Kotlin

Kotlin Kotlin is a statically typed programming language Officially supported language by Google for android Core features of Kotlin: Concise: Reduces the amount of boilerplate code Safe: Avoid NullPointerException Interoperable: Use existing libraries of Java, Android, etc Tool friendly: Use any existing IDE Hello world! fun main() { println("Hello world!") } Variables ... val a = "Hello" //type inference val a: Double = 23.44 ... val vs var val is value and var is variable val is nothing but a final value(like Java) which can not be reassigned. var can be reassigned. Can not change the type of a var. It is bounded by its defined type. We can use lateinit to initialise var later. ... lateinit var a: String; ... a = "Aditya" ... We can not set a null value to a var. We need to use ? operator. This is due to strong null safety check which comes attached with Kotlin. ... var a: String? = null; ... To get a NPE for a var in Kotlin, we need to use !! operator. It makes sure that we receive NPE if it occurs. ... var a: String? = null ... val size: Int = a?.length!! //throws NPE We have and elvis operator in Kotlin which is similar to conditional operator. (?:) ... var a: String? = null ... val size: Int = a?.length ?: 0 //returns 0 if a.length is null If we need to concatenate a string to another, there is a smart way to do it in kotlin ... var a: String = "Aditya" println(a + "Singh") //prints Aditya Singh println("$a Singh") //prints Aditya Singh ... Arrays We can define array in following ways: val a = arrayOf(1,2,3,4,"Aditya") /* arrayOf() uses vararg underneath which tells that we can have n arguements inside a function. */ val a = arrayOf<Int>(1,2,3,4) /* This will bind our array with Int type. We cant add any other variable of any other type inside it. */ val num = Array(5, {i -> i*1}) //0 1 2 3 4 /* Using Array constructor */ println(a[3]) println(a.get(3)) Collections By default in kotlin list is immutable ... val myList = listOf<String>("Hello","World","Aditya") ... myList.add("India") //cant do that as there is no function add() If we want to add items we need to explicitly define it as mutable. We can use arrayListOf() also in place of mutableListOf(). ... val myList = mutableListOf<String>("Hello","World") ... myList.add("Aditya") Maps are immutable. ... val myMap = mapOf<Int,String>(1 to "Aditya", 2 to "Singh") ... println(myMap[1]) //this will print "Aditya" val myMap = mapOf(1 to "Aditya", "surname" to "Singh") //to use different keys If we want mutable map, we use hash map. ... val myMap = hashMapOf(1 to "Aditya", "surname" to "Singh") ... myMap.set("country", "India") //add a new key value pair myMap["country"] = "India" //we can do something like this as well Loops Foreach loops works with list, array, map. It iterates over the entire set of items and give the output. val myList = listOf<String>("Hello","World","Aditya") myList.forEach(it -> println(it)) //prints all items for loop for (name in listOfNames) { println(name) //print every item in list } for ( x in 0..10) { println(x) //prints all value from 0 to 10 inclusive } for ( x in 0 until 10) { println(x) //prints all value from 0 to 10, excludes 10 } for ( x in 0 until 10 step 3) { println(x) //prints 0 3 6 9 } for ( x in 10 downTo 0 step 3) { println(x) //prints 10 7 4 1 } Keywords in is check inclusiveness When It is similar to switch statement of C++. We dont iterate over all the condition like we do in if-else. ... var a:String = "Aditya" when(a) { "Aditya" -> { println("Aditya") //will print this } "Singh" -> { println("Singh") } else { println("Other") } } ... When can return a value as well. Functions The structure of functions is very similar to C++ or Java. fun functionName(variable: Int): String { return "$variable was passed" } Types of arguements: ...