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This module shows you how to use the sort method of the Collections class along with a Comparator object to sort the contents of a List into reverse natural order.

Revised: Fri Apr 08 10:52:12 CDT 2016

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Table of contents

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 the sort method of the Collections class along with a Comparator object to sort the contents of a List into reverse natural order.

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

Preview

In this module, I will teach you how to use the sort method of the Collections class along with a Comparator object to sort the contents of a List into reverse natural order .

The methodology that I will teach you is completely general, and can be used to sort a list in a wide variety of ways, depending on how you define the compare method of a Comparator object.

Furthermore, the same sort method and the same Comparator object can be used to sort any implementation of a list, so long as the listproperly implements the List interface.

Generics

The code in this series of modules is written with no thought given to Generics . As a result, if you copy and compile the code, you will probably get warnings 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.

Discussion and sample code

Beginning with a quiz

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

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

  • A. Compiler Error
  • B. Runtime Error
  • C. BILL Bill JOE Joe TOM Tom
  • D. Tom TOM Joe JOE Bill BILL
  • E. Joe Bill Tom
  • F. None of the above.
Listing 1 . The program named Comparator06 .
//File Comparator06.java //Copyright 2001, R.G.Baldwinimport java.util.*; import java.io.Serializable;public class Comparator06{ public static void main(String args[]){ new Worker().doIt();}//end main() }//end class Comparator06class Worker{ public void doIt(){Iterator iter; Collection ref;ref = new LinkedList(); Populator.fillIt(ref);Collections.sort((List)ref, new TheComparator()); iter = ref.iterator();while(iter.hasNext()){ System.out.print(iter.next() + " ");}//end while loop System.out.println();}//end doIt() }// end class Workerclass Populator{ public static void fillIt(Collection ref){ref.add("Joe"); ref.add("Bill");ref.add("Tom"); ref.add("JOE");ref.add("BILL"); ref.add("TOM");}//end fillIt() }//end class Populatorclass TheComparator implements Comparator,Serializable{ public int compare(Object o1,Object o2){if(!(o1 instanceof String)) throw new ClassCastException();if(!(o2 instanceof String)) throw new ClassCastException();//Do a comparison int result = ((String)o1).compareTo(((String)o2));return result*(-1); }//end compare()}//end class TheComparator

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Source:  OpenStax, Object-oriented programming (oop) with java. OpenStax CNX. Jun 29, 2016 Download for free at https://legacy.cnx.org/content/col11441/1.201
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