Last updated about 3 years ago

Kotlin

• Official site

• Android Dev Docs

Notes

• main function is the primary entry point

• Print to standard output: print("Hello, World!")

• Datatypes (almost same as Java)

  • Int Long Short Byte
  • Double Float
  • String Char
  • Boolean

• Variables and Constants

  • Constants: val <name> [: <type>] = <value>
  • Variables: var <name> [: <type>] = <value>

• Operators

  • Logical: && ||
  • Bitwise: and or
  • Equality: === !==

• Data Structures

  • List: val lst = listOf("a", "b", "c") List
  • Map: val mp = mapOf("a" to 1, "b" to 2, "c" to 3) Map<String, Int>
  • Set: val st = setOf("a", "b", "c") Set
  • Add mutable as prefix for variables: val mutList = mutableListOf("a")

• Functions

  // Syntax
  // fun <name>([... <param>: <type>]): <type> { ... }
  // fun <name>([... <param>: <type>]) = ...
  
  // Example 1
  fun sum(a: Int, b: Int) = a + b
  
  // Example 2
  // Unit means void, can be omitted
  fun sum(a: Int, b: Int): Unit {
      print("$a and $b")
  }

• Loops

• Branching

  • when is like switch:

    when (x) {
      1 -> print("1")
      2 -> print("2")
      else -> print("None")
    }

• Optional

  • Use ? for nullable type: var x: String? = null
  • Use ? in call chain for safe calls: print(x?.length)
  • Use !! to throw if null (like force unwrap x! in Swift): print(x!!.length)
  • Use ?: to get a default value (like x ?? <default> in Swift): print(x?.length ?: 0)
  • Use as? for safe casting: val anInt: Int? = x as? Int

• Class/Objects