4.18 Java4100: the comparator interface, part 2  (Page 5/5)

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.

Summary

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.

Miscellaneous

This section contains a variety of miscellaneous information.

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