嵌套类

在类的内部可以嵌套其他的类

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

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

内部类

为了能被外部类访问一个类可以被标记为内部类（“inner” 关键词）。内部类会带有一个来自外部类的对象的引用：

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

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

参阅this-expressions.html中“this”关键词用法来学习在内部类中如何消除“this”关键词的歧义。

Anonymous inner classes

Anonymous inner class instances are created using an object expression:

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

If the object is an instance of a functional Java interface (i.e. a Java interface with a single abstract method), you can create it using a lambda expression prefixed with the type of the interface:

val listener = ActionListener { println("clicked") }