As you can see, both mybox1 and mybox2 were initialized by the Box( ) constructor when
they were created. Since the constructor gives all boxes the same dimensions, 10 by 10 by 10,
both mybox1 and mybox2 will have the same volume. The println( ) statement inside Box( )
is for the sake of illustration only. Most constructors will not display anything. They will
simply initialize an object.
Before moving on, let's reexamine the new operator. As you know, when you allocate an
object, you use the following general form:
class-var = new classname( );
Now you can understand why the parentheses are needed after the class name. What is actually
happening is that the constructor for the class is being called. Thus, in the line
Box mybox1 = new Box();
new Box( ) is calling the Box( ) constructor. When you do not explicitly define a constructor
for a class, then Java creates a default constructor for the class. This is why the preceding line
of code worked in earlier versions of Box that did not define a constructor. The default
constructor automatically initializes all instance variables to zero. The default constructor is
often sufficient for simple classes, but it usually won't do for more sophisticated ones. Once
you define your own constructor, the default constructor is no longer used.
Parameterized Constructors
While the Box( ) constructor in the preceding example does initialize a Box object, it is not
very useful--all boxes have the same dimensions. What is needed is a way to construct Box
objects of various dimensions. The easy solution is to add parameters to the constructor. As
you can probably guess, this makes them much more useful. For example, the following version
of Box defines a parameterized constructor that sets the dimensions of a box as specified by
those parameters. Pay special attention to how Box objects are created.
/* Here, Box uses a parameterized constructor to
initialize the dimensions of a 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;
}
}
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