Kotlin Android Syntax (Collections, Class, Instance)

Using collections

Iterating over a collection:

for (item in items)

{

    println(item)

}

Checking if a collection contains an object using in operator:

when

{

    "orange" in items -> println("juicy")

    "apple" in items -> println("apple is fine too")

}

Using lambda expressions to filter and map collections:

fruits

.filter { it.startsWith("a") }

.sortedBy { it }

.map { it.toUpperCase() }

.forEach { println(it) }

Creating basic classes and their instances:

val rectangle = Rectangle(5.0, 2.0) //no 'new' keyword required

val triangle = Triangle(3.0, 4.0, 5.0)

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Kotlin Android Syntax ( for, while, range, when)

Using a for loop

val items = listOf("apple", "banana", "kiwi")

for (item in items)

{

    println(item)

}

or

val items = listOf("apple", "banana", "kiwi")

for (index in items.indices)

{

    println("item at $index is ${items[index]}")

}

Using a while loop

val items = listOf("apple", "banana", "kiwi")

var index = 0

while (index < items.size)

{

    println("item at $index is ${items[index]}")

    index++

}

Using when expression

fun describe(obj: Any): String =

when (obj)

{

    1          -> "One"

    "Hello"    -> "Greeting"

    is Long    -> "Long"

    !is String -> "Not a string"

    else       -> "Unknown"

}

Using ranges

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

val x = 10

val y = 9

if (x in 1..y+1)

{

    println("fits in range")

}

Check if a number is out of range:

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")

}

Iterating over a range:

for (x in 1..5)

{

    print(x)

}

or over a progression:

for (x in 1..10 step 2)

{

    print(x)

}

println()

for (x in 9 downTo 0 step 3)

{

    print(x)

}

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Kotlin Android Null Pointer Safe

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 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")

    }   

}

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)

Null Safety : 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:

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

}

or

fun getStringLength(obj: Any): Int?

{

    if (obj !is String) return null

​

    // `obj` is automatically cast to `String` in this branch

    return obj.length

}

or even

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

}

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Basic Syntax - Kotlin Programming Language If, Loop, Functions

Defining functions

Function having two Int parameters with Int return type:

fun sum(a: Int, b: Int): Int 

{

    return a + b

}

Function with an expression body and inferred return type:

fun sum(a: Int, b: Int) = a + b

Function returning no meaningful value:

fun printSum(a: Int, b: Int): Unit 

{

    println("sum of $a and $b is ${a + b}")

}

Unit return type can be omitted:

fun printSum(a: Int, b: Int) 

{

    println("sum of $a and $b is ${a + b}")

}

Defining variables

Assign-once (read-only) local variable:

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

Mutable variable:

var x = 5 // `Int` type is inferred

x += 1

Top-level variables:

val PI = 3.14

var x = 0

​fun incrementX()

{

    x += 1

}

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

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"

Using conditional expressions

fun maxOf(a: Int, b: Int): Int

{

    if (a > b) {

        return a

    } else {

        return b

    }

}

Using if as an expression:

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

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You can comment if any confusion in syntax or if need more explanation we'll reply you ASAP.

Thank you Safe & Happy Coding. :)