4.12 Java4070: the comparable interface, part 1  (Page 6/6)

What happened to the Comparable interface?

By now, you are probably wondering what all of this has to do with the Comparable interface, because I haven't mentioned that interface since the introductory comments at the beginning of the module.

Comparable interface is not required for a List

Actually, the purpose of this module is to illustrate the lack of any requirement to make use of the Comparable interface with List objects. In particular, the purpose is to illustrate that this is one of thefeatures that differentiates between a List object and a Set or SortedSet object.

A List can be used as a container for other objects regardless of whether or not those objects implement the Comparable interface. However, in the next module, we will see that objects must implement the Comparable interface in order to be eligible for inclusion in collections that implementthe SortedSet interface.

This and the next several modules are intended to provide you with an understanding of the interaction between the Comparable interface, the Comparator interface, and the Collections Framework.

No requirement to compare

Because a List makes no attempt to eliminate duplicate elements, or to sort the elements on the basis of their values, there is no requirement to compare objects when placing them in a List . Therefore, objects whose references are stored in a List are not required to implement the Comparable interface (but they may implement the Comparable interface without causing any harm) .

Comparison is required For a sortedset

Because a SortedSet does eliminate duplicates and does sort the elements on the basis of their values, there is a requirement to compare each new element with the existing elements in a SortedSet whenever a new element is added to the collection. Therefore, objects whose references arestored in a SortedSet are required to implement the Comparable interface.

Run the program

I encourage you to copy the code from Listing 1 , Listing 3 , and Listing 6 . Paste the code into your Java editor. Thencompile and execute it.

Run the program and observe the results. Experiment with the code. Make changes, run the program again, and observe the results of your changes. Make certain that youcan explain why your changes behave as they do.

Summary

In this module, I began discussing the interaction between the Comparable interface and the Collections Framework.

I provided a concrete example of the specialization that occurs when moving down the interface hierarchy from Collection to List . I showed an example of using two different overloaded versions of the add method to add new elements to an ArrayList object. One version is declared in the Collection interface and both versions are declared in the List interface.

I illustrated the use of a cast to change the type of a reference from Collection to List , in order to call a version of the add method that is declared only in the List interface. This version makes it possible for the user to control the location of each individual element addedto a List .

I illustrated that a cast is not required on a reference being treated as type Collection in order to call the version of the add method that is declared in the Collection interface. This version of the add method supports the addition of new elements only at the end of the List .

Finally, I explained that it is not necessary for objects to implement the Comparable interface to make them eligible for inclusion in a List .

Although I didn't demonstrate it, I told you that it is necessary for objects to implement the Comparable interface to make them eligible for inclusion in a SortedSet .

What's next?

The next module will begin exploring the interaction between the Comparable interface and the SortedSet interface of the Collections Framework.

Miscellaneous

This section contains a variety of miscellaneous information.

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