Kotlin Cheat Sheet - Basics - 1

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Đào An viết ngày 19/12/2018

Basic Types

• In Kotlin, everything is an object.

Numbers

• Types: Double, Float, Long, Int, Short, Byte
• Tags: Long (L), Float (f, F) -> 123.5f
• Underscores: 999_99_9999L -> readable
• Optional - Boxing = null || value

val a: Int = 10000
println(a === a) // Prints 'true'
val boxedA: Int? = a
val anotherBoxedA: Int? = a
println(boxedA === anotherBoxedA) // !!!Prints 'false'!!!

• Casting

val b: Byte = 1 
val i: Int = b.toInt() // OK: explicitly widened
val l = 1L + 3 // Long + Int => Long

Arrays

• Arrays in Kotlin are immutable.
• Use [ ] / get(), set() method

val numbers: IntArray = intArrayOf(10, 20, 30, 40, 50)
/* Creates an Array<String> 
with values ["0", "1", "4", "9", "16"] */
val asc = Array(5) { i -> (i * i).toString() }
asc.forEach { println(it) }
val x: IntArray = intArrayOf(1, 2, 3)
x[0] = x[1] + x[2]

Strings

Strings are immutable.

• Raw string

val text = """
    for (c in "foo")
        print(c)
"""

• String Templates

val i = 10
println("i = $i") /* prints "i = 10" */
val s = "abc"
println("$s.length is ${s.length}") /* prints "abc.length is 3" */

Packages

package foo.bar

fun baz() {...}
class Goo {...}

Imports

• Single:

import foo.Bar 

• Multiple

import foo.*  
/*everything  (package, class, object etc) 
in 'foo' becomes accessible*/

• Namesake

import foo.Bar 
/* Bar is accessible*/
import bar.Bar as bBar
/* bBar stands for 'bar.Bar'*/

Control Flow

• Don't have ternary operator (condition ? then : else), use if else instead.

// As expression 
val max = if (a > b) a else b
val max = if (a > b) {
    print("Choose a")
    a
} else {
    print("Choose b")
    b
}

• Don't have Switch

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

when (x) {
    in 1..10 -> print("x is in the range")
    in validNumbers -> print("x is valid")
    !in 10..20 -> print("x is outside the range")
    else -> print("none of the above")
}

fun hasPrefix(x: Any) = when(x) { /* is | !is */
    is String -> x.startsWith("prefix")
    else -> false
}

when { /* replace if-else if chain */
    x.isOdd() -> print("x is odd")
    x.isEven() -> print("x is even")
    else -> print("x is funny")
}

fun Request.getBody() =
        when (val response = executeRequest()) {
            is Success -> response.body
            is HttpError -> throw HttpException(response.status)
        }

• For Loops

for (item in collection) print(item)

for (i in 1..3) {
    println(i)
}
for (i in 6 downTo 0 step 2) {
    println(i)
}

for (i in array.indices) {
    println(array[i])
}

for ((index, value) in array.withIndex()) {
    println("the element at $index is $value")
}

• while and do..while work as usual