fun main(){
    println("hello world")
}
 　↓　
hello world

class MainActivity : AppCompatActivity() {
 
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
 
        val but = findViewById(R.id.button) as Button
        but.setOnClickListener {
            Toast.makeText(this, "テストメッセージです", Toast.LENGTH_SHORT).show()
        }
    }
}

fun maxOf(a: Int, b: Int): Int {
    if (a > b) {
        return a
    } else {
        return b
    }
}

　↓同じ

fun main() {
    println("max of 0 and 42 is ${maxOf(0, 42)}")
}

val list = listOf("a", "b", "c")

println(list.lastIndex)
println(list.indices)
　↓
2
0..2

fun main() {
    val items = listOf("apple", "banana", "kiwifruit")
    
    for (index in items.indices) {
        println("item at $index is ${items[index]}")
    }
    
    for (item in items) {
        println(item)
    }
    var index = 0
    while (index < items.size) {
        println("item at $index is ${items[index]}")
        index++
    }
    ↓
item at 0 is apple
item at 1 is banana
item at 2 is kiwifruit
apple
banana
kiwifruit
item at 0 is apple
item at 1 is banana
item at 2 is kiwifruit
}
 ↓
item at 0 is apple
item at 1 is banana
item at 2 is kiwifruit
apple
banana
kiwifruit

fun describe(obj: Any): String =
    when (obj) {
        1          -> "One"
        "Hello"    -> "Greeting"
        is Long    -> "Long"
        !is String -> "Not a string"
        else       -> "Unknown"
    }

fun main() {
    println(describe(1))
    println(describe("Hello"))
    println(describe(1000L))
    println(describe(2))
    println(describe("other"))
    ↓
One
Greeting
Long
Not a string
Unknown
}

  val fruits = listOf("banana", "avocado", "apple", "kiwifruit")
  fruits
    .filter { it.startsWith("a") }
    .sortedBy { it }
    .map { it.toUpperCase() }
    .forEach { println(it) }
    ↓
APPLE
AVOCADO

自動でGetter　Setter　が作られる

class Data(var param: Int){

}

fun main() {
    val data = Data(5)
    println(data.param) // called getter
    data.param = 1 // called setter
    println(data.param) // called getter
}
 　↓
5
1

Getter　Setter をカスタマイズする場合

class Animal(){
   var name:String = ""
   var hello:String 
    get(){
        return name
        }
    set(value){
        name="${value} Chan"
    }

}

fun main() {
    val cat = Animal()
    cat.hello = "HaNa"
    println(cat.name) // called getter

}
　↓
HaNa Chan

Kotlin の Collection まとめ ～List編～
Kotlin のコレクション使い方メモ

不変のリストは listOf
可変のリストは mutableListOf
文字列の結合joinToString

fun joinOptions(options: Collection<String>) = options.joinToString(prefix = "[", postfix = "]")

fun main() {
    val list = listOf("t", "e", "s", "t")
    println(joinOptions(list))

}
　↓
[t, e, s, t]