Completed Coursera: Kotlin for developers course

.gitignore

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+### IntelliJ IDEA ###
+out/
+!**/src/main/**/out/
+!**/src/test/**/out/
+
+### Kotlin ###
+.kotlin
+
+### Eclipse ###
+.apt_generated
+.classpath
+.factorypath
+.project
+.settings
+.springBeans
+.sts4-cache
+bin/
+!**/src/main/**/bin/
+!**/src/test/**/bin/
+
+### NetBeans ###
+/nbproject/private/
+/nbbuild/
+/dist/
+/nbdist/
+/.nb-gradle/
+
+### VS Code ###
+.vscode/
+
+### Mac OS ###
+.DS_Store

.idea/.gitignore

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+# Default ignored files
+/shelf/
+/workspace.xml
+# Editor-based HTTP Client requests
+/httpRequests/
+# Environment-dependent path to Maven home directory
+/mavenHomeManager.xml
+# Datasource local storage ignored files
+/dataSources/
+/dataSources.local.xml

.idea/codeStyles/Project.xml

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+<component name="ProjectCodeStyleConfiguration">
+  <code_scheme name="Project" version="173">
+    <JetCodeStyleSettings>
+      <option name="CODE_STYLE_DEFAULTS" value="KOTLIN_OFFICIAL" />
+    </JetCodeStyleSettings>
+    <codeStyleSettings language="JAVA">
+      <option name="ALIGN_MULTILINE_BINARY_OPERATION" value="true" />
+    </codeStyleSettings>
+    <codeStyleSettings language="kotlin">
+      <option name="CODE_STYLE_DEFAULTS" value="KOTLIN_OFFICIAL" />
+    </codeStyleSettings>
+  </code_scheme>
+</component>

.idea/codeStyles/codeStyleConfig.xml

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+<component name="ProjectCodeStyleConfiguration">
+  <state>
+    <option name="USE_PER_PROJECT_SETTINGS" value="true" />
+  </state>
+</component>

.idea/inspectionProfiles/Project_Default.xml

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+<component name="InspectionProjectProfileManager">
+  <profile version="1.0">
+    <option name="myName" value="Project Default" />
+    <inspection_tool class="GrazieInspection" enabled="false" level="GRAMMAR_ERROR" enabled_by_default="false" />
+    <inspection_tool class="LanguageDetectionInspection" enabled="false" level="WARNING" enabled_by_default="false" />
+    <inspection_tool class="SpellCheckingInspection" enabled="false" level="TYPO" enabled_by_default="false">
+      <option name="processCode" value="true" />
+      <option name="processLiterals" value="true" />
+      <option name="processComments" value="true" />
+    </inspection_tool>
+  </profile>
+</component>

.idea/kotlinc.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="Kotlin2JvmCompilerArguments">
+    <option name="jvmTarget" value="1.8" />
+  </component>
+  <component name="KotlinCommonCompilerArguments">
+    <option name="apiVersion" value="2.1" />
+    <option name="languageVersion" value="2.1" />
+  </component>
+</project>

.idea/libraries/KotlinJavaRuntime.xml

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+<component name="libraryTable">
+  <library name="KotlinJavaRuntime" type="repository">
+    <properties maven-id="org.jetbrains.kotlin:kotlin-stdlib:2.1.21" />
+    <CLASSES>
+      <root url="jar://$MAVEN_REPOSITORY$/org/jetbrains/kotlin/kotlin-stdlib/2.1.21/kotlin-stdlib-2.1.21.jar!/" />
+      <root url="jar://$MAVEN_REPOSITORY$/org/jetbrains/annotations/13.0/annotations-13.0.jar!/" />
+    </CLASSES>
+    <JAVADOC>
+      <root url="jar://$MAVEN_REPOSITORY$/org/jetbrains/kotlin/kotlin-stdlib/2.1.21/kotlin-stdlib-2.1.21-javadoc.jar!/" />
+      <root url="jar://$MAVEN_REPOSITORY$/org/jetbrains/annotations/13.0/annotations-13.0-javadoc.jar!/" />
+    </JAVADOC>
+    <SOURCES>
+      <root url="jar://$MAVEN_REPOSITORY$/org/jetbrains/kotlin/kotlin-stdlib/2.1.21/kotlin-stdlib-2.1.21-sources.jar!/" />
+      <root url="jar://$MAVEN_REPOSITORY$/org/jetbrains/annotations/13.0/annotations-13.0-sources.jar!/" />
+    </SOURCES>
+  </library>
+</component>

