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Revised: Thu Apr 07 11:52:29 CDT 2016
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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.
The module shows you how to use a Comparator to cause a TreeSet collection to be sorted in descending order while preserving the impact of differences in case.
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.
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.
In this module, I will teach you how to use a Comparator to cause a TreeSet collection to be sorted in descending order while preserving the impact of differences in case. We might refer to this as reverse natural order. In other words, the sorting order is the same as the natural order except that the order is descending instead of ascending.
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.
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 ?
| Listing 1 . The program named Comparator04. |
|---|
//File Comparator04.java
//Copyright 2001, R.G.Baldwinimport java.util.*;
import java.io.Serializable;public class Comparator04{
public static void main(String args[]){
new Worker().doIt();}//end main()
}//end class Comparator04class Worker{
public void doIt(){Iterator iter;
Collection ref;ref = new TreeSet(new TheComparator());
Populator.fillIt(ref);iter = ref.iterator();
while(iter.hasNext()){System.out.print(iter.next() + " ");
}//end while loopSystem.out.println();
}//end doIt()}// end class Worker
class 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 Populator
class 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();
int result = ((String)o1).compareTo(((String)o2));return result*(-1);
}//end compare()public boolean equals(Object o){
if(!(o instanceof TheComparator))return false;
else return true;}//end overridden equals()
}//end class TheComparator |
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