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2024-08-06 09:38:15 +03:00
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"source": [
"* Official list of idioms: https://kotlinlang.org/docs/idioms.html\n",
"\n",
"* Original repository: https://code.mipt.ru/SPC/education/ks-materials"
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"## Preamble/Important notes\n",
"\n",
"* \"Syntactic sugar\" is a false concept. Each language feature affects how we use the language.\n",
"* One can adopt different programming styles in Kotlin. Idiomatic kotlin is an unofficial combination of practices developed by experienced Kotlin users.\n",
"* One of the key features of idiomatic kotlin is to use scopes instead of objects to provide incapsulation and limit visible mutable state.\n",
"\n",
"### What is idiomatic Kotlin (roughly)?\n",
"\n",
"* Less \"ceremony\". Pragmatic approach.\n",
"* Hybrid object/functional paradigm (objects for data, functions for interactions).\n",
"* Don't use mutable state wherever read-only is enough.\n",
"* Don't use classes when functions are enough.\n",
"* Don't use inheritance when composition is enough.\n",
"* Function is a first-class citizen.\n",
"* Scope is a first-class citizen.\n",
"* Use extensions to separate inner workings from utilities."
],
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"source": [
"## String interpolation\n",
"\n",
"Interpolated string allows constructing string fast"
],
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"println(\"This is a plain string\")"
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"cell_type": "code",
"source": [
"val arg = \"Interpolated\"\n",
"println(\"This is an $arg string\")"
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"val number = 1\n",
"\n",
"fun increment(num: Int) = num + 1\n",
"println(\"This is an $arg string number ${increment(number) + 1}\")"
],
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{
"cell_type": "code",
"source": [
"println(\"This is a string with \\$\")"
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{
"cell_type": "code",
"source": [
"println(\"\"\"\n",
" This is a \n",
" multi\n",
" line\n",
" raw\n",
" string\n",
"\"\"\".trimIndent())"
],
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{
"cell_type": "code",
"source": [
"println(\"\"\"This is a raw string number ${number+1} with \\ and ${'$'} \"\"\")"
],
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"source": [
"## Make `val`, not `var`!\n",
"\n",
"`val` means read-only property\n",
"\n",
"`var` means writable property\n",
"\n",
"Unnecessary use of `var` is not recommended.\n",
"\n",
"**NOTE:** In performance critical cases, explicit cycles are better, but mutable state should be encapsulated."
],
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"cell_type": "code",
"source": [
"/* Not recommended */\n",
"var counter = 0\n",
"for(i in 0..20){\n",
" counter += i\n",
"}\n",
"counter"
],
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{
"cell_type": "code",
"source": [
"/* recommended */\n",
"val sum = (0..20).sum()\n",
"sum"
],
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{
"cell_type": "code",
"source": [
"val intArray: IntArray = IntArray(21){ it }\n",
"intArray.sum()"
],
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"cell_type": "markdown",
"source": [
"## Top level functions\n",
"\n",
"Don't create a class when a function is enough.\n",
"\n",
"Use access modifiers for incapsulation."
],
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"/**\n",
" * Functions could be defined everywhere in kotlin code. This is a so-called top-level function.\n",
" */\n",
"fun doSomething(){\n",
" println(\"I did it\")\n",
"}\n",
"\n",
"doSomething()"
],
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"\n",
"object AnObject{\n",
" /**\n",
" * This is a member-function\n",
" */\n",
" fun doSomethingInAnObject(){\n",
" println(\"I did it in an object\")\n",
" }\n",
"}\n",
"\n",
"AnObject.doSomethingInAnObject()"
],
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"fun doSomethingSpecial(){\n",
" val special = \"special\"\n",
" /**\n",
" * This one is inside another function\n",
" */\n",
" fun doSomethingInside(){\n",
" println(\"I did $special inside another function\")\n",
" }\n",
" doSomethingInside()\n",
"}\n",
"\n",
"doSomethingSpecial()"
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{
"cell_type": "code",
"source": [
"fun returnFunction(): (String) -> Unit{\n",
