Java 中的单个 Inheritance 问题

Question

I tried to execute the below java code which demonstrates single inheritance.

class Box {
    double width;
    double height;
    double depth;
    Box(double w, double h, double d) {
        width = w;
        height = h;
        depth = d;
    }
    double volume() {
        return width*height*depth;
    }
}

class BoxWeight extends Box {
    double mass;
    BoxWeight(double w, double h, double d, double m) {
        width = w;
        height = h;
        depth = d;
        mass = m;
    }
}

class BoxDemo {
    public static void main(String[] args) {
        BoxWeight box = new BoxWeight(10,20,15,32.3);
        double vol;
        vol = box.volume();
        System.out.println("Volume = " + vol);
    }
}

But this raised the following error:

C:\User\Java>javac BoxWeight.java
BoxWeight.java:16: error: constructor Box in class Box cannot be applied to given types;
    BoxWeight(double w, double h, double d, double m) {
                                                      ^
  required: double,double,double
  found:    no arguments
  reason: actual and formal argument lists differ in length
1 error

Again when I modified the code in sub class constructor as below

BoxWeight(double w, double h, double d, double m) {
        super(w,h,d);
        mass = m;
    }

it gave the desired output.

Why the first way didn't work?

Answer 1

You have to pass the width, height and depth to the super class constructor:

BoxWeight(double w, double h, double d, double m) {
    super(w,h,d);
    mass = m;
}

When the sub-class constructor doesn't contain the super(...) call to the super-class constructor, the compiler implicitly adds a super() call to the super-class parameter-less constructor, which doesn't exist in your Box class. That's the reason your code didn't pass compilation.

As Amongalen commented, if you don't define any constructor in your class, the compiler adds a default parameter-less constructor. However, once you define any constructor, the default constructor is not added. This means that your code could have passed compilation if you just removed the Box(double w, double h, double d) constructor (or kept that constructor and added an explicit Box() constructor).

Answer 2

Always remember a simple rule when working with inheritance

Whenever any constructor of a child class is called the default constructor of the parent class is called automatically

Now in our case when we invoked BoxWeight(double w, double h, double d, double m) constructor the default constructor of parent class ie Box() needs to be called automatically. But in our case Box() doesn't exist so we need to manually call super(w,h,d); . Here super is used to refer to the parent class.