topic: Basic Syntax & Types

Hard Prerequisites
IMPORTANT: Please review these prerequisites, they include important information that will help you with this content.
  • TOPICS: Set Up

    • var us val:

    • val and var both are used to declare a variable.
    • var is like general variable and it's known as a mutable variable in Kotlin and can be assigned multiple times.
    • val is like Final variable and it's known as immutable in Kotlin and can be initialized only single time., after it become read only. The IllegalAccessorError will occur when you try to reassign the value.
    • You can enforce a type called strongtyping. This is the opposite of statically typed.

    Syntaxt : var book:String = "Maths" // This should only be used when necessary.

    Numbers:

    • Kotlin handles numbers in a way close to Java, but not exactly the same.
    • Kotlin provides the following built in types representing numbers (this is close to Java):
    Type Bit Width
    Double 64
    Float 32
    Long 64
    Int(Default data type in Kotlin) 32
    Short 16
    Byte 8

    Characters:

    Fun check(c: Char){

If (c == 1) {

// Error : incomaptable types will occur

}

}

    Booleans:

    The type Boolean represents booleans, has a true or false value.

    Arrays:

    Arrays in Kotlin are represented by the array class, that has get and set functions (that turn into [] by operator overloading conventions), and size property, along with a few other useful member functions:

    Class Array private constructor() {

val size: int

operator fun get (index : Int) T

operator fun set(index : Int, value: T ) : Unit

}

    Strings

    Strings represented by the type String. Strings are immutable.

    They are immutable in nature.

    Should be written in double quotes.

    Elements of a string are characters that can be accessed by the indexing operation:

    s[I]

A string can be iterated over with a for loop:

for (c in str){

prinln(c)

}

    Defining packages

    Package specification should be at the top of the source file:

    package my.demo

import java.util.*

// ...

    It is not required to match directories and packages: source files can be placed arbitrarily in the file system.

    Defining functions

    Function having two Int parameters with Int return type:

    //sampleStart
fun sum(a: Int, b: Int): Int {
    return a + b
}
//sampleEnd

fun main(args: Array<String>) {
    print("sum of 3 and 5 is ")
    println(sum(3, 5))
}

    Function with an expression body and inferred return type:

    //sampleStart
fun sum(a: Int, b: Int) = a + b
//sampleEnd

fun main(args: Array<String>) {
    println("sum of 19 and 23 is ${sum(19, 23)}")
}

    Function returning no meaningful value:

    //sampleStart
fun printSum(a: Int, b: Int): Unit {
    println("sum of $a and $b is ${a + b}")
}
//sampleEnd

fun main(args: Array<String>) {
    printSum(-1, 8)
}

    Unit return type can be omitted:

    //sampleStart
fun printSum(a: Int, b: Int) {
    println("sum of $a and $b is ${a + b}")
}
//sampleEnd

fun main(args: Array<String>) {
    printSum(-1, 8)
}

    Defining local variables

    Assign-once (read-only) local variable:

    fun main(args: Array<String>) {
//sampleStart
    val a: Int = 1  // immediate assignment
    val b = 2   // `Int` type is inferred
    val c: Int  // Type required when no initializer is provided
    c = 3       // deferred assignment
//sampleEnd
    println("a = $a, b = $b, c = $c")
}

    Mutable variable:

    fun main(args: Array<String>) {
//sampleStart
    var x = 5 // `Int` type is inferred
    x += 1
//sampleEnd
    println("x = $x")
}

    Comments

    Just like Java and JavaScript, Kotlin supports end-of-line and block comments.

    // This is an end-of-line comment

/* This is a block comment
   on multiple lines. */

    Unlike Java, block comments in Kotlin can be nested.

    Using string templates

    fun main(args: Array<String>) {
//sampleStart
    var a = 1
    // simple name in template:
    val s1 = "a is $a"

    a = 2
    // arbitrary expression in template:
    val s2 = "${s1.replace("is", "was")}, but now is $a"
//sampleEnd
    println(s2)
}

    Using conditional expressions

    //sampleStart
fun maxOf(a: Int, b: Int): Int {
    if (a > b) {
        return a
    } else {
        return b
    }
}
//sampleEnd

fun main(args: Array<String>) {
    println("max of 0 and 42 is ${maxOf(0, 42)}")
}

    Using if as an expression:

    //sampleStart
fun maxOf(a: Int, b: Int) = if (a > b) a else b
//sampleEnd

fun main(args: Array<String>) {
    println("max of 0 and 42 is ${maxOf(0, 42)}")
}

    Using nullable values and checking for null

    A reference must be explicitly marked as nullable when null value is possible.