" fun printStringWithPrefix(str: String){\n",
" println(\"Prefixed: $str\")\n",
" }\n",
" return ::printStringWithPrefix\n",
" // return { printStringWithPrefix(it) }\n",
" // return { println(\"Prefixed: $it\") }\n",
"}\n",
"\n",
"returnFunction()(\"Haskel, Boo!\")"
],
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"source": [
"## Unit"
],
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"\n",
"fun returnUnit(): Unit{\n",
" // no return statement `Unit` is returned implicitly\n",
" val b = Unit\n",
"}"
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"source": [
"## Default parameters"
],
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"source": [
"fun doSomethingWithDefault(a: Int = 0, b: String = \"\") {\n",
" println(\"A string with a == $a and b == \\\"$b\\\"\")\n",
"}\n",
"\n",
"doSomethingWithDefault()\n",
"doSomethingWithDefault(2, \"aaa\")\n",
"doSomethingWithDefault(b = \"fff\", a = 8)\n",
"doSomethingWithDefault(4, b = \"fff\")\n",
"doSomethingWithDefault(a = 2, \"fff\")// don't do that"
],
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"cell_type": "code",
"source": [
"fun integrate(\n",
" from: Double = 0.0,\n",
" to: Double = 1.0,\n",
" step: Double = 0.01,\n",
" function: (Double) -> Double,\n",
"): Double {\n",
" require(to > from)\n",
" require(step > 0)\n",
" var sum = 0.0\n",
" var pos = from\n",
" while (pos < to) {\n",
" sum += function(pos)\n",
" pos += step\n",
" }\n",
" return sum*step\n",
"}\n",
"\n",
"integrate { x ->\n",
" x * x + 2 * x + 1\n",
"}\n",
"integrate(0.0, PI) { sin(it) }\n",
"integrate(0.0, step = 0.02, to = PI) { sin(it) }\n",
"//integrate(0.0, step = 0.02, PI) { sin(it) }"
],
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"cell_type": "markdown",
"source": [
"## Function body shortcut"
],
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"cell_type": "code",
"source": [
"fun theSameAsBefore(a: Int = 0, b: String = \"\"): String = \"A string with a == $a and b == $b\"\n",
"\n",
"class Context(val a: String, val b: String)\n",
"\n",
"fun somethingWith(context: Context) = with(context){\n",
" println(a+b)\n",
"}"
],
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"cell_type": "markdown",
"source": [
"## Trailing lambda\n",
"\n",
"Scope is a separate important concept in Kotlin"
],
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"cell_type": "code",
"source": [
"data class BuildResult(val str: String, val int: Int)\n",
"\n",
"class Builder(var str: String = \"\", var int: Int = 0){\n",
" fun build() = BuildResult(str, int)\n",
"}\n",
"\n",
"fun doWithBuilder(defaultInt: Int, block: Builder.() -> Unit): BuildResult = \n",
" Builder(int = defaultInt).apply(block).build()\n",
"\n",
"doWithBuilder(2){\n",
" str = \"ff\"\n",
"}"
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"source": [
"## Inheritance"
],
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"source": [
"//Interfaces and objects\n",
"\n",
"interface AnInterface {\n",
" val a: Int\n",
" get() = 4\n",
" // set(value){\n",
" // println(value)\n",
" // }\n",
" fun doSomething() //= println(\"From interface\")\n",
"}\n",
"\n",
"abstract class AnAbstractClass(override val a: Int): AnInterface{\n",
" override final fun doSomething() = println(\"From a class\")\n",
" abstract fun doSomethingElse()\n",
"}\n",
"\n",
"class AClass(override val a: Int) : AnInterface {\n",
" override fun doSomething() = println(\"From a class\")\n",
"}\n",
"\n",
"class BClass(a: Int) : AnAbstractClass(a) {\n",
" override fun doSomethingElse() = println(\"something else\")\n",
"}\n",
"\n",
"/**\n",
" * A singleton (object) is a type (class) which have only one instance\n",
" */\n",
"object AnObject : AnInterface {\n",
" override fun doSomething(): Unit = TODO(\"Not yet implemented\")\n",
"}\n",
"\n",
"/**\n",
"* Creating an instance\n",
"*/\n",
"val instance = AClass(3)\n",
"\n",
"/**\n",
"* Using singleton reference without constructor invocation\n",
"*/\n",
"val obj: AnInterface = AnObject\n",
"\n",
"/**\n",
"* Anonymous object\n",
"*/\n",
"val anonymous = object : AnInterface {\n",
" override fun doSomething(): Unit = TODO(\"Not yet implemented\")\n",
"}\n",
"\n",
"/**\n",
"* The one that should not be named\n",
"*/\n",
"val voldemort = object {\n",
" fun doSomething(): Unit = TODO()\n",
"}\n",
"\n",
"//voldemort.doSomething()"
],
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"cell_type": "markdown",
"source": [
"## Declaration site variance\n",
"\n",
"In consumes, out produces."
