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You will recall that in several previous modules, I have written a class from which I instantiated a Comparator object that was used to sort elements into reverse natural order . I chose that sorting order simply because I needed to illustrate how to define such a class, and in my specificcases, reverse natural order was relatively easy to implement. (With a little more effort, I could have implemented a variety of different sorting orders.)
In my design of those classes, I made no attempt to write a general class that could do the job independently of the type of the elements to be sorted.Rather, my Comparator objects tended to be very type specific.
What we see here is much more general and sophisticated. The Comparator object returned by the reverseOrder method can be used to impose a reverse natural order on any collection of objects that implement the Comparable interface. Thus, the class from which the objects are instantiated doesn't matter, as long as those classes implement the Comparable interface. (I also discussed the Comparable interface in some detail in an earlier module. You may want to refer backto that module to learn more about it.)
Here again, we see a manifestation of the benefits of polymorphism as implemented using the Java interface. (I frequently tell my students that if they don't understand interfaces, they can't possibly understand Java.)
The code in Listing 6 is not new to this module. An earlier module discussed the use of the sort method of the Collections class, along with a Comparator object to sort a list.
| Listing 6 . Sorting the list. |
|---|
Collections.sort((List)ref, aComparator); |
The thing that is new to this module is the source of the Comparator object provided to the sort method in Listing 6 .
In the previous modules, the Comparator object was obtained by instantiating an object from a class of my own design. Those classesimplemented the Comparator interface.
In this case, a reference to a Comparator object was returned by the call to the reverseOrder method of the Collections class, and that reference was passed as a parameter to the sort method.
The sort method doesn't care where the Comparator object comes from, as long as it properly implements the Comparator interface.
Regardless of the source of the Comparator object, the sort method will use that object to impose the sorting rules imposed by the compare method of the object. In this case, the sorting rules cause the list to be sorted into reverse natural order .
The code in Listing 7 gets and uses an iterator to display the contents of the list following the call to the sort method in Listing 6 .
| Listing 7 . Produce the output. |
|---|
iter = ref.iterator();
while(iter.hasNext()){System.out.print(iter.next() + " ");
}//end while loop |
The output produced by the code in Listing 7 is shown below:
Tom TOM Joe JOE Bill BILL
You will recognize this as reverse natural order for the elements in the list.
I encourage you to copy the code from Listing 1 . Paste the code into your Java editor. Thencompile and execute it.
Run the program and observe the results. Experiment with the code. Make changes, run the program again, and observe the results of your changes. Make certain that youcan explain why your changes behave as they do.
In this module, I taught you how to use a Comparator created by the reverseOrder method of the Collections class to sort a list into reverse natural order . The Comparator object is generic, and can be used to sort any list of objects that implement the Comparable interface.
I also taught you how to use the reverse method of the Collections class to reverse the order of the elements in a list.
In the next module, I am going to dig a little deeper into the implications of using the toArray method declared in the Collection interface.
This section contains a variety of miscellaneous information.
Financial : Although the Connexions site makes it possible for you to download a PDF file for thismodule at no charge, and also makes it possible for you to purchase a pre-printed version of the PDF file, you should beaware that some of the HTML elements in this module may not translate well into PDF.
I also want you to know that, I receive no financial compensation from the Connexions website even if you purchase the PDF version of the module.
In the past, unknown individuals have copied my modules from cnx.org, converted them to Kindle books, and placed them for sale on Amazon.com showing me as the author. Ineither receive compensation for those sales nor do I know who does receive compensation. If you purchase such a book, please beaware that it is a copy of a module that is freely available on cnx.org and that it was made and published withoutmy prior knowledge.
Affiliation : I am a professor of Computer Information Technology at Austin Community College in Austin, TX.
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