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()
        }
    }
}