],
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"source": [
"interface Producer<out T>{\n",
" fun produce(): T\n",
"}\n",
"\n",
"interface Consumer<in T>{\n",
" fun consume(value: T)\n",
"}"
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"cell_type": "markdown",
"source": [
"## Return on when\n",
"https://kotlinlang.org/docs/idioms.html#return-on-when-statement"
],
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"cell_type": "code",
"source": [
"fun transform(color: String): Int = when (color) {\n",
" \"Red\" -> 0\n",
" \"Green\" -> 1\n",
" \"Blue\" -> 2\n",
" else -> throw IllegalArgumentException(\"Invalid color param value\")\n",
"}"
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"source": [
"## Runtime type dispatch\n",
"\n",
"In Java it is called DOP (Data Oriented Programming).\n",
"\n",
"Also sometimes (wrongly) called patter matching."
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"source": [
"/**\n",
" * Matching by type\n",
" */\n",
"fun checkType(arg: Any): Unit = when(arg){\n",
" is String -> println(\"I am a String\")\n",
" is Int -> println(\"I am an Int\")\n",
" is Double -> println(\"I am a Double\")\n",
" // 2==2 -> println(\"Wat?\")\n",
" else -> println(\"I don't know who am I?\")\n",
"}\n",
"\n",
"fun checkType2(arg: Any): Unit = when{\n",
" arg is String -> println(\"I am a String\")\n",
" arg is Int -> println(\"I am an Int\")\n",
" arg is Double -> println(\"I am a Double\")\n",
" 2==2 -> println(\"Wat?\")\n",
" else -> println(\"I don't know who am I?\")\n",
"}\n",
"\n",
"checkType(true)"
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"hide_output_from_viewers": true
}
},
"outputs": [],
"execution_count": null
},
{
"cell_type": "markdown",
"source": [
"## Try-catch expression\n",
"https://kotlinlang.org/docs/idioms.html#try-catch-expression"
],
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"cell_type": "code",
"source": [
"fun tryCatch() {\n",
" val result = try {\n",
" count()\n",
" } catch (e: ArithmeticException) {\n",
" throw IllegalStateException(e)\n",
" }\n",
"\n",
" // Working with result\n",
"}"
],
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"node_id": "qT7cCbqiG1c4Agyc9qD5I3",
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"hide_output_from_viewers": true
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},
"outputs": [],
"execution_count": null
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{
"cell_type": "markdown",
"source": [
"## Data class"
],
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"cell_type": "code",
"source": [
"\n",
"class SimpleClass(val a: Int, val b: Double, c: Double){\n",
" override fun toString() = \"SimpleClass(a=$a, b=$b)\"\n",
"}\n",
"\n",
"\n",
"data class DataClass(val a: Int, val b: Double) {\n",
" val c get() = b + 1\n",
"}\n",
"\n",
"val simple = SimpleClass(1, 2.0, 3.0)\n",
"\n",
"println(simple)\n",
"\n",
"val data = DataClass(2, 2.0)\n",
"val copy = data.copy(b = 22.2)\n",
"println(copy)"
],
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"node_id": "B7unRz7w7U8OhXejJeFspj",
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"hide_output_from_viewers": true
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"outputs": [],
"execution_count": null
},
{
"cell_type": "markdown",
"source": [
"## Nothing"
],
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"cell_type": "code",
"source": [
"/**\n",
" * The declaration if valid because [TODO] returns Nothing\n",
" */\n",
"fun getSomething(): Int? = TODO()\n",
"\n",
"fun findSomething(): Int {\n",
" // early return is valid because `return` operator result is Nothing\n",
" val found: Int = getSomething() ?: return 2\n",
" return found + 2\n",
"}\n",
"\n",
"fun checkCondition(): Int {\n",
" fun conditionSatisfied() = false\n",
"\n",
" return if (conditionSatisfied()) {\n",
" 1\n",
" } else {\n",
" //error is Nothing\n",
" error(\"Condition is not satisfied\")\n",
" }\n",
"}"
],
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"datalore": {
"node_id": "h11q8FWGKYIcuFOnFOLmBl",
"type": "CODE",
"hide_input_from_viewers": true,
"hide_output_from_viewers": true
}
},
"outputs": [],
"execution_count": null
},
{
"cell_type": "markdown",
"source": [
"## Nullable truth"
],
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"cell_type": "code",
"source": [
"\n",
"/**\n",
" * idiom 10\n",
" * Nullable truth.\n",
" */\n",
"fun idiom10() {\n",
