Kotlin Cheat Sheet

Source : Google

Hi Everyone, this time I come up with something new rather than Understanding concept. I prepared Cheat Sheet for Kotlin Programming Language for quick reference.

Kotlin is a cross-platform, statically typed, general-purpose programming language with type inference. Kotlin is a new programming language for the JVM. It produces Java bytecode, supports Android and generates JavaScript.

Source : Google

Basics

• You do not need ; to break statements.
• There are two keywords for variable declaration, var and val.
• var : var is like a general variable and can be assigned multiple times and is known as the mutable variable in Kotlin
• val : val is a constant variable and can not be assigned multiple times and can be Initialized only single time and is known as the immutable variable in Kotlin.
• The keyword void common in Java or C# is called Unit in Kotlin.
• Unlike Java or C#, you declare the type of a variable after the name, e.g. var firstName : String

1. Strings :

val name = "Peter"
val newVar = "Hello, " + name
val helloName = "Hello, $name"
val upercaseName = "Hello, ${name.toUpperCase()

2. Booleans :

val trueBoolean = true
val falseBoolean = false
val andCondition = trueBoolean && falseBoolean
val orCondition = trueBoolean || falseBoolean

3. Floats :

val intNumber = 10
val doubleNumber = 10.0
val longNumbe = 10L
val floatNum = 10.0F

4. Mutability :

var mutableString: String = "Hello"
val immutableString: String = Hello"

5. Companion objects Syntax :

class MyClass {
    companion object Factory {
        fun create(): MyClass = MyClass()
    }
}

6. Null Safety :

Nullable properties

val cannotBeNull: String = null // Invalid
val canBeNull: String? = null // Valid
val cannotBeNull: Int = null // Invalid
val canBeNull: Int? = null // Valid

Null Checking

val name: String? = "Hello"
if (name != null && name.length > 0) {
    print("String length is ${name.length}")
} else {
    print("String is empty.")
}

!! operator

val opt = name!!.length

the not-null assertion operator (!!) converts any value to a non-null type and throws an exception if the value is null. You can write name!!, and this will return a non-null value of name (for example, a String in our example) or throw an NPE if name is null

Safe Casts

val opt: Int? = a as? Int

Regular casts may result in a ClassCastException if the object is not of the target type. Another option is to use safe casts that return null if the attempt was not successful.

7. Classes :

Classes in Kotlin are declared using the keyword class

class Student{ /*...*/ }

Primary Constructor

class Student(val name: String, val age: Int)
val student1 = Student("Peter", 11)

Secondary Constructor

class Student(val name: String) {
    private var age: Int? = null
    constructor(name: String, age: Int) : this(name) {
        this.age = age
    }
}
// Above can be replaced with default params
class Student(val name: String, val age: Int? = null)

Enum Class

enum class Color(val rgb: Int) {
    RED(0xFF0000),
    GREEN(0x00FF00),
    BLUE(0x0000FF)
}

Data Class

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

8. Functions :

Kotlin functions are declared using the fun keyword:

fun printStudentName() {
    print("Richa")
}

Parameters & Return Types

fun printStudentName(student: Student) {
    print(student.name)
}

Default Parameters

fun getStudentName(student: Student, intro: String = "Hello,") {
    return "$intro ${student.name}"
}

9. Collections

List

val numbers = listOf("one", "two", "three", "four")

Set

val numbers = setOf(1, 2, 3, 4)

Map

A Map stores key-value pairs (or entries); keys are unique, but different keys can be paired with equal values.

val numbersMap = mapOf("key1" to 1, "key2" to 2, "key3" to 3, "key4" to 1)

10. Control Flows

Conditions and loops

“if” Statement

if is an expression: it returns a value

var max = a
if (a < b) max = b

// With else
var max: Int
if (a > b) {
    max = a
} else {
    max = b
}

// As expression
val max = if (a > b) a else b

when

when defines a conditional expression with multiple branches.

when (x) {
    1 -> print("x == 1")
    2 -> print("x == 2")
    else -> {
        print("x is neither 1 nor 2")
    }
}

for

for (item in collection) print(item)

If you want to know more about null concept please go through my blog Kotlin: Null Safety Concept for basic understanding.

Happy Reading :)