.idea/misc.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectRootManager" version="2" languageLevel="JDK_24" default="true" project-jdk-name="openjdk-24" project-jdk-type="JavaSDK">
+    <output url="file://$PROJECT_DIR$/out" />
+  </component>
+</project>

.idea/modules.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectModuleManager">
+    <modules>
+      <module fileurl="file://$PROJECT_DIR$/kotlin-for-java-developers.iml" filepath="$PROJECT_DIR$/kotlin-for-java-developers.iml" />
+    </modules>
+  </component>
+</project>

.idea/vcs.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="$PROJECT_DIR$" vcs="Git" />
+  </component>
+</project>

kotlin-for-java-developers.iml

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+<?xml version="1.0" encoding="UTF-8"?>
+<module type="JAVA_MODULE" version="4">
+  <component name="NewModuleRootManager" inherit-compiler-output="true">
+    <exclude-output />
+    <content url="file://$MODULE_DIR$">
+      <sourceFolder url="file://$MODULE_DIR$/resources" type="java-resource" />
+      <sourceFolder url="file://$MODULE_DIR$/src" isTestSource="false" />
+      <sourceFolder url="file://$MODULE_DIR$/test" isTestSource="true" />
+      <sourceFolder url="file://$MODULE_DIR$/testResources" type="java-test-resource" />
+    </content>
+    <orderEntry type="inheritedJdk" />
+    <orderEntry type="sourceFolder" forTests="false" />
+    <orderEntry type="library" name="KotlinJavaRuntime" level="project" />
+  </component>
+</module>

src/Also.kt

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+fun main() {
+    // Takes a regular lambda without a receiver
+    val uppercase = "Hello".also { it.uppercase() }
+}

src/Apply.kt

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+fun main() {
+    // Returns the receiver as the result
+    val apply = "Hello".apply { print(this) }
+}

src/Classes.kt

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+// Cannot initialize class, must be extended
+abstract class Classes(val name: String) {
+
+    // Makes it possible to override method
+    open fun finalFunction() = 1
+
+    // Must be overridden when extending
+    abstract fun overrideMe()
+}
+
+// Primary constructor
+class ExtendedClass(name: String, val age: Int) : Classes(name) {
+
+    // Called after primary constructor
+    init {
+        println(age)
+    }
+
+    // secondary constructor
+    constructor(name: String) : this(name, age = 0)
+
+    // Overrides a method
+    override fun overrideMe() {
+        TODO("Not yet implemented")
+    }
+
+}

src/Conditionals.kt

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+import Color.*;
+
+fun main() {
+    when (RED) {
+        RED -> println("red")
+        BLUE -> println("blue")
+        GREEN -> println("green")
+    }
+
+    when ("y") {
+        "yes", "y" -> println("Yes")
+        else -> println("No")
+    }
+
+    val b: A = B()
+
+    when (b) {
+        // Smart cast b to the type B
+        is B -> println("B")
+        else -> println("Unknown")
+    }
+}
+
+enum class Color {
+    BLUE, GREEN, RED
+}
+
+open class A
+
+class B : A()

src/Constants.kt

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+// Compile time constant. Is inlined by the compiler. Only works for primitives and Strings. static method in Java
+const val MAGIC_NUMBER = 42
+
+// Is converted into a static final in Java. Supports all types
+@JvmField
+val JAVA_CONSTANT = MyClass()
+
+class MyClass