" class A(val b: Boolean?)\n",
"\n",
" val a: A? = A(null) // Compilator warning here\n",
" //use\n",
" a?.b == true\n",
" //instead of\n",
" a?.b ?: false\n",
"\n",
" // The old way\n",
" val res = if (a == null) {\n",
" false\n",
" } else {\n",
" if (a.b == null) {\n",
" false\n",
" } else {\n",
" a.b\n",
" }\n",
" }\n",
"}"
],
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"node_id": "5BN3u1gHkSamtLIJiQicsV",
"type": "CODE",
"hide_input_from_viewers": true,
"hide_output_from_viewers": true
}
},
"outputs": [],
"execution_count": null
},
{
"cell_type": "markdown",
"source": [
"## Safe call"
],
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"cell_type": "code",
"source": [
"\n",
"\n",
"//TODO move up\n",
"/**\n",
" * Safe call and elvis operator\n",
" */\n",
"fun idiom12() {\n",
" val files = File(\"Test\").listFiles()\n",
" println(files?.size ?: \"empty\")\n",
"}\n",
"idiom12()"
],
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"node_id": "v4cP31zyrmBkHP7lnYZWN0",
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"hide_output_from_viewers": true
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},
"outputs": [],
"execution_count": null
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{
"cell_type": "markdown",
"source": [
"## Nullable assignment"
],
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"cell_type": "code",
"source": [
"\n",
"/**\n",
" * Dart-like (?=) nullable assignment\n",
" */\n",
"fun idiom11() {\n",
" var x: String? = null\n",
" x = x ?: \"Hello\"\n",
"}"
],
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"node_id": "di2EXkn35CFEja9KMK1Ur5",
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"hide_input_from_viewers": true,
"hide_output_from_viewers": true
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},
"outputs": [],
"execution_count": null
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{
"cell_type": "markdown",
"source": [
"## Safe calls"
],
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"cell_type": "code",
"source": [
"fun printNotNull(any: Any) = println(any)\n",
"\n",
"// printNotNull(null) does not work\n",
"\n",
"val value: Int? = 2\n",
"//val value: Int? by lazy{ 2 }\n",
"\n",
"//printNotNull(value) // Error\n",
"if (value != null) {\n",
" //not guaranteed to work with mutable variable\n",
" printNotNull(value)\n",
"}\n",
"\n",
"var value1: Int? = 2\n",
"\n",
"// Safe call here\n",
"value1?.let {\n",
" printNotNull(it) // execute this block if not null\n",
" //printNotNull(value1) // value is not null here\n",
"}"
],
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"node_id": "hGZ9h4H8TaYZefCkAmP3Gd",
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"hide_output_from_viewers": true
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"outputs": [],
"execution_count": null
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{
"cell_type": "markdown",
"source": [
"## Extension functions"
],
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"cell_type": "code",
"source": [
"/**\n",
" * Extension function on a String. It is not monkey-patching.\n",
" */\n",
"fun String.countOs(): Int = count { it == 'о' } // implicit this points to String\n",
"\n",
"fun Int.printMe() = println(this) // explicit this\n",
"\n",
"fun Any?.doSomething() = TODO(\"Don't do that\")\n",
"\n",
"\"вылысыпыдыстычка\".countOs().printMe()"
],
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"node_id": "li65OumXRClbEIwURW9DEE",
"type": "CODE",
"hide_input_from_viewers": true,
"hide_output_from_viewers": true
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{
"cell_type": "code",
"source": [
"infix fun <T: Comparable<T>> ClosedRange<T>.intersect(other: ClosedRange<T>): ClosedRange<T>?{\n",
" val start = maxOf(this.start, other.start)\n",
" val end = minOf(this.endInclusive, other.endInclusive)\n",
" return if(end>=start) start..end else null\n",
"}\n",
"\n",
"(0..8).intersect(6..12)\n",
"0..8 intersect 6..12"
],
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"datalore": {
"node_id": "cyD7D8tmZJex8pJ3NsufHN",
"type": "CODE",
"hide_input_from_viewers": true,
"hide_output_from_viewers": true
}
},
"outputs": [],
"execution_count": null
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{
"cell_type": "markdown",
"source": [
"## Extension properties\n",
"\n",
"Remember [property usage guidelines](https://kotlinlang.org/docs/coding-conventions.html#functions-vs-properties)."
