4.18 Java4100: the comparator interface, part 2

Revised: Thu Apr 07 10:44:22 CDT 2016

This page is included in the following Books:

• ITSE 2321 - Object-Oriented Programming (Java)
• Object-Oriented Programming (OOP) with Java

Table of contents

• Preface
  • Viewing tip
    • Listings
• Preview
• Discussion and sample code
  • Generics
  • Beginning with a quiz
    • The program output
    • From the previous module
    • Doing more with a Comparator
  • Two steps in the program
    • The first step
    • The second step
    • Duplicate names eliminated from the set
    • Does Joe equal JOE?
  • Let's see some code
    • The Populator code
    • Populating the collection with String objects
    • Populating the TreeSet collection
    • Beginning of the Comparator class
    • The interesting code
    • Convert to upper-case
    • Making the comparison
    • Just what I was looking for
    • The results
• Run the program
• Summary
• What's next?
• Miscellaneous

Preface

This module is one of a series of modules designed to teach you about Object-Oriented Programming (OOP) in general and the Java Collectionsframework in particular.

This module shows you how to use a Comparator object to achieve natural (ascending) order on a set of names added as String objects to a TreeSet collection while ignoring the case used to write the names.

In addition to studying these modules, I strongly recommend that you study the Collections Trail in Oracle's Java Tutorials . The modules in this collection are intended to supplement and not to replace those tutorials.

Viewing tip

I recommend that you open another copy of this module in a separate browser window and use the following links to easily find and view the listings while you are reading about them.

Listings

• Listing 1 . The program named Comparator03
• Listing 2 . Create, populate, and display a TreeSet collection.
• Listing 3 . The class named Populator.
• Listing 4 . A TreeSet with a Comparator.
• Listing 5 . Populating the TreeSet collection.
• Listing 6 . Beginning of the Comparator class.
• Listing 7 . The interesting code in the compare method.

Preview

In this module, I will show 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. (The natural ordering for String objects is ascending.)

Discussion and sample code

Generics

The code in this module is written with no thought given to Generics . As a result, if you copy and compile this code, you will probably get a warning about unchecked or unsafe operations .

While you will ultimately need to understand how to use Generics, that is a very complex topic. An understanding of Generics is beyond the scope of thiscourse. Therefore, for purposes of this course, you can simply ignore those warnings.

Beginning with a quiz

Let's begin with a quiz to test your prior knowledge of the Java Collections Framework.

What output is produced by the program shown in Listing 1 ?

• A. Compiler Error
• B. Runtime Error
• C. Joe Bill Tom JOE BILL TOM
• D. Tom TOM Joe JOE Bill BILL
• E. Joe Bill Tom
• F. None of the above.