java - 如果我将一个类的默认构造函数声明为私有(private)，为什么它不能被子类化

Question

In a class i can have as many constructors as I want with different argument types. I made all the constructors as private,it didn't give any error because my implicit default constructor was public But when i declared my implicit default constructor as private then its showing an error while extending the class. WHY?

这很好用

public class Demo4  {
    private String name;
    private int age;
    private double sal;

    private Demo4(String name, int age) {
        this.name=name;
        this.age=age;   
    }

    Demo4(String name) {
        this.name=name;
    }

    Demo4() {
        this("unknown", 20);
        this.sal=2000;
    }

    void show(){
        System.out.println("name"+name);
        System.out.println("age: "+age);
    }
}

不能继承

public class Demo4  {
    private String name;
    private int age;
    private double sal;

    private Demo4(String name, int age) {
        this.name=name;
        this.age=age;
    }

    Demo4(String name) {
        this.name=name;
    }

    private Demo4() {
        this("unknown", 20);
        this.sal=2000;
    }

    void show() {
        System.out.println("name"+name);
        System.out.println("age: "+age);
    }
}

Answer 1

why a class can not be subclassed if i declare its default constructor as private

子类中的构造函数必须调用 super 构造函数(可以是隐式的或显式的)，以便完全构造对象。 super 构造函数调用链一直向上直到 Object 类，它是 Java 中所有类的父类(super class)。

如果任何 super 构造函数对子类不可见，则无法完全构造 then 对象。

解决这个问题的一种方法是使父类(super class)中的构造函数 protected 。这样 super 构造函数仅对子类可见。