],
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"cell_type": "code",
"source": [
"\n",
"/**\n",
" * Extension property (must be virtual)\n",
" */\n",
"val List<Int>.odd get() = filter { it % 2 == 1 }\n",
"\n",
"List(10){it}.odd"
],
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"node_id": "L7qpVNVtW0Fh6dMX4pR3il",
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"cell_type": "code",
"source": [
"\n",
"/**\n",
" * Extension variable (also must be virtual)\n",
" */\n",
"var Array<Int>.second: Int\n",
" get() = this[1]\n",
" set(value) {\n",
" this[1] = value\n",
" }\n",
"\n",
"\n",
"val array = Array(5){it}\n",
"array.second = 9\n",
"array"
],
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"datalore": {
"node_id": "hAjUSxhPqTgmnz9WQYXg3U",
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"hide_output_from_viewers": true
}
},
"outputs": [],
"execution_count": null
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{
"cell_type": "markdown",
"source": [
"## Scope functions (run, with, let)"
],
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"cell_type": "code",
"source": [
"object AClass{\n",
" val a = \"a\"\n",
" val b = \"b\"\n",
" val c = \"c\"\n",
"}\n",
"\n",
"fun getAClass(): AClass? = null\n",
"\n",
"/**\n",
" * [with]/[run]/[let] are used to create a scope with given argument or receiver\n",
" */\n",
"\n",
"\n",
"// Simple print of AClass\n",
"println(\"a = ${AClass.a}, b = ${AClass.b}, c = ${AClass.c}\")\n",
"\n",
"// Using `with`\n",
"val res = with(AClass){\n",
" // AClass type is the receiver in this scope\n",
" println(\"a = $a, b = $b, c = $c\")\n",
" /*return@with*/ \"some value\"\n",
"}\n",
"\n",
"res"
],
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"hide_output_from_viewers": true
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"cell_type": "code",
"source": [
"\n",
"//using `run`\n",
"getAClass()?.run {\n",
" // the same as above\n",
" println(\"a = $a, b= $b, c = $c\")\n",
"}\n",
"\n",
"//Using `let` to compose result. Not recommended using without a need\n",
"val letResult = getAClass()?.let { arg ->\n",
" arg.c + arg.a\n",
"}\n"
],
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"node_id": "aJGJ3ukmhCchnZwJEzLV3K",
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"hide_output_from_viewers": true
}
},
"outputs": [],
"execution_count": null
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{
"cell_type": "markdown",
"source": [
"## Scope functions (also, apply)"
],
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"source": [
"/**\n",
" * Using apply/also to add a finalizing action\n",
" */\n",
"var i = 2\n",
"\n",
"/**\n",
"* [also] block does not affect the result\n",
"*/\n",
"fun getAndIncrement() = i.also { i += 1 }\n",
"\n",
"println(getAndIncrement())\n",
"println(i)"
],
"metadata": {
"datalore": {
"node_id": "c0kY56DJne2lbwGShOgur6",
"type": "CODE",
"hide_input_from_viewers": true,
"hide_output_from_viewers": true
}
},
"outputs": [],
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"cell_type": "code",
"source": [
"\n",
"class Rectangle{\n",
" var length: Number = 0\n",
" var breadth: Number = 0\n",
" var color: Int = 0xffffff\n",
"}\n",
"\n",
"/**\n",
" * Configure properties of an object (apply)\n",
" * https://kotlinlang.org/docs/idioms.html#configure-properties-of-an-object-apply\n",
" */\n",
"val myRectangle = Rectangle().apply {\n",
" length = 4\n",
" breadth = 5\n",
" color = 0xFAFAFA\n",
"}\n"
],
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"datalore": {
"node_id": "m9HH2IzTy1dbYkBRUeFjh9",
"type": "CODE",
"hide_input_from_viewers": true,
"hide_output_from_viewers": true
}
},
"outputs": [],
"execution_count": null
},
{
"cell_type": "markdown",
"source": [
"## Lists\n",
"\n",
"Do not cast List to MutableList!"
