類別中可以包含其他類別

class Outer {
    private val bar: Int = 1
    class Nested {
        fun foo() = 2
    }
}

val demo = Outer.Nested().foo() // == 2

Inner classes

類別可以使用inner宣告以便存取外部類別的成員

class Outer {
    private val bar: Int = 1
    inner class Inner {
        fun foo() = bar
    }
}

val demo = Outer().Inner().foo() // == 1

Anonymous inner classes

使用object expression建立匿名內部類別的實例

window.addMouseListener(object: MouseAdapter() {
    override fun mouseClicked(e: MouseEvent) {
        // ...
    }
                                                                                                            
    override fun mouseEntered(e: MouseEvent) {
        // ...
    }
})