Kotlin Idioms

Idioms

Random and frequently used in kotlin.

Creating Data classes in kotlin (POJOs/DTOs)

Data classes are model classes which are created to hold data. In Java, we usually create POJO classes with getter and setter methods for each member variable of a class. But in Kotlin, POJO class can be created within a single line of code with keywords: 'data', 'class', 'class name' and its member variables.

data class Student(val name: String, val age: Int)

provides a Student class with the following functionality:
- getters(and setters in case of vars) for all properties
- equals()
- hashCode()
- toString()
- copy()
- object1(), object2(), ..., for all properties.

Filter a list in kotlin

There are several ways to filter a list in kotlin.

For example:

val fruits = listOf("Apple", "Banana", "Orange")                                                              

And we will filter values matching "Apple".

fruits.filter { fruit  ->  fruit == "Apple" }                                                                        

And If we do not want Apple in our list we will use filterNot.

fruits.filterNot { it == "Apple" }                                                                                      

Lambda expressions are accepted by filter standard library and such methods are called Higher-order functions in Kotlin.

Expressions in kotlin

Use 'when' as expression body:

fun parseNum(number: String): Int?{

       when (number) { 

                 "one" -> return 1

                 "two" -> return 2

                 else -> return null

       }

} 

Alternative way:

fun parseNum(number: String) =

       when (number) {

                "one" -> 1

                "two" -> 2

                 else -> null

       }                                                                                                        

Use 'try' as expression body:

fun tryParse(number: String): Int? {

      try{

             return Integer.parseInt(number)

     }

     catch (e: NumberFormatException) {

            return null

     }

}

Alternative way:

fun tryParse(number: String) =

      try{

           return Integer.parseInt(number)

       }

       catch (e: NumberFormatException) {

         return null

      }                                                                                                                                                

Ranges in kotlin

Using double dot operator, a range can be defined in kotlin as examples below: 

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.. 20 step 2) { ... }
for (x in 10 downTo 1) { ... }
if (x in 1..10) { ... }

Use range checks instead of comparison pairs:

fun isLatinUppercase(c: Char) =
       c >= 'A'  && c <= 'Z'

Alternative way:

fun isLatinUppercase(c: Char) =
       c in 'A' .. 'Z'

Accessing a map in kotlin

println(map["key"])
map["key"] = 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() }

Consuming a nullable Boolean

val b: Boolean? = ...
if (b == true) { ... } else { // `b` is false or null }

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