4.14 Java4080: the comparable interface, part 2

Revised: Wed Apr 06 22:21:03 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
  • Begin with a quiz
    • What caused the runtime error?
    • Why did this code produce a runtime error?
    • What does this mean?
    • The compareTo method
    • A possible exception
    • The SortedSet interface
    • Natural ordering of the elements
    • Conclusion regarding traversal
    • The bottom line
  • The solution
    • The corrected code
    • The compareTo method
    • Consistent with equals
    • Meeting the consistent with equals requirement
    • The program output
• 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 explains why the elements stored in a TreeSet collection must be references to objects instantiated from a class that implements the Comparable interface. The module also briefly discusses an alternative approach using the Comparator interface.

The module shows you how to implement the Comparable interface for a new class definition, explains the "natural ordering of the elements" for a class, and discusses the "consistent with equals" requirement. Finally,the module shows you how to define a new class whose objects are eligible for inclusion in a TreeSet collection.

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 Comparable04.
• Listing 2 . The code with the problem.
• Listing 3 . The program named Comparable05.
• Listing 4 . Beginning of the class named MyClass.
• Listing 5 . The compareTo method.
• Listing 6 . The overridden equals method.

Preview

In this module, I will teach you why the elements stored in a TreeSet collection must be references to objects instantiated from a class thatimplements the Comparable interface. ( In a subsequent module, I will teach you about an alternative approach that makes use of the Comparator interface.)

I will provide an example of implementing the Comparable interface for a new class definition, and will teach you about the natural ordering of the elements for a class.