Java构造函数继承？

Question

我一直以为构造函数不是继承的，但是看看这段代码:

class Parent {
    Parent() {
        System.out.println("S1");
    }
}

class Child extends Parent {
    Child() {
        System.out.println("S2");
    }
}

public class Test5 {
    public static void main(String[] args) {
        Child child = new Child();
    }
}

//RESULT:
//S1
//S2

说明Child继承了构造函数。为什么结果上有S1？是否有可能创建 2 个不带参数的构造函数并且在结果上只有子构造函数而没有基本构造函数(仅 S2)？

Answer 1

无论您在这里看到什么，都称为构造函数链。现在什么是构造函数链接:

Constructor chaining occurs through the use of inheritance. A subclass constructor method's first task is to call its superclass' constructor method. This ensures that the creation of the subclass object starts with the initialization of the classes above it in the inheritance chain.

There could be any number of classes in an inheritance chain. Every constructor method will call up the chain until the class at the top has been reached and initialized. Then each subsequent class below is initialized as the chain winds back down to the original subclass. This process is called constructor chaining.(Source)

这就是您的程序中发生的事情。当你编译你的程序时，你的 Child 被 javac 编译成这样:

class Child extends Parent 
{ 
  Child()
  {
    super();//automatically inserted here in .class file of Child
    System.out.println("S2");
  }
}

并且您的父类转换为以下内容:

Parent() 
{
    super();//Constructor of Object class
    System.out.println("S1");
}

这就是为什么您的输出显示为:

S1 //output from constructor of super class Parent
S2 //output from constructor of child Class Child