between the primitive types and classes, or object types.

They have different runtime representations: a primitive value is stored directly in the field, which can only hold one of the corresponding primitive type values (e.g. only true and false for boolean) and has no special representation for a missing value. In comparison, a class-typed field stores an indirect reference to an instance that is allocated somewhere in the heap, and a class-typed field can hold a special null reference that doesn't point to any instance.

The Kotlin lateinit properties are compiled to a single field and they internally use the null value to indicate that the property has not yet been initialized. As primitive-typed JVM fields can't have a null value, Kotlin doesn't allow lateinit properties of types that are normally mapped to Java primitives, as that would require a different representation, and no acceptable solution existed, as the other answer says.

Instead, you can use by Delegates.notNull() to achieve similar behavior with the mapped types.

Upvotes: 5

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