How deeply does Kotlin in Action, 2E cover boxing?

Distinguishes boxed and primitive array representations, then stops short of broader boxing rules outside arrays.

Boxing

Boxing is the representation of a primitive value as an object instance such as java.lang.Integer.

Boxing represents a primitive value as an object. In Kotlin, a generic Array<Int> becomes a Java java.lang.Integer[] and stores boxed integers. section 8.2.5

Choose the array type according to interoperability and storage needs:

Use Array<Int> when an object array is required, such as when calling Java APIs that expect object arrays. section 8.2.5
Use IntArray to compile to a Java int[] and avoid boxing, storing values in the primitive representation. section 8.2.5
Create primitive arrays with IntArray(size), intArrayOf(...), or an initialization lambda. section 8.2.5

Boxed versus primitive arrays

val boxed = Array<Int>(3) { it } val unboxed = IntArray(3) { it } The first declaration produces boxed integers; the second produces a primitive int[] representation. section 8.2.5

val fiveZeros = IntArray(5)
val fiveZerosToo = intArrayOf(0, 0, 0, 0, 0)

Compare two concise ways to create an IntArray without boxing. section 8.2.5

How Kotlin in Action, 2E covers it

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Also explained in

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100 Java Mistakes and How to Avoid ThemA boxing conversion converts a Java primitive value to an object of its corresponding wrapper type, either explicitly or implicitly.
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Clojure: The Essential ReferenceAutoboxing is the implicit conversion of primitive values to object representations and, where applicable, the reverse conversion.

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Related concepts

dynamic typingused by · Dynamic typing is a typing discipline in which type information and checks are determined primarily at runtime rather than fixed statically for variables.
primitive arraycontrasts with · A primitive array is an indexed collection whose elements are stored as values of a primitive type rather than references to objects.
unchecked operatorscontrasts with · Unchecked operators are Clojure numeric functions that perform basic arithmetic without integer overflow checks and accept restricted numbers of arguments.

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.