],
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"cell_type": "code",
"source": [
"/**\n",
" * Lists and mutable lists\n",
" */\n",
"\n",
"\n",
"/**\n",
" * This method creates a read-only list of strings. One should note that the type of the list is not specified\n",
" */\n",
"val list = listOf(\"a\", \"b\", \"c\")\n",
"\n",
"println(list[0])\n",
"println(list.get(1))\n",
"//println(list.get(4))\n",
"println(list.getOrNull(4) ?: \"nothing\")"
],
"metadata": {
"datalore": {
"node_id": "pP9PTSy6IXeTC6HITFtBoR",
"type": "CODE",
"hide_input_from_viewers": true,
"hide_output_from_viewers": true
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"cell_type": "code",
"source": [
"\n",
"/**\n",
"* This one creates a mutable list\n",
"*/\n",
"val mutableList: MutableList<String> = mutableListOf(\"a\", \"b\", \"c\")\n",
"mutableList[2] = \"d\"\n",
"mutableList.add(\"e\")\n",
"mutableList += \"f\"\n",
"mutableList"
],
"metadata": {
"datalore": {
"node_id": "Kx565m7VjuF2RncqW7gw2i",
"type": "CODE",
"hide_input_from_viewers": true,
"hide_output_from_viewers": true
}
},
"outputs": [],
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"cell_type": "code",
"source": [
"\n",
"/**\n",
" * This one creates a mutable ArrayList.\n",
" */\n",
"val arrayList: ArrayList<String> = arrayListOf(\"a\", \"b\", \"c\")\n",
"\n",
"//Danger zone\n",
"\n",
"val newList: List<String> = list + \"f\" + mutableList\n",
"\n",
"println(newList)"
],
"metadata": {
"datalore": {
"node_id": "qSaaZGyHiH9dsL4dtQ8UyE",
"type": "CODE",
"hide_input_from_viewers": true,
"hide_output_from_viewers": true
}
},
"outputs": [],
"execution_count": null
},
{
"cell_type": "code",
"source": [
"\n",
"//Bonus\n",
"\n",
"val lambdaList = List(3){ it.toString() }\n",
"println(lambdaList)\n",
"\n",
"\n",
"val builderList: List<Int> = buildList{\n",
" add(2)\n",
" add(8)\n",
" remove(8)\n",
"}\n",
"builderList"
],
"metadata": {
"datalore": {
"node_id": "JPSYaCAgOZRveNdFPO15SG",
"type": "CODE",
"hide_input_from_viewers": true,
"hide_output_from_viewers": true
}
},
"outputs": [],
"execution_count": null
},
{
"cell_type": "code",
"source": [
"// val sequentialList = List(20){it}\n",
"// sequentialList.zipWithNext{ l, r -> r - l }.forEach{println(it)}"
],
"metadata": {
"datalore": {
"node_id": "fQBhhTBHCdc9JkWiMZ6uEh",
"type": "CODE",
"hide_input_from_viewers": true,
"hide_output_from_viewers": true
}
},
"outputs": [],
"execution_count": null
},
{
"cell_type": "markdown",
"source": [
"## Shortcut collection builders"
],
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"cell_type": "code",
"source": [
"/**\n",
" * Use shortcut function to provide default values\n",
" */\n",
"fun doSomething(additionalArguments: List<String> = emptyList()){\n",
" TODO()\n",
" emptyArray<String>()\n",
" emptySet<String>()\n",
" emptyMap<String,String>()\n",
"}\n",
"emptyList<String>()::class"
],
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"datalore": {
"node_id": "F5RQ5108YJe3pzfN0eAWwm",
"type": "CODE",
"hide_input_from_viewers": true,
"hide_output_from_viewers": true
}
},
"outputs": [],
"execution_count": null
},
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"cell_type": "markdown",
"source": [
"## Maps"
],
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"cell_type": "code",
"source": [
"val map = mutableMapOf(\n",
" \"key\" to \"a\",\n",
" \"key2\" to \"b\",\n",
")\n",
"\n",
"//The map could be accessed via indexing operation\n",
"println(map[\"key\"])\n",
"map[\"key\"] = \"fff\""
],
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"node_id": "cZd2JPB72uAfnx1o8AYjOs",
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"hide_output_from_viewers": true
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"cell_type": "code",
"source": [
"\n",
"//val entry: MutableMap.MutableEntry<String, String> = map.iterator().next()\n",
"\n",
"/**\n",
" * The destructuring declaration for maps and other objects that support `operator fun componentN`\n",
" */\n",
"for ((k, v) in map) {\n",
"//val (k, v) = entry\n",
"// val k = entry.component1()\n",
"// val v = entry.component2()\n",
" println(\"$k -> $v\")\n",
"}"
],
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"cell_type": "code",
"source": [
"map.forEach { (k, v) -> println(\"$k -> $v\")}"
],
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"node_id": "s1Q65aNWb9YNBZOgxmjotW",
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"hide_output_from_viewers": true
}
},
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{
"cell_type": "code",
"source": [
"\n",
"val (a, b) = Pair(1, 2)\n",
"\n",
"val coord = doubleArrayOf(0.0, 1.0, 2.0)\n",
"\n",
"val (x,y,z) = coord\n",
"\n",
"data class Coordinate(val x: Double, val y: Int)\n",
"\n",
"val (x1, y1) = Coordinate(1.0,2)"
