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The output produced by this program is shown below:
Tom TOM Joe JOE Bill BILL
If that was your answer, you probably already understand most of the material covered in this module. In that case, you might consider skipping this moduleand moving on to the next module. If that wasn't your answer, you should probably continue with your study of this module.
The overall structure of the program in Listing 1 is similar to programs that I have discussed in previous modules. Therefore, I will concentrate onthose aspects of this program that differentiate it from the programs in previous modules.
The code in Listing 2 instantiates a new LinkedList object and passes that object's reference to a method named fillIt where it is populated with the names of several people.
| Listing 2 . A new LinkedList object. |
|---|
ref = new LinkedList();
Populator.fillIt(ref); |
The LinkedList class is one of the concrete implementation classes of the Collections Framework. This class implements the Collection interface and the List interface. Here is part of what Oracle has to say about the LinkedList class:
"Linked list implementation of the List interface. Implements all optional list operations, and permits all elements (including null). Inaddition to implementing the List interface, the LinkedList class provides uniformly named methods to get, remove and insert an element at thebeginning and end of the list. These operations allow linked lists to be used as a stack, queue, or double-ended queue (deque)."
The code in Listing 3 shows the fillIt method that is used to populate the list with references to six different String objects.
The add method is used to add each new element to the end of the list. As you can see, the elements are added to the list in no particular order withrespect to their values.
| Listing 3 . The fillIt method. |
|---|
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 |
Although I didn't bother to do so in this program, if an iterator were to be used to access and display the elements in the list following the invocation ofthe fillIt method, the result would be as shown below:
Joe Bill Tom JOE BILL TOM
As you can see, this is the same as the order in which the elements are added to the list. The first element is added to the list at index value 0 and thesixth element is added to the list at index value 5.
The code shown in Listing 4 is new to this module. This code uses the sort method of the Collections class, along with a Comparator object to sort the contents of the list.
| Listing 4 . Sort the list. |
|---|
Collections.sort((List)ref, new TheComparator()); |
The sort method expects to receive an incoming parameter of type List . Therefore, it was necessary to cast the reference from type Collection to type List .
Unlike the programs in previous modules that simply extracted the contents of the collection into an array and sorted the array, this codeactually rearranges the contents of the list according to the sorting rules.
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