Kotlin beginners cheatsheet

cheatsheet.kt

fun main(args: Array<String>) {

    // when statement
    val firstName = "Dan"
    val job = when (firstName) {
        "Dan", "Jay", "Philip" -> "programmer"
        "Jamie" -> "designer"
        else -> "boss"
    }
    println("$firstName is a $job")
    // Output: Dan is a programmer

    // one line functions
    fun fullName(firstName: String, lastName: String) = "$firstName $lastName"
    println(fullName("Philip", "Kotlin"))
    // Output: Philip Kotlin

    // strings

    println("what happened".capitalize())
    // Output: What happened
    println("Kotlin".equals("KOTLIN", ignoreCase = true))
    // Output: true
    println("Kotlin".contains("LIN", ignoreCase = true))
    // Output: true

    var name: String? = null;
    println(name.equals("Philip", ignoreCase = true))
    // Output: false
    println(name.isNullOrEmpty())
    // Output: true
    println(name.isNullOrBlank())
    // Output: true

    name = "     ";
    println(name.isNullOrEmpty())
    // Output: false
    println(name.isNullOrBlank())
    // Output: true

    var names = "Dan\nJay\nPhilip"
    for (name in names.lines()) {
        println("> $name")
    }
    // > Dan
    // > Jay
    // > Philip

    for ((index, name) in names.lines().withIndex()) {
        println("${index + 1}. $name")
    }
    // 1. Dan
    // 2. Jay
    // 3. Philip

    // maps

    // readonly map
    var legs = mapOf(Pair("dog", 4), Pair("penguin", 2))
    println(legs.size)
    // Output: 2
    println(legs["dog"])
    // Output: 4
    for ((animal, count) in legs) {
        println("$animal = $count")
    }
    // dog = 4
    // penguin = 2

    // another way to create the readonly map
    legs = mapOf("dog" to 4, "penguin" to 2)
    println(legs.size)
    // Output: 2

    // editable map using a HashMap
    legs = hashMapOf("dog" to 4, "penguin" to 2)
    println(legs["dog"])
    // Output: 4
    legs["dog"] = 5
    println(legs["dog"])
    // Output: 5

    // list

    // readonly list
    listOf(1, 2, 3, 4)

    // mutable list
    val array = arrayListOf("Hello", "World")
    array.add("1")

    // set

    // readonly set
    setOf(1, 2, 3)

    // mutable set
    hashSetOf(1, 2, 3)
    sortedSetOf(1, 2, 3)

    // null safety

    var message: String? = null
    message?.let { println(it) }
    // Output nothing
    message = "Hello"
    message?.let { println(it) }
    // Output: Hello
    message?.let { name ->
        println(name)
    }
    // Output: Hello

    println(null ?: "Default value")
    // Output: Default value
    println("Hello" ?: "Default value")
    // Output: Hello
}