],
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"hide_output_from_viewers": true
}
},
"outputs": [],
"execution_count": null
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{
"cell_type": "markdown",
"source": [
"## Mutable type access decorator\n",
"\n",
"To be replaced with qualified getter types soon"
],
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"cell_type": "code",
"source": [
"class AClassWithList{\n",
" var b: Double = 2.0\n",
" private set\n",
"\n",
" init{\n",
" b = 5.0\n",
" }\n",
"\n",
" private val _list: MutableList<Int> = ArrayList<Int>()\n",
" val list: List<Int> get() = _list\n",
"}\n",
"\n",
"\n",
"\n",
"val obj = AClassWithList()\n",
"\n",
"// obj.b = 10.0 //error\n",
"obj.list\n"
],
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"cell_type": "markdown",
"source": [
"## Contains operator and ranges"
],
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"cell_type": "code",
"source": [
"\n",
"val emailsList = emptyList<String>()\n",
"\n",
"// When used directly infix in operator checks if the element is contained in a collection\n",
"//using `operator fun contains`\n",
"\n",
"if (\"john@example.com\" in emailsList) {\n",
" println(\"is in list\")\n",
"}\n",
"\n",
"if (\"jane@example.com\" !in emailsList) {\n",
" println(\"not in list\")\n",
"}"
],
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"cell_type": "code",
"source": [
"\n",
"\n",
"class DateRange(val start: Instant, val end: Instant)\n",
"\n",
"operator fun DateRange.contains(value: Instant): Boolean = value > start && value < end\n",
"\n",
"\n",
"println(Instant.now() in DateRange(Instant.EPOCH, Instant.MAX))"
],
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"source": [
"\n",
"// Another (different) use of `in` is iteration over range or collection using\n",
"// using `operator fun iterator`\n",
"\n",
"for (i in 1..100) {\n",
" println(i)\n",
"} // closed range: includes 100\n",
"\n",
"(1..100).forEach { i ->\n",
" println(i)\n",
"} //the same, but with boxing\n",
"\n",
"for (i in 1..<100) {\n",
" println(i)\n",
"} // half-open range: does not include 100\n",
"\n",
"for (x in 2..10 step 2) {\n",
" println(x)\n",
"}\n",
"for (x in 10 downTo 1) {\n",
" println(x)\n",
"}\n",
"\n",
"infix fun ClosedRange<Double>.step(step: Double): Sequence<Double> {\n",
" //TODO check arguments\n",
" var current = start\n",
" return sequence {\n",
" do {\n",
" yield(current)\n",
" current += step\n",
" } while (current <= endInclusive)\n",
" }\n",
"}\n",
"\n",
"for (x in 0.0..10.0 step 0.5){\n",
" println(x)\n",
"}"
],
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"cell_type": "markdown",
"source": [
"## Map-reduce"
],
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"cell_type": "code",
"source": [
"val list: List<Int> = listOf(1, 2, 3, 4, 5, 6)\n",
"\n",
"val result = list\n",
" //.stream().parallel()\n",
" //.asSequence()\n",
" .filter { it % 2 == 0 } //select even numbers\n",
" .map { it * it } // get square of each element\n",
" //.onEach { println(it) }\n",
" //.sumOf { it } //use one of reduce operations\n",
" .reduce { acc: Int, i: Int -> acc + i }\n",
"\n",
"result"
],
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"cell_type": "code",
"source": [
"val sequence = sequence{\n",
" var counter = 1\n",
" while(true){\n",
" yield(counter++)\n",
" // println(counter)\n",
" }\n",
"}\n",
"\n",
"val result = sequence\n",
" .take(6)\n",
" .filter { it % 2 == 0 } //select even numbers\n",
" .map { it * it } // get square of each element\n",
" //.onEach { println(it) }\n",
" //.sumOf { it } //use one of reduce operations\n",
" .reduce { acc: Int, i: Int -> acc + i }\n",
"\n",
"result"
],
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},
"outputs": [],
"execution_count": null
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{
"cell_type": "markdown",
"source": [
"## Wrap mutable logic"
],
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"cell_type": "code",
"source": [
"val list = buildList {\n",
" repeat(10){\n",
" add(it)\n",
" }\n",
"}\n",
"println(list)\n"
],
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},
"outputs": [],
"execution_count": null
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{
"cell_type": "markdown",
"source": [
"## Scoped resource usage"
],
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"cell_type": "code",
"source": [
"val stream = Files.newInputStream(Paths.get(\"file.txt\"))\n",
"// The resource is automatically closed when leaving the scope\n",
"stream.bufferedReader().use { reader ->\n",
" println(reader.readText())\n",
"}"
],
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{
"cell_type": "markdown",
"source": [
"## Factory as parameter and companion factories"
],
