How deeply does Kotlin in Action, 2E cover out keyword?

Defines preserved subtyping, restricts T to out positions, and contrasts declaration-site variance with Java wildcards.

Out keyword

The out modifier declares covariant use of a Kotlin type parameter so a parameterized type can accept subtype arguments.

The out modifier declares covariance: if Cat is a subtype of Animal, Herd<Cat> can be used where Herd<Animal> is expected. This is useful for collection-like classes that produce values without accepting replacements. section 11.3.3

Passing a herd of cats to a broader API

An invariant Herd<T> rejects feedAll(cats) because Herd<Cat> is not a subtype of Herd<Animal>. Marking the parameter covariant fixes the call: class Herd<out T : Animal> { /* ... */ } fun takeCareOfCats(cats: Herd<Cat>) { for (i in 0..<cats.size) cats[i].cleanLitter() feedAll(cats) } The declaration is safe because Herd exposes T only through operations that produce animals.

Use out when the type parameter appears only in out positions:

A covariant class can return T but cannot consume T, preserving type safety. section 11.3.3

Covariant class references

The annotation parameter KClass<out Any> accepts KClass<CompanyImpl>, allowing a concrete class reference to stand in for a reference to any class:

Kotlin declares variance on the class itself, unlike Java's use-site wildcards. The related in modifier reverses subtyping for consumer-like types; out applies to producer-like types. section 11.3.4

How Kotlin in Action, 2E covers it

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Related concepts

keywordkind of · A keyword is a language-defined word or token with a predefined syntactic or semantic role, often restricted from use as an ordinary identifier.
covarianceconfigures · Covariance is a type-system property in which subtyping of a type argument induces the same-direction subtyping relationship for its parameterized type.
out positionconfigures · An out position is a type-parameter usage position in which values of the parameterized type are produced or returned rather than consumed.
genericsconfigures · Generics are programming-language constructs that parameterize or generalize types and operations so they can apply to multiple concrete types or input forms.

This explainer was generated by Manning's knowledge graph from the text of Kotlin in Action, 2E. The section links point to the parts of the book it draws on.