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Listing 1 . The program named Comparator03.
//File Comparator03.java //Copyright 2001 R.G.Baldwinimport java.util.*; import java.io.Serializable;public class Comparator03{ public static void main(String args[]){ new Worker().doIt();}//end main() }//end class Comparator03class Worker{ public void doIt(){Iterator iter; Collection ref;System.out.println("Natural ordering"); ref = new TreeSet();Populator.fillIt(ref); iter = ref.iterator();while(iter.hasNext()){ System.out.print(iter.next() + " ");}//end while loop System.out.println();System.out.println("Comparator in use"); ref = new TreeSet(new TheComparator());Populator.fillIt(ref); 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 an upper-case comparisonint result = ((String)o1).toUpperCase().compareTo(((String)o2). toUpperCase());return result; }//end compare()public boolean equals(Object o){ if(!(o instanceof TheComparator))return false; else return true;}//end overridden equals() }//end class TheComparator

If your answer was None of the above , you are correct.

The program output

The output produced by the program shown in Listing 1 is four lines long as shown below. (Note that the bullets shown below do not appear in the actual program output.)

  • Natural ordering
  • BILL Bill JOE Joe TOM Tom
  • Comparator in use
  • Bill Joe Tom

From the previous module

In the previous module, I introduced you to the essentials of defining and using a Comparator for controlling the sort order of the elements contained in a TreeSet collection.

In that module, I explained the difference between natural ordering and the sort ordering produced through the use of a Comparator object.

However, what I showed you generally replicated the natural ordering , and therefore, wasn't too exciting.

Doing more with a Comparator

In this and several subsequent modules, I am going to show you some of the things that you can do with a Comparator object. By using a Comparator object, you can achieve comparisons and sort orders that are different from the natural ordering for a given element type.

Two steps in the program

The program shown in Listing 1 goes through two major steps.

The first step

First it populates a TreeSet collection with the names of six people without using a Comparator . Then it displays the contents of that collection using an iterator. That produces the following output (without the bullets) :

  • Natural ordering
  • BILL Bill JOE Joe TOM Tom

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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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