4.16 Java4090: the comparator interface, part 1

Revised: Wed Apr 06 23:01:46 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
  • The Comparable interface
  • The Comparator interface
  • Beginning with a quiz
  • Eligibility for inclusion in a TreeSet
    • Using a Comparator object
    • Passing Comparator to TreeSet constructor
    • Passing the TreeSet to a Populator method
    • Similar to previous program
    • MyClass does not implement Comparable
    • Comparator eliminates requirement for Comparable
  • The class named TheComparator
    • Implementing the Comparator interface
    • Implementing the Serializable interface
    • Methods of the Comparator interface
    • The compare method
  • Specialization is required
    • Must gain access to instance variables
    • Specialized for type MyClass
    • General behavior of compare method
    • Implementation of required behavior
    • Other stipulations
  • The equals method
    • Overridden equals method
  • 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 discusses and illustrates the use of the Comparator interface. The sorting order established by a Comparator may be different or may be the same as the natural order. A Comparator can be used to establish a sorting order for objects that don't have a natural ordering. The use of a Comparator is an alternative to the implementation of the Comparable interface.

This module is also the first of several modules on the Comparator interface. The purpose of the modules in this group is to teach you about theinteractions between the Comparator interface and the Collections Framework , particularly with respect to the Set , SortedSet , and SortedMap interfaces of the Collections Framework. This module discusses Set and SortedSet . A discussion of SortedMap will be deferred to a future module.

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 Comparator02.
• Listing 2 . Passing Comparator to TreeSet constructor.
• Listing 3 . The fillIt method.
• Listing 4 . The class named MyClass.
• Listing 5 . Beginning of the class named TheComparator.
• Listing 6 . Beginning of the compare method.
• Listing 7 . Implementation of required behavior.
• Listing 8 . The overridden equals method.
• Listing 9 . Display the contents of the TreeSet object.