一、修饰符
Kotlin中有四种可见修饰符，可为类、对象、接口、构造函数、方法、属性及属性的访问器添加可见修饰符
private：当前文件可见protected：与private类似，但它在子类中也是可见的。不适用于顶层声明，如顶层类、顶层属性、顶层方法等internal：当前模块可见public: 默认值，任何位置可见
二、修饰符覆盖
如果去覆盖一个protected或internal成员，并且没有显式的指定可见性，则它仍然具有原始成员的可见性
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open class Outer {
private val a = 1
protected open val b = 2
internal open val c = 3
val d = 4 // public by default
protected class Nested {
public val e: Int = 5
}
}
class Subclass : Outer() {
// a is not visible
// b, c and d are visible
// Nested and e are visible
override val b = 5 // 'b' is protected
override val c = 7 // 'c' is internal
}
class Unrelated(o: Outer) {
// o.a, o.b are not visible
// o.c and o.d are visible (same module)
// Outer.Nested is not visible, and Nested::e is not visible either
}
三、构造函数可见性
类指定构造函数可见性时需要明确指定constructor关键字
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class C private constructor(a: Int) { ... }