Kotlin实例初始化时,构造函数、成员变量、init块等的初始化顺序必须弄清楚,否则容易出bug:
Example:
open class Parent {
private val a = println("Parent.a")
constructor(arg: Unit=println("Parent primary constructor default argument")) {
println("Parent primary constructor")
}
init {
println("Parent.init")
}
private val b = println("Parent.b")
}
class Child : Parent {
val a = println("Child.a")
init {
println("Child.init 1")
}
constructor(arg: Unit=println("Child primary constructor default argument")) : super() {
println("Child primary constructor")
}
val b = println("Child.b")
constructor(arg: Int, arg2:Unit= println("Child secondary constructor default argument")): this() {
println("Child secondary constructor")
}
init {
println("Child.init 2")
}
}
Output:
Child primary constructor default argument
Parent primary constructor default argument
Parent.a
Parent.init
Parent.b
Parent primary constructor
Child.a
Child.init 1
Child.b
Child.init 2
Child primary constructor
Conclusion:
- 按照父类->子类的顺序执行初始化(同Java)
- 构造函数在成员变量初始化之后执行(同Java)
- init与成员变量初始化按照代码位置顺序执行 (重点!)
因此,下面的代码会出现NPE
class Foo {
init {
initList()
}
val list : MutableList<String> = mutableListOf()
fun initList() {
list.clear()
}
}