Java Reference
In-Depth Information
Randomizer.class
and
Randomizer$Test.class
. Running this nested
Random-
izer.Test
class is a little tricky. You
ought
to be able to do so like this:
% java com.davidflanagan.examples.classes.Randomizer.Test
However, current versions of the Java SDK don't correctly map from the class
name
Randomizer.Test
to the class file
Randomizer$Test.class
. So, to run the test
program, you must invoke the Java interpreter using a
$
character instead of a
.
character in the class name:
% java com.davidflanagan.examples.classes.Randomizer$Test
On a Unix system, however, you should be aware that the
$
character has special
significance and must be escaped. Therefore, on such a system, you have to type:
% java com.davidflanagan.examples.classes.Randomizer\$Test
or:
% java 'com.davidflanagan.examples.classes.Randomizer$Test'
You need to use this technique whenever you need to run a Java program that is
defined as an inner class.
Computing Statistics
Example 2-7 shows a class that computes some simple statistics for a set of num-
bers. As numbers are passed to the
addDatum()
method, the
Averager
class
updates its internal state so that its other methods can easily return the average
and standard deviation of the numbers that have been passed to it so far. Like
Randomizer
, the
Averager
class doesn't represent any kind of real-world object or
abstract concept. Nevertheless,
Averager
does maintains some state (this time as
private
fields), and it has methods that operate on that state, so it is implemented
as a class.
Like Example 2-6, Example 2-7 defines an inner
Test
class that contains a
main()
method that implements a test program for
Averager
.
Example 2•7: Averager.java
package com.davidflanagan.examples.classes;
/**
* A class to compute the running average of numbers passed to it
**/
public class Averager {
// Private fields to hold the current state.
private int n = 0;
private double sum = 0.0, sumOfSquares = 0.0;
/**
* This method adds a new datum into the average.
**/
public void addDatum(double x) {
n++;
sum += x;
sumOfSquares += x * x;
}
