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)}")
}