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Convert to upper-case

The method named toUpperCase is used to produce a version of each of the incoming strings that consists of upper-case characters only. In otherwords, lower-case characters in each of the two strings are replaced by the corresponding upper-case characters. This conversion occurs before the stringsare compared.

For example, the string Joe is temporarily converted to JOE inside the compare method, before the actual comparison is made. This results in the two strings containing Joe and JOE being considered to be duplicates. If one of them is already in the collection when an attempt is madeto add the other, the second will be rejected as a duplicate.

Making the comparison

Then the compareTo method of the String class is used to make the actual comparison. (Note that this is the same method that is used to make the comparison in the absence of a Comparator object. However, in the case of the Comparator object, the case of the strings is modified before they are passed to the compareTo method.)

This code calls the compareTo method on the upper-case version of the string represented by o1 , passing the upper-case version of the string represented by o2 as a parameter. Here is part of what Oracle has to say about the behavior of the compareTo method.

"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."

Just what I was looking for

That is exactly the behavior that I was looking for, so all that I needed to do after calling the compareTo method on the upper-case versions of the two strings was to return the value that was returned by the compareTo method.

(Note, while writing this module and explaining the behavior of this program, I discovered that I could have used a method of the String class named compareToIgnoreCase to accomplish the same thing with a little less work.)

The results

When the TreeSet object used the Comparator object to compare and arrange the elements in the collection, the three duplicate names were eliminated and the iterator delivered references to the remaining threenames in the following order:

Bill Joe Tom

Run the program

I encourage you to copy the code from Listing 1 and paste it 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 showed you how to use a Comparator object to achieve a natural ordering of a set of names ( String objects) added to a TreeSet collection while ignoring the case used to write the names. (Natural ordering for String objects is ascending.)

What's next?

In the next module, I will show you how to use a Comparator to cause a TreeSet collection containing references to String objects to be sorted in descending order while preservingdifferences in case.


This section contains a variety of miscellaneous information.

Housekeeping material
  • Module name: Java4100: The Comparator Interface, Part 2
  • File: Java4100.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

Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
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Crow Reply
what about nanotechnology for water purification
RAW Reply
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I think
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Brian Reply
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scanning tunneling microscope
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what is differents between GO and RGO?
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Bob Reply
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nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
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biomolecules are e building blocks of every organics and inorganic materials.
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sciencedirect big data base
Introduction about quantum dots in nanotechnology
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nano basically means 10^(-9). nanometer is a unit to measure length.
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