java - java 中得到意外的输出

Question

我有一些java代码:

class Protected{
    int n = 1;

    public Protected(){
        System.out.println("Constructor of protected");
        System.out.println("========================");
        System.out.println("n = "+n);
        System.out.println();
    }
}
class Derived extends Protected{
    Derived(){
        System.out.println("Constructor of derived");
        System.out.println("======================");
        System.out.println("n = "+(n+1));
    }
}

public class Demo{
    public static void main(String args[]){
        Derived ob2 = new Derived();
    }
}

我得到的输出为:

constructor of protected
========================
n=1

constructor of Derived
========================
n=2

这就是我想要的:

constructor of Derived
========================
n=2

Answer 1

必须为子类的每个新实例调用父类(super class)构造函数。如果您没有在子类构造函数中提供对父类(super class)构造函数的显式调用，那么 Java 将插入对无参数父类(super class)构造函数的隐式调用。这解释了为什么您会看到父类(super class)构造函数的输出。

要删除该输出，您可以执行以下操作之一:

1) 从父类(super class)构造函数中删除输出语句。

2)在父类(super class)中创建另一个不输出任何内容的构造函数，并在子类构造函数中显式调用它。

Section 8.8.7 of the JLS状态:

If a constructor body does not begin with an explicit constructor invocation and the constructor being declared is not part of the primordial class Object, then the constructor body implicitly begins with a superclass constructor invocation "super();", an invocation of the constructor of its direct superclass that takes no arguments.