[JAVA] Kotlin cheat sheet

At first

Kotlin is convenient! But now I have to use Java as well. How do you do that in Kotlin? So I made a cheat sheet. Comes with code that can be executed immediately.

What is Kotlin?

Kotlin data type

Kotlin variables and constants

Variable var

var a: Float
a = 12345.6f

Constant val

val a: Int = 10000

val b = 10_000L //Separated by underscore to make digits easier to see

Declaration and initialization do not have to be simultaneous.

val c: Double
c = 12345.6

Kotlin control syntax

If expression

if(true)
    println("Hello")

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if(false){
    println("Hello")
} else if(true) {
    println("Kotlin")
} else {
    println("World!")
}

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Since it is an expression, the result can be assigned. The last evaluated formula is returned.

val max = if (a > b) {
    print("a is greater than b")
    a
} else {
    print("a is less than b")
    b
}

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When expression

when (a) {
    1    -> println("a == 1")
    2,3  -> println("a == 2 or a == 3")
    else -> {
        println("otherwise")
    }
}

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Specify the range with in.

when (a) {
    in 1..10   -> println("a is in the range")
    !in 10..20 -> println("a is outside the range")
    else       -> println("otherwise")
}

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For loop

for (a in 1..10) println(a)

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for (a in 'Z' downTo 'A') print(a)

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collection

val array = arrayOf(297456, 119243,9,30)
for (a in array) {
    println(a)
}

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If you want to use an index

val array = arrayOf(297456, 119243,9,30)
for (i in array.indices) {
    println(array[i])
}

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While loop

Same as Java.

var x = 10
while (x > 0) {
    println(x--)
}

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do-while

do{
  println("i = " + i--)
}while(i > 4)

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Kotlin data conversion

String to number

val intVal: Int = str.toInt()
val intValOrNull: Int? = str.toIntOrNull()
val doubleVal = str.toDouble()

From each type to a string

val intStr = num.toString()

Cast of numbers (eg Int to Long conversion)

val long= int.toLong()

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Date processing

Date processing

Kotlin arithmetic operator

Same as Java

Kotlin Boolean operator

Same as Java

a && b (and) a || b (or) !a (not)

Kotlin string

    val multiline = """
    for (c in "foo"){
        print(c)
    }
    """

    val withTrim = """
    |for (c in "foo"){
    |    print(c)
    |}
    """.trimMargin()

Kotlin string template

val str = "Hello Kotlin!"
val templete = " $str is ${str.length} characters."
// >Hello Kotlin! is 13 characters.

When you want to output $ as it is

val price = 200
val multiline = """
|This is ${'$'}$price.
""".trimMargin()

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Kotlin comparison operator

Almost the same as Java, but "==" is equivalent to Java equals. Object comparison is "===" in Kotlin.

Kotlin string function

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Kotlin collection

List

Read-only List

val items = listOf(1, 2, 3)
println(items)

Modifiable List

val numbers: MutableList<Int> = mutableListOf(1, 2, 3)
numbers.add(4)

Set

Read-only Set

var strings: Set<String> = setOf("A","B","C")

Changeable Set

var stringSet: MutableSet<String> = mutableSetOf("x","y","z")
stringSet.add("a") 
stringSet.filter { it != "x"}

Map

Read-only Map

val fruits = mapOf("apple" to 1, "orange" to 2, "banana" to 3)

Changeable Map

val readWriteMap = mutableMapOf("apple" to 1, "orange" to 2, "banana" to 3)
readWriteMap.put("melon",4)

Kotlin function

Kotlin functions are defined with the keyword fun

fun times(a: Int, b: Int): Int {
    return a * b
}

Function execution

val result = times(2,5)

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If the function body consists of one expression, you can omit the curly braces and write the expression after =. The return value can be omitted by inference.

fun times(a: Int, b: Int) = a * b

Default value can be specified for the argument. You can specify the name of the argument when calling the function.

val result = makeString(base = "world ", concl = "of Kotlin!")

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Kotlin class

open class Parent{
    open fun show() = println("This is Parent")
}
class Child:Parent(){
    override fun show() = println("This is Child")
}

Initializer: Initialization code uses init block

class Person(val name: String, val isMan:Boolean, var favorite:String ){
    init {
        val s = if(isMan) "He" else "She"
        favorite =  "$s liles $favorite"
    }
}

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Kotlin data class

equals, hashCode, toString, copy are automatically generated by defining as data class.

data class Dog(val name: String)

Object expression

A function that corresponds to an anonymous class in Java. object: Inheritance Class (Interface) name {implementation}

view.setOnClickListener(object: View.OnClickListener {
override fun onClick(v: View?) {
        view.text = "clicked"
    }
})

Kotlin lambda expression

{variable:Mold->processing}

Lambda expressions can be stored in variables

val minus = {x: Int, y: Int -> x - y }

If the lambda expression has one argument, you can omit the argument and use the implicit argument it

val double:(Int)->Int = {it * 2}

Interoperability with Java

Use ClassName :: class.java or instance :: class.java to get java.lang.Class

val clazz = Activity::class.java
val clazzz = this::class.java