Help a Beginner? cannot find symbol error?

Question

class Box {
     double width;
     double height;
     double depth;
Box(Box ob) {
          width = ob.width;
          height = ob.height;
          depth = ob.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 weight;
     BoxWeight(double w, double h, double d, double m) {
          width = w;
          height = h;
          depth = d;
          weight = m;
class DemoBoxWeight {
     public static void main(String[] args) {
          BoxWeight mybox1 = new BoxWeight(10, 20, 35, 34);
          double vol;
          vol = mybox1.volume();
          System.out.println("Weight of mybox1 is " + mybox1.weight);
okay, this is basically from a book, but when I compile it in TextPad, it reads
C:\Documents and Settings\Guest 1\Desktop\EPGY C015\Box\Box.java:27: cannot find symbol
symbol : constructor Box()
location: class Box
     BoxWeight(double w, double h, double d, double m) {
     ^
1 error
Tool completed with exit code 1
Why is that and how can I fix it? :P

Answer 1

Whenever you extend a class (here you're extending Box with BoxWeight), and create the subclass's constructor, the parent class's (or super class's) constructor gets called first whether it is explicitly called or not. So here in the BoxWeight constructor, the Box constructor is implicitly called:
BoxWeight(double w, double h, double d, double m) {
  // even though you don't specify it, the Box() constructor is called here. 
  // it's as if you have here the call:
  super();
  width = w;
  height = h;
  depth = d;
  weight = m;
}If you don't specify the super(), it get's implicitly called as the first call in the BoxWeight constructor. Notice that the implicit constructor has no parameters. Since Box does not have a constructor without parameters, this will throw an error. To get around this, you should explicitly call the super constructor with its proper parameters as the first method called in your BoxWeight constructor:
BoxWeight(double w, double h, double d, double m) {
  super(w, h, d);  // *** here ***
  width = w;
  height = h;
  depth = d;
  weight = m;
}