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To put what we know about classes to use, we can use our Sphere class in an example.
You will be creating two source files. The first is the file CreateSpheres.java, which
will contain the definition of the CreateSpheres class that will have the method
main() defined as a static method. As usual, this is where execution of the program
starts. The second file will be the file Sphere.java that contains the definition of the
class Sphere that we have been assembling.
Both files will need to be in the same directory or folder--I suggest you name the directory
CreateSpheres. Then copy or move the last version of Sphere.java to this
directory.
Enter the following code for the file CreateSpheres.java:
class CreateSpheres {
public static void main(String[] args) {
System.out.println("Number of objects = " + Sphere.getCount());
Sphere ball = new Sphere(4.0, 0.0, 0.0, 0.0); // Create a sphere
System.out.println("Number of objects = " + ball.getCount());
Sphere globe = new Sphere(12.0, 1.0, 1.0, 1.0); // Create a sphere
System.out.println("Number of objects = " + Sphere.getCount());
// Output the volume of each sphere
System.out.println("ball volume = " + ball.volume());
System.out.println("globe volume = " + globe.volume());
}
}Compile the source files and then run CreateSpheres, and you should get the
output:
Number of objects = 0
Number of objects = 1
Number of objects = 2
ball volume = 267.94666666666666
globe volume = 7234.559999999999This is the first time we have run a program involving two source files. If you are using the
JDK compiler, then compile CreateSpheres.java with the current directory as
CreateSpheres using the command:
javac CreateSpheres.javaThe compiler will find and compile the Sphere.java source file automatically. If all
the source files for a program are in the current directory, then compiling the file containing a
definition of main() will compile all the source files for the program.
Note that by default, the .class files generated by the compiler will be stored in
the current directory, that is, the directory containing your sourcecode. If you want the .class
files stored in a different directory, then you can use the –d option with the Java compiler
to specify where they should go. For example, to store the class files in a directory called
C:\classes you would type:
javac –d C:/classes CreateSpheres.javaHow It Works
The Sphere class definition includes a constructor and the method volume()
to calculate the volume of a particular sphere. It also contains the static method, getCount(),
we saw earlier, which returns the current value of the class variable count. We need to define
this method as static since we want to able to call it regardless of how many objects have
been created, including the situation when there are none.
The method main() in the CreateSpheres class puts the class Sphere
through its paces. When the program is compiled, the compiler will look for a file Sphere.java
to provide the definition of the class Sphere. As long as this file is in the current directory
the compiler will be able to find it.
The first thing the program does is to call the static method getCount(). Because no objects
exist, you must use the class name to call it at this point. We then create the object ball,
which is a Sphere object, with a radius of 4.0 and its center at the origin point,
(0.0, 0.0, 0.0). The method getCount() is called again, this time using the object name
to demonstrate that you can call a static method through an object. Another Sphere object,
globe, is created with a radius of 12.0. The getCount() method is called again,
this time using the class name. Static methods are usually called using the class name
because in most situations, where you would use such a method, you cannot be sure that any objects exist.
After all, the reason for calling this particular method would be to find out how many objects exist.
A further reason to use the class name when calling a static method is that it makes it quite clear in
the source code that it is a static method that is being called. You can't call a non-static method
using the class name.
Our program finally outputs the volume of both objects by calling the volume() method for
each, from within the expressions, specifying the arguments to the println() method calls.
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Created: July 1, 2002
Revised: July 1, 2002
URL: http://webreference.com/programming/java/beginning/chap5/2/3.html
