How deeply does Kotlin in Action, 2E cover protected member?

Defines the visibility rule, demonstrates an extension-function failure, and contrasts Kotlin with Java package behavior.

Protected member

As explained in Kotlin in Action, 2E

The protected modifier restricts access to a declaration to its containing class and subclasses in Kotlin.

A protected member is visible inside its containing class and its subclasses, but not to unrelated callers or same-package code. Kotlin deliberately differs from Java here: package membership does not grant protected access. section 4.1.3

Subclass access, extension rejection

open class TalkativeButton { protected fun whisper() = println("Let's talk!") } class Child : TalkativeButton() { fun speak() = whisper() // allowed } fun TalkativeButton.giveSpeech() { whisper() // error: extensions gain no protected access } section 4.1.3

Table 4.2 Kotlin visibility modifiers (view table figure)

Modifier	Class member	Top-level declaration
`public` (default)	Visible everywhere	Visible everywhere
`internal`	Visible in a module	Visible in a module
`protected`	Visible in subclasses	—
`private`	Visible in a class	Visible in a file

Use the visibility table to compare protected with public, internal, and private declarations, including its absence for top-level declarations. section 4.1.3

How Kotlin in Action, 2E covers it

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Also explained in

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Essential TypeScript 5, Third EditionAn access control keyword specifies which code can access a class member or other program element.
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Python How-ToProtected access is an access level in which a class attribute or method is available to the defining class and its subclasses but not external code.
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100 C++ Mistakes and How to Avoid ThemA protected member is a class member accessible from the class and its derived classes, subject to C++ access rules, but not generally from unrelated client code.

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Related concepts

private membercontrasts with · A private member is a program element whose direct access is restricted to its defining class, module, file, or other language-defined scope.
leading underscore conventionconfigured by · The leading underscore convention prefixes an identifier with an underscore to signal intended internal or non-public use without enforcing access restrictions.
inheritanceused by · Inheritance is a mechanism by which a child type or element receives properties, values, or behavior from a parent, subject to system rules and possible overrides.
invariantcontrasts with · An invariant is a property or condition that remains true across the valid states, operations, or specified transformations of a system or computation.
instance variableproduces · An instance variable is data associated with a particular object, rather than shared across all instances of its class or type.
selfuses · self is Ruby's implicit reference to the object that is the current method-execution receiver.

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.