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"cell_type": "code",
"source": [
"interface Factory<T : Any> {\n",
" fun build(str: String): T\n",
"}\n",
"\n",
"data class IntContainer(val arg: Int) {\n",
" \n",
" companion object : Factory<IntContainer> {\n",
" override fun build(str: String) = IntContainer(str.toInt())\n",
" }\n",
"}\n",
"\n",
"data class DoubleContainer(val arg: Double) {\n",
" \n",
" companion object : Factory<DoubleContainer> {\n",
" override fun build(str: String) = DoubleContainer(str.toDouble())\n",
" }\n",
"}\n",
"\n",
"fun <T : Any> buildContainer(str: String, factory: Factory<T>): T = factory.build(str)\n",
"\n",
"\n",
"buildContainer(\"22\", IntContainer)\n"
],
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{
"cell_type": "markdown",
"source": [
"## Initialization"
],
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"cell_type": "code",
"source": [
"\n",
"open class Bad{\n",
" val value: Int\n",
"\n",
" init {\n",
" value = requestValue()\n",
" }\n",
"\n",
" open fun requestValue(): Int {\n",
" doSomethingElse()\n",
" return 2\n",
" }\n",
"\n",
" private fun doSomethingElse(){\n",
" println(value)\n",
" }\n",
"}\n",
"\n",
"Bad()"
],
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"source": [
"\n",
"\n",
"//Factory functions are preferred to the initialization logic\n",
"\n",
"data class Good internal constructor(val value: Int){\n",
" init {\n",
" //Initialization block is there to check arguments\n",
" require(value >= 0)\n",
" }\n",
"\n",
" companion object\n",
"}\n",
"\n",
"fun requestValue(): Int = TODO()\n",
"\n",
"// This is the factory-function\n",
"fun Good() = Good(requestValue())\n",
"\n",
"\n",
"// additional constructor-like builders could be added to the companion\n",
"\n",
"@OptIn(ExperimentalUnsignedTypes::class)\n",
"fun Good.Companion.build(value: UInt) = Good(value.toInt())\n",
"\n",
"Good.build(32U)"
],
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"cell_type": "markdown",
"source": [
"## Delegates"
],
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"cell_type": "code",
"source": [
"class ClassWithALazyProperty{\n",
" //Use lazy delegate for something that should be calculated ones on first call\n",
" val lazyValue by lazy {\n",
" //Do dome heavy logic here\n",
" println(\"Initialized\")\n",
" 22\n",
" }\n",
"\n",
" val getterValue: Int get(){\n",
" println(\"got\")\n",
" return 33\n",
" }\n",
"}\n",
"val lazyClass = ClassWithALazyProperty()\n",
"lazyClass.lazyValue\n",
"lazyClass.lazyValue\n",
"lazyClass.getterValue\n",
"lazyClass.getterValue"
],
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"cell_type": "code",
"source": [
"\n",
"//Using other delegates\n",
"val map = mapOf(\"a\" to 1, \"b\" to 2)\n",
"\n",
"val a: Int by map\n",
"\n",
"println(a)"
],
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"cell_type": "markdown",
"source": [
"## Inline functions"
],
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"cell_type": "code",
"source": [
"\n",
"/**\n",
" * Definition of inline function\n",
" */\n",
"inline fun List<Int>.forEachOdd(block: (Int) -> Unit) = forEach {\n",
" if (it % 2 == 1) block(it)\n",
"}\n",
"\n",
"\n",
"/**\n",
" * The demonstration of use of inline [forEach] function with non-local return\n",
" */\n",
"fun foo() {\n",
" listOf(1, 2, 3, 4, 5).forEach {\n",
" if (it == 3) return // non-local return directly to the caller of foo()\n",
" print(\"$it, \")\n",
" }\n",
" println(\"this point is unreachable\")\n",
"}\n",
"foo()"
],
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"source": [
"\n",
"/**\n",
" * Using inline function for type reification during the compile time\n",
" */\n",
"inline fun <reified T> List<T>.prettyPrint() = forEach {\n",
" when (T::class) {\n",
" Double::class -> println(\"Double: ${it as Double}\")\n",
" Int::class -> println(\"Int: ${it as Int}\")\n",
" else -> it.toString()\n",
" }\n",
"}\n",
"\n",
"inline fun <reified T> prettyPrintOne(arg: T) = listOf(arg).prettyPrint()\n",
"\n",
"/**\n",
" * **WARNING** inline functions are an advanced feature and should be used only for\n",
" * reification or non-local return\n",
" * NOT for optimization.\n",
" */"
],
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"cell_type": "markdown",
"source": [
"## Collections and boxing"
],
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"/**\n",
" * Values are boxed. Each call is indirect\n",
" */\n",
"val list: List<Double> = List(20) { it.toDouble() }\n",
"\n",
"/**\n",
" * Still boxed\n",
" */\n",
"val genericArray: Array<Double> = Array(20) { it.toDouble() }\n",
"\n",
"/**\n",
" * Non-boxed\n",
" */\n",
"val specializedArray: DoubleArray = DoubleArray(20) { it.toDouble() }"
],
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