You should call the file that contains this program BoxDemo.java, because the main( ) method
is in the class called BoxDemo, not the class called Box. When you compile this program, you
will find that two .class files have been created, one for Box and one for BoxDemo. The Java
compiler automatically puts each class into its own .class file. It is not necessary for both the
Box and the BoxDemo class to actually be in the same source file. You could put each class
in its own file, called Box.java and BoxDemo.java, respectively.
To run this program, you must execute BoxDemo.class. When you do, you will see the
following output:
Volume is 3000.0
As stated earlier, each object has its own copies of the instance variables. This means that
if you have two Box objects, each has its own copy of depth, width, and height. It is important
to understand that changes to the instance variables of one object have no effect on the instance
variables of another. For example, the following program declares two Box objects:
// This program declares two Box objects.
class Box {
double width;
double height;
double depth;
}
class BoxDemo2 {
public static void main(String args[]) {
Box mybox1 = new Box();
Box mybox2 = new Box();
double vol;
// assign values to mybox1's instance variables
mybox1.width = 10;
mybox1.height = 20;
mybox1.depth = 15;
/* assign different values to mybox2's
instance variables */
mybox2.width = 3;
mybox2.height = 6;
mybox2.depth = 9;
// compute volume of first box
vol = mybox1.width * mybox1.height * mybox1.depth;
System.out.println("Volume is " + vol);
// compute volume of second box
vol = mybox2.width * mybox2.height * mybox2.depth;
System.out.println("Volume is " + vol);
}
}
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