src/DataClass.kt

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+// data adds extra methods like toString, equals and hashcode, only uses data from primary constructor
+data class DataClass(val name: String, val age: Int)

src/Exceptions.kt

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+import java.io.IOException
+
+fun main() {
+    val value = try {
+        throw NullPointerException("Whoops")
+    } catch (e: NullPointerException) {
+        println(e.message)
+        "Caught NullPointerException"
+    }
+    println(value)
+}
+
+// Like the throws keyword in Java. Only needed when calling from Java
+@Throws(IOException::class)
+fun catchedException() {
+    throw IOException("Whoops")
+}

src/Extensions.kt

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+import kotlin.random.Random
+
+fun main() {
+    println(9.random())
+    listOf(1, 2, 3).sum()
+}
+
+// Defines an extension function on the Int type, must be imported outside the file
+// This refers to the value we extend
+// Static function under the hood
+fun Int.random(): Int = Random.nextInt(this)
+
+// Add extension to List class of Int
+fun List<Int>.sum(): Int {
+    var result = 0
+    for (i in this) {
+        result += i
+    }
+    return result
+}

src/FunctionTypes.kt

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+fun main() {
+    // Defines a function type with two int args and one int result
+    val sum: (Int, Int) -> Int = { a, b -> a + b }
+    println(run { "This is a function which is called immediately" })
+    val nullableFunctionType: (() -> Unit)? = null
+    nullableFunctionType?.invoke() // Safe call a nullable function type
+
+    val isEvenVariable = ::isEven // Store a ref to a function
+}
+
+fun isEven(x: Int) = x % 2 == 0

src/Functions.kt

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+// Specify the name to use when calling from another language
+@file:JvmName("fun")
+
+fun main() {
+    print(max(2, 3))
+
+    println(
+        // Named arguments are optional
+        listOf("a", "b", "c").joinToString(
+            separator = "",
+            prefix = "(",
+            postfix = ")"
+        )
+    )
+
+    // Using default value
+    println(repeatString("*"))
+    println(repeatString("*", 2))
+    // Flip the arguments by using named arguments
+    println(repeatString(times = 5, s = "*"))
+}
+
+// Top-level function
+fun max(a: Int, b: Int): Int = if (a > b) a else b
+
+// Unit return type
+fun print(value: Int) = println("Max is $value")
+
+// Times has a default value
+fun repeatString(s: String, times: Int = 10): String = s.repeat(times)
+
+// Generates 4 overloaded functions for the JVM. Not needed when only using Kotlin
+@JvmOverloads
+fun sum(a: Int = 0, b: Int = 0, c: Int = 0) = a + b + c

src/Generics.kt

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+// T? marks type T as nullable
+fun <T> List<T>.filterNullable(predicate: (T) -> Boolean): T? {
+    TODO()
+}
+
+fun <T> List<T>.filterCanBeNull(predicate: (T) -> Boolean) {
+    TODO()
+}
+
+// T cannot be nullable
+fun <T : Any> List<T>.filterNotNull(predicate: (T) -> Boolean) {
+    TODO()
+}
+
+// Define bounds using where clause
+fun <T> ensureTrailingPeriod(seq: T) where T : CharSequence, T : Appendable {
+    if (seq.endsWith(".")) {
+        seq.append('.')
+    }
+}
+
+fun List<Int>.avg(): Double {
+    TODO()
+}
+
+// Java doesn't allow overloaded methods that differ only by generic types. We add @JvmName to create a new name when used from Java
+@JvmName(name = "avgDouble")
+fun List<Double>.avg(): Double {
+    TODO()
+}

src/HelloWorld.kt

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+fun main() = println("Hello World")

src/In.kt

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+import java.time.Instant
+
+fun main() {
+    // Check if a is in abc
+    println('a' in "abc")
+    // Check if a is not in abc
+    println('a' !in "abc")
+    // Check if now is within the bounds, using comparable under the hood
+    println(Instant.now() in Instant.MIN..Instant.MAX)
+}
+
+/// Playground
+fun isValidIdentifier(s: String): Boolean {
+    fun isValidCharacter(ch: Char): Boolean = ch == '_' || ch.isLetterOrDigit()
+    if (s.isEmpty() || s[0].isDigit()) {
+        return false
+    }
+    for (ch in s) {
+        if (!isValidCharacter(ch)) {
+            return false
+        }
+    }
+    return true
+}

