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The code in Listing 4 isn't particularly interesting. That code simply throws an exception if either of the references passed to the compare method refer to an object of some type other than String .

The interesting code

The most interesting code is shown in Listing 5 . The first statement in Listing 5 uses the compareTo method of the String class to compare the two objects in an ascending natural ordering sense. The behavior of this method is more formally described as follows:

"Returns: the value 0 if the argument is a string lexicographically equal to this string; a value less than 0 if the argument is a stringlexicographically greater than this string; and a value greater than 0 if the argument is a string lexicographically less than this string."
Listing 5 . The most interesting code.
int result = ((String)o1).compareTo(((String)o2)); return result*(-1);

Converting to reverse natural order

The most interesting line of code in this entire program is the return statement shown in Listing 5 . This line of code changes the sign on the value returned by the compareTo method before returning it as the return value for the compare method.

The effect of changing the sign is to return a value that causes the TreeSet collection to arrange the elements in reverse natural order instead of the normal ascending natural order.

As a result, the use of an iterator to access and display the contents of the collection produces the following output:


For comparison, if the names were arranged in ascending natural order, the output would be as shown below:


Run the program

I encourage you to copy the code from Listing 1 and paste it into your Javatext editor. Then compile 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 you can explain why your changes behave as they do.


In this module, I taught you how to use a Comparator to cause a TreeSet collection to be sorted in reverse natural order. In other words, the sorting order is the same as the natural order except that the order is descending instead of ascending.

What's next?

In the next module, I will show you how to use a Comparator object to sort the contents of an array.


This section contains a variety of miscellaneous information.

Housekeeping material
  • Module name:Java4110: The Comparator Interface, Part 3
  • File: Java4110.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.


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