A Closer Look at Methods and Classes - Overloading Constructors

(Page 2 of 12 )

In addition to overloading normal methods, you can also overload constructor methods. In fact, for most real-world classes that you create, overloaded constructors will be the norm, not the exception. To understand why, let’s return to the Box class developed in the preceding chapter. Following is the latest version of Box:

class Box{
  double width;
  double height;
  double depth;
  // This is the constructor for Box.
  Box(double w, double h, double d) {
    width = w;
    height = h;
    depth = d;
  }
  // compute and return volume
  double volume() {
    return width * height * depth;
  }
}

As you can see, the Box( ) constructor requires three parameters. This means that all declarations of Box objects must pass three arguments to the Box( ) constructor. For example, the following statement is currently invalid:

Box ob = new Box();

Since Box( ) requires three arguments, it’s an error to call it without them. This raises some important questions. What if you simply wanted a box and did not care (or know) what its initial dimensions were? Or, what if you want to be able to initialize a cube by specifying only one value that would be used for all three dimensions? As the Box class is currently written, these other options are not available to you.

Fortunately, the solution to these problems is quite easy: simply overload the Box constructor so that it handles the situations just described. Here is a program that contains an improved version of Box that does just that:

/* Here, Box defines three constructors to initialize
   the dimensions of a box various ways.
*/
class Box {
  double width;
  double height;
  double depth;
  // constructor used when all dimensions specified
  Box(double w, double h, double d) {
    width = w;
    height = h;
    depth = d;
  }
  // constructor used when no dimensions specified
  Box() {
    width = -1; // use -1 to indicate
    height = -1; // an uninitialized
    depth = -1; // box
  }
  // constructor used when cube is created
  Box(double len) {
    width = height = depth = len;
  }
  // compute and return volume
  double volume() {
    return width * height * depth;
  }
}
class OverloadCons {
  public static void main(String args[]) {
    // create boxes using the various constructors
    Box mybox1 = new Box(10, 20, 15);
    Box mybox2 = new Box();
    Box mycube = new Box(7);
    double vol;
    // get volume of first box
    vol = mybox1.volume();
    System.out.println("Volume of mybox1 is " + vol);
    // get volume of second box
    vol = mybox2.volume();
    System.out.println("Volume of mybox2 is " + vol);
    // get volume of cube
    vol = mycube.volume();
    System.out.println("Volume of mycube is " + vol);
  }
}

The output produced by this program is shown here:

Volume of mybox1 is 3000.0
Volume of mybox2 is -1.0
Volume of mycube is 343.0

As you can see, the proper overloaded constructor is called based upon the parameters specified when new is executed.

Next: Using Objects as Parameters >>
 

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This article is excerpted from chapter 7 of Java: the Complete Reference, J2SE 5 Edition, written by Herbert Schildt (McGraw-Hill/Osborne, 2004; ISBN: 0072230738). Check it out at your favorite bookstore. Buy this book now.