    Return null if str does not hold an integer:

    fun parseInt(str: String): Int? {
    // ...
}
Use a function returning nullable value:

fun parseInt(str: String): Int? {
    return str.toIntOrNull()
}

//sampleStart
fun printProduct(arg1: String, arg2: String) {
    val x = parseInt(arg1)
    val y = parseInt(arg2)

    // Using `x * y` yields error because they may hold nulls.
    if (x != null && y != null) {
        // x and y are automatically cast to non-nullable after null check
        println(x * y)
    }
    else {
        println("either '$arg1' or '$arg2' is not a number")
    }
}
//sampleEnd


fun main(args: Array<String>) {
    printProduct("6", "7")
    printProduct("a", "7")
    printProduct("a", "b")
}

    or

    fun parseInt(str: String): Int? {
    return str.toIntOrNull()
}

fun printProduct(arg1: String, arg2: String) {
    val x = parseInt(arg1)
    val y = parseInt(arg2)

//sampleStart
    // ...
    if (x == null) {
        println("Wrong number format in arg1: '${arg1}'")
        return
    }
    if (y == null) {
        println("Wrong number format in arg2: '${arg2}'")
        return
    }

    // x and y are automatically cast to non-nullable after null check
    println(x * y)
//sampleEnd
}

fun main(args: Array<String>) {
    printProduct("6", "7")
    printProduct("a", "7")
    printProduct("99", "b")
}

    Using type checks and automatic casts

    The is operator checks if an expression is an instance of a type. If an immutable local variable or property is checked for a specific type, there’s no need to cast it explicitly:

    //sampleStart
fun getStringLength(obj: Any): Int? {
    if (obj is String) {
        // `obj` is automatically cast to `String` in this branch
        return obj.length
    }

    // `obj` is still of type `Any` outside of the type-checked branch
    return null
}
//sampleEnd


fun main(args: Array<String>) {
    fun printLength(obj: Any) {
        println("'$obj' string length is ${getStringLength(obj) ?: "... err, not a string"} ")
    }
    printLength("Incomprehensibilities")
    printLength(1000)
    printLength(listOf(Any()))
}

    or

    //sampleStart
fun getStringLength(obj: Any): Int? {
    if (obj !is String) return null

    // `obj` is automatically cast to `String` in this branch
    return obj.length
}
//sampleEnd


fun main(args: Array<String>) {
    fun printLength(obj: Any) {
        println("'$obj' string length is ${getStringLength(obj) ?: "... err, not a string"} ")
    }
    printLength("Incomprehensibilities")
    printLength(1000)
    printLength(listOf(Any()))
}

    or even

    //sampleStart
fun getStringLength(obj: Any): Int? {
    // `obj` is automatically cast to `String` on the right-hand side of `&&`
    if (obj is String && obj.length > 0) {
        return obj.length
    }

    return null
}
//sampleEnd


fun main(args: Array<String>) {
    fun printLength(obj: Any) {
        println("'$obj' string length is ${getStringLength(obj) ?: "... err, is empty or not a string at all"} ")
    }
    printLength("Incomprehensibilities")
    printLength("")
    printLength(1000)
}

    Using a for loop

    fun main(args: Array<String>) {
//sampleStart
    val items = listOf("apple", "banana", "kiwi")
    for (item in items) {
        println(item)
    }
//sampleEnd
}

    or

    fun main(args: Array<String>) {
//sampleStart
    val items = listOf("apple", "banana", "kiwi")
    for (index in items.indices) {
        println("item at $index is ${items[index]}")
    }
//sampleEnd
}

    Using a while loop

    fun main(args: Array<String>) {
//sampleStart
    val items = listOf("apple", "banana", "kiwi")
    var index = 0
    while (index < items.size) {
        println("item at $index is ${items[index]}")
        index++
    }
//sampleEnd
}

    Using when expression

    //sampleStart
fun describe(obj: Any): String =
    when (obj) {
        1          -> "One"
        "Hello"    -> "Greeting"
        is Long    -> "Long"
        !is String -> "Not a string"
        else       -> "Unknown"
    }
//sampleEnd

fun main(args: Array<String>) {
    println(describe(1))
    println(describe("Hello"))
    println(describe(1000L))
    println(describe(2))
    println(describe("other"))
}

    Using ranges

    Check if a number is within a range using in operator:

    fun main(args: Array<String>) {
//sampleStart
    val x = 10
    val y = 9
    if (x in 1..y+1) {
        println("fits in range")
    }
//sampleEnd
}

    Check if a number is out of range:

    fun main(args: Array<String>) {
//sampleStart
    val list = listOf("a", "b", "c")

    if (-1 !in 0..list.lastIndex) {
        println("-1 is out of range")
    }
    if (list.size !in list.indices) {
        println("list size is out of valid list indices range too")
    }
//sampleEnd
}

    Iterating over a range:

    fun main(args: Array<String>) {
//sampleStart
    for (x in 1..5) {
        print(x)
    }
//sampleEnd
}
or over a progression:

fun main(args: Array<String>) {
//sampleStart
    for (x in 1..10 step 2) {
        print(x)
    }
    for (x in 9 downTo 0 step 3) {
        print(x)
    }
//sampleEnd
}

    Using collections

    Iterating over a collection:

    fun main(args: Array<String>) {
    val items = listOf("apple", "banana", "kiwi")
//sampleStart
    for (item in items) {
        println(item)
    }
//sampleEnd
}

    Checking if a collection contains an object using in operator:

    fun main(args: Array<String>) {
    val items = setOf("apple", "banana", "kiwi")
//sampleStart
    when {
        "orange" in items -> println("juicy")
        "apple" in items -> println("apple is fine too")
    }
//sampleEnd
}

    Using lambda expressions to filter and map collections:

    fun main(args: Array<String>) {
    val fruits = listOf("banana", "avocado", "apple", "kiwi")
//sampleStart
    fruits
        .filter { it.startsWith("a") }
        .sortedBy { it }
        .map { it.toUpperCase() }
        .forEach { println(it) }
//sampleEnd
}
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