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Reverse natural order

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.

A type-independent Comparator

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.)

The wonderful world of the Java interface

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.)

Sorting the list

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);

Source of Comparator object is new

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.

Don't know, don't care

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 output

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:


You will recognize this as reverse natural order for the elements in the list.

Run the program

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.

What's next?

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.

Housekeeping material
  • Module name: Java4140: The Comparator Interface, Part 6
  • File: Java4140.htm
  • Published: 05/07/13

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.


Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
Nasa has use it in the 60's, copper as water purification in the moon travel.
nanocopper obvius
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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