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Beyond this, there are a number of additional stipulations that I won't repeat here. You can view them in the Oracle documentation if you areinterested in that level of detail.
Listing 5 shows my implementation of the compareTo method. Although this implementation satisfies the general description givenabove, I haven't taken the time to test it fully to confirm that it meets all of the additional stipulations provided by Oracle.
| Listing 5 . The compareTo method. |
|---|
public int compareTo(Object o){
if(!(o instanceof MyClass))throw new ClassCastException();
if(((MyClass)o).data<data)
return 1;
if(((MyClass)o).data>data)
return -1;else return 0;
}//end compareTo() |
The Oracle documentation strongly emphasizes the need to make certain that a class' natural ordering is consistent with equals, and provides the rules for meeting that requirement.
Further, the documentation for the TreeSet class reads partially as follows:
"Note that the ordering maintained by a set (whether or not an explicit comparator is provided) must be consistent with equals if it is to correctly implement the Set interface. ..."
In order to satisfy the rules and to cause the natural ordering of the MyClass class to be consistent with equals , it was necessary to override the equals method inherited from the Object class. My overridden version of the equals method is shown in Listing 6 .
| Listing 6 . The overridden equals method. |
|---|
public boolean equals(Object o){
if(!(o instanceof MyClass))return false;
if(((MyClass)o).data == data)return true;
else return false;}//end overridden equals()
}//end MyClass |
As was the case in defining the compareTo method, there are also a large number of stipulations involved in properly overriding the equals method. I will simply refer you to the Oracle documentation if you areinterested in reading about those stipulations.
Given all of the above, this program compiles and executes correctly, producing the following output.
1234
Note that duplicate elements were eliminated, and the iterator traversed the set in ascending element order, sorted according to the natural ordering of theelements, as required for a SortedSet collection.
I encourage you to copy the code from Listing 1 and Listing 3 . Paste the code into your Java editor. Then compile 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.
I explained why the elements stored in a TreeSet collection must be references to objects instantiated from a class that implements the Comparable interface. ( In a future module, I will teach you about an alternative approach that makes use of the Comparator interface.)
I provided an example of implementing the Comparable interface for a new class definition, and I taught you about the natural ordering of the elements for a class.
I taught you the meaning of the consistent with equals requirement and showed you how to satisfy that requirement for a new class definition.
I showed you how to define a new class whose objects are eligible for inclusion in a TreeSet collection.
In the next module, I will discuss the use of the Comparator interface in order to achieve a sorting order that is different from the natural ordering of the elements in a sorted collection.
This section contains a variety of miscellaneous information.
Financial : Although the Connexions site makes it possible for you to download a PDF file for thismodule at no charge, and also makes it possible for you to purchase a pre-printed version of the PDF file, you should beaware that some of the HTML elements in this module may not translate well into PDF.
I also want you to know that, I receive no financial compensation from the Connexions website even if you purchase the PDF version of the module.
In the past, unknown individuals have copied my modules from cnx.org, converted them to Kindle books, and placed them for sale on Amazon.com showing me as the author. Ineither receive compensation for those sales nor do I know who does receive compensation. If you purchase such a book, please beaware that it is a copy of a module that is freely available on cnx.org and that it was made and published withoutmy prior knowledge.
Affiliation : I am a professor of Computer Information Technology at Austin Community College in Austin, TX.
-end-
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