If you selected the following, you are correct:
D. Tom TOM Joe JOE Bill BILL
Similar to previous programs
The overall structure of this program is very similar to programs that I have discussed in previous modules. Therefore, I will concentrate on thoseaspects of this program that differentiate it from the programs in previous modules.
A new TreeSet object with a Comparator
The code in Listing 2 instantiates a new TreeSet object, by providing a reference to an anonymous object that implements the Comparator interface. That object is instantiated from the class named TheComparator . It is the Comparator object that will be of most interest to us in this module.
| Listing 2 . A new TreeSet object with a Comparator. |
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Collection ref;
ref = new TreeSet(new TheComparator());Populator.fillIt(ref);
iter = ref.iterator();while(iter.hasNext()){
System.out.print(iter.next() + " ");}//end while loop |
Populating the TreeSet collection
After the TreeSet object is instantiated, it is passed to a method named fillIt where the TreeSet collection is populated with the names of several people.
Display the contents of the TreeSet collection
As shown by the code in Listing 2 , after the TreeSet collection is populated, an Iterator is obtained for that collection and used to display the contents of the collection. The output produced by the programis shown below :
Tom TOM Joe JOE Bill BILL
Analyzing the contents of the TreeSet collection
We will need to compare this output with the names used to populate thecollection to appreciate the true significance of the use of the Comparator object.
At this point, it is worth pointing out that the six names contained in the collection are returned by the iterator in descending order , taking the significance of upper and lower case into account. In other words, namesbeginning with letters that are high in the alphabet occur before names beginning with letters that are lower in the alphabet. In addition, namescontaining lower case characters appear before the same names containing only upper case characters.
Method used to populate the collection
Listing 3 shows the method named fillIt that was used to populate the collection with references to six String objects. As you can see, the names weren't added in any particular order.
As you can also see by comparing Listing 3 with the output shown above , all six names that were added to the collection were displayed in the output, but ina different order from the order in which they were added. (Names with the same spelling but different case were not considered to be duplicates insofar asthe contract for the set was concerned.)
| Listing 3 . The fillIt method. |
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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() |
Implementing the Comparator interface
That brings us to the class from which the Comparator object was instantiated. The beginning portion of that class is shown in Listing 4 .
| Listing 4 . Beginning of the class named TheComparator. |
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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(); |