src/Inline.kt

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+fun main() {
+    thisIsInlined { true }
+}
+
+// The function is replaced with the contects of the function
+// No performance overhead
+inline fun thisIsInlined(predicate: (Int) -> Boolean): Int? = 42.takeIf(predicate)

src/Lambdas.kt

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+fun main() {
+    listOf(1, 2, 3)
+        // Lambda, it is inferred argument, only used for single argument lambdas
+        .filter { it % 2 == 0 }
+        // Explicitly define args
+        .map { value -> value * value }
+
+    mapOf(1 to 2)
+        // Automatic destructuring of mep entries
+        .mapValues { (key, value) -> "$key $value" }
+}
+
+fun returnFromLambda(): List<Int> {
+    return listOf(3, 0, 5)
+        .flatMap {
+            if (it == 0) return emptyList() // Returns from the function, will always return an empty list when 0
+            listOf(it, it)
+        }
+}
+
+fun returnFromLambda2(): List<Int> {
+    return listOf(3, 0, 5)
+        .flatMap {
+            if (it == 0) return@flatMap emptyList() // Returns from the lambda
+            listOf(it, it)
+        }
+}
+
+fun returnFromLambda3(): List<Int> {
+    return listOf(3, 0, 5)
+        .flatMap l@{ // Define custom label
+            if (it == 0) return@l emptyList() // Returns from the lambda
+            listOf(it, it)
+        }
+}

src/Lateinit.kt

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+fun main() {
+    val init = Init()
+    init.name = ""
+}
+
+class Init {
+    // Allows setting the value at a later time without making it nullable. We can't use primitives
+    lateinit var name: String
+
+    /// Checks if name is initialized
+    fun isInitialized(): Boolean {
+        return ::name.isInitialized
+    }
+}

src/Lazy.kt

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+fun main() {
+
+    // Computes only when called and only once
+    val lazyValue: String by lazy {
+        println("computed!")
+        "Hello"
+    }
+}

src/Let.kt

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+fun main() {
+    val string = getString()
+    // Lambda is only called if string is not null
+    string?.let { println(it) }
+    val uppercaseMaybe = string
+        // Return value if predicate is true, otherwise null
+        ?.takeIf { it.length > 4 }
+        ?.let { it.uppercase() }
+}
+
+fun getString(): String? = "Hello"

src/Loops.kt

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+fun main() {
+    while (true) {
+        println("While")
+        break
+    }
+
+    // Including upper bound
+    for (x in 1..10) {
+        print(x)
+    }
+
+    // Excluding upper bound
+    for (x in 1 until 10) {
+        print(x)
+    }
+
+    // Unpack the map to key and value
+    for ((key, value) in mapOf("key" to 1, "value" to 2, "key2" to 3)) {
+        println("$key = $value")
+    }
+}

src/Main.kt

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+//TIP To <b>Run</b> code, press <shortcut actionId="Run"/> or
+// click the <icon src="AllIcons.Actions.Execute"/> icon in the gutter.
+fun main() {
+    val name = "Kotlin"
+    //TIP Press <shortcut actionId="ShowIntentionActions"/> with your caret at the highlighted text
+    // to see how IntelliJ IDEA suggests fixing it.
+    println("Hello, $name!")
+
+    for (i in 1..5) {
+        //TIP Press <shortcut actionId="Debug"/> to start debugging your code. We have set one <icon src="AllIcons.Debugger.Db_set_breakpoint"/> breakpoint
+        // for you, but you can always add more by pressing <shortcut actionId="ToggleLineBreakpoint"/>.
+        println("i = $i")
+    }
+}

