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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:

Tom TOM Joe JOE Bill BILL

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.

Summary

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.

Miscellaneous

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
Disclaimers:

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.

-end-

Questions & Answers

can someone help me with some logarithmic and exponential equations.
Jeffrey Reply
sure. what is your question?
ninjadapaul
20/(×-6^2)
Salomon
okay, so you have 6 raised to the power of 2. what is that part of your answer
ninjadapaul
I don't understand what the A with approx sign and the boxed x mean
ninjadapaul
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
Salomon
I'm not sure why it wrote it the other way
Salomon
I got X =-6
Salomon
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
ninjadapaul
oops. ignore that.
ninjadapaul
so you not have an equal sign anywhere in the original equation?
ninjadapaul
Commplementary angles
Idrissa Reply
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Sherica
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Sherica
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Tamia
what is a good calculator for all algebra; would a Casio fx 260 work with all algebra equations? please name the cheapest, thanks.
Kevin Reply
a perfect square v²+2v+_
Dearan Reply
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Abdirahman Reply
algebra 2 Inequalities:If equation 2 = 0 it is an open set?
Kim Reply
or infinite solutions?
Kim
The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
Al
y=10×
Embra Reply
if |A| not equal to 0 and order of A is n prove that adj (adj A = |A|
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rolling four fair dice and getting an even number an all four dice
ramon Reply
Kristine 2*2*2=8
Bridget Reply
Differences Between Laspeyres and Paasche Indices
Emedobi Reply
No. 7x -4y is simplified from 4x + (3y + 3x) -7y
Mary Reply
is it 3×y ?
Joan Reply
J, combine like terms 7x-4y
Bridget Reply
im not good at math so would this help me
Rachael Reply
yes
Asali
I'm not good at math so would you help me
Samantha
what is the problem that i will help you to self with?
Asali
how do you translate this in Algebraic Expressions
linda Reply
Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
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. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa's original weight?
Chris Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
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Yasmin
what is the function of carbon nanotubes?
Cesar
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
AMJAD
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
AMJAD
what is system testing
AMJAD
what is the application of nanotechnology?
Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
Azam
anybody can imagine what will be happen after 100 years from now in nano tech world
Prasenjit
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
Azam
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
Prasenjit
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
Damian
silver nanoparticles could handle the job?
Damian
not now but maybe in future only AgNP maybe any other nanomaterials
Azam
can nanotechnology change the direction of the face of the world
Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
Ali Reply
the Beer law works very well for dilute solutions but fails for very high concentrations. why?
bamidele Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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