java - 这个子类是如何隐式调用父类的构造函数的呢？

Question

我有这两个类:

public class Document {
    private static int quantity = 0;
    public Document() {
        quantity = quantity + 1;
    }
    public static int getQuantity() {
        return quantity;
    }
}

public class Book extends Document {
    private int numChapters;
    public Book(int numChapters) {
        this.numChapters = numChapters;
    }
    public static void main(String[] args) {
        Book b1 = new Book(4);
        Book b2 = new Book(6);
        Document d1 = new Document();
        System.out.println(Document.getQuantity());
    }
}

这里的程序返回 3，但我希望它返回 1。有人能告诉我代码中发生了什么吗？似乎在创建 b1 和 b2 时隐式调用了父类的构造函数。据我了解，子类不继承其父类的构造函数。

Answer 1

参见 docs :

Note: If a constructor does not explicitly invoke a superclass constructor, the Java compiler automatically inserts a call to the no-argument constructor of the superclass. If the super class does not have a no-argument constructor, you will get a compile-time error. Object does have such a constructor, so if Object is the only superclass, there is no problem.

你有一个自动调用的无参数构造函数。

这种行为是有道理的，构造函数是用来创建对象的。所以它应该包含其类的正确初始化代码。如果您的类(class)扩展了另一个类(class)怎么办？它不应该包含构造该对象所需的所有代码吗？