[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"me":3,"catalog:en:mobile\u002Fkt-kotlin-type-system-null-safety":4,"config":211},null,{"field_key":5,"field_name":6,"seniority":7,"topic_key":8,"topic_name":9,"spec_key":7,"spec_name":7,"locale":10,"cell_total":11,"field_total":12,"seniorities":13,"topics":17,"specs":112,"samples":126},"mobile","Mobile","","kt-kotlin-type-system-null-safety","Kt Kotlin Type System Null Safety","en",75,2400,[14,15,16],"junior","mid","senior",[18,21,24,27,30,33,36,39,42,45,48,51,54,57,60,63,66,69,70,73,76,79,82,85,88,91,94,97,100,103,106,109],{"key":19,"name":20,"count":11},"fl-dart-async-isolates","Fl Dart Async Isolates",{"key":22,"name":23,"count":11},"fl-dart-language-null-safety","Fl Dart Language Null Safety",{"key":25,"name":26,"count":11},"fl-flutter-persistence-storage","Fl Flutter Persistence Storage",{"key":28,"name":29,"count":11},"fl-flutter-platform-channels-lifecycle","Fl Flutter Platform Channels 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Automation",{"key":92,"name":93,"count":11},"mobile-ui-lifecycle","Mobile Ui Lifecycle",{"key":95,"name":96,"count":11},"rn-javascript-typescript-fundamentals","Rn Javascript Typescript Fundamentals",{"key":98,"name":99,"count":11},"rn-native-bridge-modules","Rn Native Bridge Modules",{"key":101,"name":102,"count":11},"rn-navigation-lifecycle","Rn Navigation Lifecycle",{"key":104,"name":105,"count":11},"rn-networking-persistence-offline","Rn Networking Persistence Offline",{"key":107,"name":108,"count":11},"rn-performance-rendering","Rn Performance Rendering",{"key":110,"name":111,"count":11},"rn-react-hooks-state-management","Rn React Hooks State Management",[113,117,120,123],{"key":114,"name":115,"count":116},"android-kotlin","Android (Kotlin)",450,{"key":118,"name":119,"count":116},"flutter","Flutter",{"key":121,"name":122,"count":116},"ios-swift","iOS (Swift)",{"key":124,"name":125,"count":116},"react-native","React Native",[127,145,158,172,185,198],{"id":128,"topic":9,"difficulty":129,"body":130,"options":131,"correct_key":139,"explanation":144},"019fa914-614e-7eeb-96c8-0a42de532ba7",1,"What does declaring a class as `data class` automatically generate?",[132,135,138,141],{"key":133,"text":134},"a","A private constructor that prevents instantiation outside the file.",{"key":136,"text":137},"b","A default no-argument constructor only.",{"key":139,"text":140},"c","`equals()`, `hashCode()`, `toString()`, and `copy()`.",{"key":142,"text":143},"d","Automatic Parcelable serialization with no annotation needed.","`data class` generates structural `equals`\u002F`hashCode`\u002F`toString`\u002F`copy` (and `componentN` functions) from the primary constructor's properties, saving boilerplate.",{"id":146,"topic":9,"difficulty":129,"body":147,"options":148,"correct_key":142,"explanation":157},"019fa914-6150-7e4e-99d0-bab52d9ed01f","```kotlin\ndata class Point(val x: Int, val y: Int)\nval p1 = Point(1, 2)\nval p2 = Point(1, 2)\nprintln(p1 == p2)\n```\nWhat is printed?",[149,151,153,155],{"key":133,"text":150},"false, because `==` checks reference identity for classes.",{"key":136,"text":152},"It depends on whether `Point` is also marked `open`.",{"key":139,"text":154},"A compile error, `data class` cannot override `==`.",{"key":142,"text":156},"true","`data class` generates a structural `equals()`, so `==` compares property values, not references; two instances with equal `x`\u002F`y` are equal.",{"id":159,"topic":9,"difficulty":160,"body":161,"options":162,"correct_key":136,"explanation":171},"019fa914-6152-7729-b9bb-545853aafc0d",2,"A `data class Account(val id: String, var balance: Double)` is put into a `HashSet`. After insertion, `balance` is mutated on the object already inside the set. What risk does this create?",[163,165,167,169],{"key":133,"text":164},"No risk, `HashSet` recomputes hash codes automatically on every mutation.",{"key":136,"text":166},"The object's hash code can change, breaking lookups in the set.",{"key":139,"text":168},"`balance` mutation triggers `copy()` internally, creating a new element.",{"key":142,"text":170},"The mutation is rejected at runtime since `HashSet` elements are immutable.","Mutable properties used in `hashCode()` are dangerous inside hash-based collections: mutating the object after insertion can move where its correct bucket would be, while the set still stores it in the old bucket, silently breaking `contains`\u002Flookup.",{"id":173,"topic":9,"difficulty":129,"body":174,"options":175,"correct_key":142,"explanation":184},"019fa914-6155-7786-a0b8-f8b2bbefc3cc","What does calling `.copy(balance = 500.0)` on a `data class Account(val id: String, val balance: Double)` instance do?",[176,178,180,182],{"key":133,"text":177},"Mutates `balance` on the original instance in place.",{"key":136,"text":179},"Throws at runtime since `balance` was declared `val`.",{"key":139,"text":181},"Deletes the original instance and replaces every reference to it.",{"key":142,"text":183},"Returns a new instance with `balance` replaced.","`copy()` (generated for `data class`) returns a new instance, letting you override selected constructor properties while reusing the rest — the original instance is untouched.",{"id":186,"topic":9,"difficulty":160,"body":187,"options":188,"correct_key":133,"explanation":197},"019fa914-6158-79d5-90f1-a1c578ff8a58","Why does the compiler allow an exhaustive `when` expression over a `sealed class` hierarchy without an `else` branch?",[189,191,193,195],{"key":133,"text":190},"The subclass set is closed, so the compiler can enumerate every case.",{"key":136,"text":192},"Because `sealed class` forces every subclass to be an `object`, so there are always exactly two cases.",{"key":139,"text":194},"Because `when` always requires an `else` branch regardless of the type.",{"key":142,"text":196},"Because sealed classes cannot be subclassed at all, so there is only one case.","A `sealed class`'s subclasses are restricted to a known, closed set the compiler can fully enumerate, so a `when` that covers every subclass is provably exhaustive and doesn't need a catch-all `else`.",{"id":199,"topic":9,"difficulty":160,"body":200,"options":201,"correct_key":139,"explanation":210},"019fa914-6159-7854-8ce7-56be49fcf7b2","```kotlin\nsealed class Result\ndata class Ok(val v: Int) : Result()\ndata class Err(val msg: String) : Result()\n\nfun describe(r: Result): String = when (r) {\n    is Ok -> \"ok: ${r.v}\"\n    is Err -> \"err: ${r.msg}\"\n}\n```\nA new subclass `object Pending : Result()` is added later. What happens to `describe`?",[202,204,206,208],{"key":133,"text":203},"Nothing, `describe` keeps compiling and silently returns null for `Pending`.",{"key":136,"text":205},"The new subclass is rejected at compile time because `Result` already has two subclasses.",{"key":139,"text":207},"The compiler now reports the `when` as non-exhaustive.",{"key":142,"text":209},"`describe` throws a runtime exception the first time it is called with any argument.","Because `Result` is sealed, the compiler tracks every subclass; adding `Pending` makes the existing `when` (used as an expression, requiring exhaustiveness) incomplete, and the compiler flags the missing branch instead of letting it compile 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