Init method inheritance

问题:

If I have abstract class A with an init method:

abstract class A(){
  init {
    println("Hello")
  }     
}

And then class B that extends A

class B(): A()

If I instantiate B like this

fun main(args: Array<String>){
  B()
}

Does the init method in A still get run and Hello gets printed?

And if not, what do I need to do to have the init method of A get run?

回答1:

Yes, an init block of a base class gets run when the derived class instance is initialized.

In Kotlin, similarly to Java, an instance of a class is constructed in the following way:

1. An object is allocated.

2. The constructor of the class is called. (a)

   1. If the class has a superclass, the superclass constructor is called before the class construction logic is executed;
      ^{(i.e., the point (a) is executed recursively for the superclass, then the execution continues from here)}

   2. If the class has property initializers or init blocks, they are executed in the same order as they appear in the class body;

   3. If the constructor has a body (i.e. it is a secondary constructor) then the body is executed.

In this description, you can see that, when B is constructed, the constructor of A is called before B initialization logic is executed, and, in particular, all init blocks of A are executed.

^{(runnable demo of this logic)}

---

A small remark on terminology: init block is not actually a separate method. Instead, all init blocks together with member property initializers are compiled into the code of the constructor, so they should rather be considered a part of the constructor.