src/Nothing.kt

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+fun main() {
+    doesNotReturn()
+    println("This is unreachable")
+}
+
+fun doesNotReturn(): Nothing {
+    throw RuntimeException("This function should always fail")
+}

src/Nullable.kt

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+fun main() {
+    var nullableString: String? = null
+    println(nullableString?.length ?: 0) // Null safe method call with elvis operator for fallback
+    nullableString = "Not Null"
+    println(nullableString)
+}
+
+fun nullableFun(value: Any?) {
+    value!! // Explicitly tell the compilar that it's not null
+}

src/Object.kt

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+// A singleton object
+object Object {
+    val name: String
+        get() {
+            TODO()
+        }
+}
+
+class Static {
+
+
+    companion object {
+        // Makes it possible to call it from Java as a static method on the class
+        // Default is a static method on the companion object
+        @JvmStatic
+        fun create(): Companion /* Companion is default name of companion object */ {
+            return Static
+        }
+    }
+}

src/OperatorOverloading.kt

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+// Implement the plus method on custom class
+// Same with other operators
+operator fun MyNewOperator.plus(other: MyNewOperator): MyNewOperator {
+    return TODO()
+}
+
+class MyNewOperator

src/Properties.kt

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+fun main() {
+    val person = Person("name", age = 5)
+    println(person.name) // Calling getter
+    person.age = 6 // Calling setter
+}
+
+// Defines a class with 2 properties.
+// val generates only getter, var generates both getters and setters
+class Person(val name: String, var age: Int)
+
+class Address {
+
+    // Defines get and set methods with custom logic
+    var place: String = ""
+        get() = field.uppercase()
+        set(value) {
+            field = value.lowercase()
+        }
+
+}
+
+// Extension property on String
+val String.name: String
+    get() = this.lowercase()

src/Run.kt

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+fun main() {
+    // Similar to with but can be used with nullable values
+    val uppercase = nullableString()?.run { uppercase() }
+}
+
+fun nullableString(): String? = "Hello"

src/SafeCast.kt

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+fun main() {
+    any("Whoops")
+}
+
+fun any(any: Any) {
+    if (any is String) {
+        println(any.length)
+    }
+    // Safe case
+    println((any as? String)?.uppercase())
+}

src/SealedClass.kt

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+sealed class SealedClass
+class OK : SealedClass()
+class NotFound : SealedClass()
+
+fun main() {
+    // Is exhaustive since we know only Ok and NotFound extends sealed class
+    when (OK() as SealedClass) {
+        is OK -> println("OK")
+        is NotFound -> println("Not found")
+    }
+}

src/Sequences.kt

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+fun main() {
+    // Lazy list, like Java Streams
+    sequenceOf(1, 2, 3)
+        .map { it * it }
+        // Will only enumerate the first 2 elements
+        .first { it > 2 }
+}
+
+fun yieldSequence(): Sequence<Int> = sequence {
+    // yield returns a value to generate a sequence
+    while (true) yield(1)
+}
+
+fun fibonacci(): Sequence<Int> = sequence {
+    yieldAll(0..1)
+    var values = Pair(0, 1)
+    while (true) {
+        val nextValue = values.first + values.second
+        yield(nextValue)
+        values = Pair(values.second, nextValue)
+    }
+}

src/With.kt

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+fun main() {
+    val sb = StringBuilder()
+    // sb becomes this within the lambda scope
+    val string = with(sb) /* Is an extention function on type T */ {
+        appendLine("Something")
+        append('a')
+        toString()
+    }
+
+    val buildString = buildString {
+        append(1)
+        append(2)
+        toString()
+    }
+
+    val words = Words()
+    with(words) {
+        // The following two lines should compile:
+        "one".record()
+        +"two"
+    }
+    words.toString() == "[one, two]"
+}
+
+class Words {
+    private val list = mutableListOf<String>()
+
+    fun String.record() = list.add(this)
+
+    operator fun String.unaryPlus() = list.add(this)
+
+    override fun toString() = list.toString()
+}