Idioms(kotlin) | Tutorial-5

Idioms

A collection of random and frequently used idioms in Kotlin. If you have a favorite idiom, contribute it by sending a pull request.

 Creating DTOs (POJOs/POCOs)  
 data class Customer(val name: String, val email: String)  
 provides a Customer class with the following functionality:  
 getters (and setters in case of vars) for all properties  
 equals()  
 hashCode()  
 toString()  
 copy()  
 component1(), component2(), …, for all properties  
 Default values for function parameters  
 fun foo(a: Int = 0, b: String = "") { ... }  
 Filtering a list  
 val positives = list.filter { x -> x > 0 }  
 Or alternatively, even shorter:  
 val positives = list.filter { it > 0 }  
 String Interpolation  
 println("Name $name")  
 Instance Checks  
 when (x) {  
       is Foo -> ...  
       is Bar -> ...  
       else  -> ...  
 }  
 Traversing a map/list of pairs  
 for ((k, v) in map) {  
       println("$k -> $v")  
 }  
 k, v can be called anything.  
 Using ranges  
 for (i in 1..100) { ... } // closed range: includes 100  
 for (i in 1 until 100) { ... } // half-open range: does not include 100  
 for (x in 2..10 step 2) { ... }  
 for (x in 10 downTo 1) { ... }  
 if (x in 1..10) { ... }  
 Read-only list  
 val list = listOf("a", "b", "c")  
 Read-only map  
 val map = mapOf("a" to 1, "b" to 2, "c" to 3)  
 Accessing a map  
 println(map["key"])  
 map["key"] = value  
 Lazy property  
 val p: String by lazy {  
       // compute the string  
 }  
 Extension Functions  
 fun String.spaceToCamelCase() { ... }  
 "Convert this to camelcase".spaceToCamelCase()  
 Creating a singleton  
 object Resource {  
       val name = "Name"  
 }  
 If not null shorthand  
 val files = File("Test").listFiles()  
 println(files?.size)  
 If not null and else shorthand  
 val files = File("Test").listFiles()  
 println(files?.size ?: "empty")  
 Executing a statement if null  
 val data = ...  
 val email = data["email"] ?: throw IllegalStateException("Email is missing!")  
 Execute if not null  
 val data = ...  
 data?.let {  
   ... // execute this block if not null  
 }  
 Return on when statement  
 fun transform(color: String): Int {  
       return when (color) {  
       "Red" -> 0  
       "Green" -> 1  
       "Blue" -> 2  
       else -> throw IllegalArgumentException("Invalid color param value")  
   }  
 }  
 'try/catch' expression  
 fun test() {  
       val result = try {  
       count()  
   } catch (e: ArithmeticException) {  
       throw IllegalStateException(e)  
   }  
       // Working with result  
 }  
 'if' expression  
 fun foo(param: Int) {  
       val result = if (param == 1) {  
       "one"  
   } else if (param == 2) {  
       "two"  
   } else {  
       "three"  
   }  
 }  
 Builder-style usage of methods that return Unit  
 fun arrayOfMinusOnes(size: Int): IntArray {  
       return IntArray(size).apply { fill(-1) }  
 }  
 Single-expression functions  
 fun theAnswer() = 42  
 This is equivalent to  
 fun theAnswer(): Int {  
       return 42  
 }  
 This can be effectively combined with other idioms, leading to shorter code. E.g. with the when-expression:  
 fun transform(color: String): Int = when (color) {  
       "Red" -> 0  
       "Green" -> 1  
       "Blue" -> 2  
       else -> throw IllegalArgumentException("Invalid color param value")  
 }  
 Calling multiple methods on an object instance ('with')  
 class Turtle {  
       fun penDown()  
       fun penUp()  
       fun turn(degrees: Double)  
       fun forward(pixels: Double)  
 }  
 val myTurtle = Turtle()  
 with(myTurtle) { //draw a 100 pix square  
       penDown()  
       for(i in 1..4) {  
       forward(100.0)  
       turn(90.0)  
   }  
       penUp()  
 }  
 Java 7's try with resources  
 val stream = Files.newInputStream(Paths.get("/some/file.txt"))  
 stream.buffered().reader().use { reader ->  
       println(reader.readText())  
 }  
 Convenient form for a generic function that requires the generic type information  
 // public final class Gson {  
 //   ...  
 //   public <T> T fromJson(JsonElement json, Class<T> classOfT) throws JsonSyntaxException {  
 //   ...  
 inline fun <reified T: Any> Gson.fromJson(json): T = this.fromJson(json, T::class.java)  
 Consuming a nullable Boolean  
 val b: Boolean? = ...  
 if (b == true) {  
   ...  
 } else {  
       // `b` is false or null  
 }