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The TreeSet class implements the SortedSet interface, which extends the Set interface, which in turn extends the Collection interface. Thus, a TreeSet object is a Collection . Therefore, a reference to a TreeSet object can be stored in a reference variable of type Collection , and can be treated as the generic type Collection .
Among other things, in CS2 courses, we worry about the time and memory cost of a collection. According to Sun, the TreeSet class guarantees that the sorted set will be in ascending element order, and provides guaranteed log(n)time cost for the basic operations ( add , remove and contains ).
Again, according to Sun, the elements will be sorted according to the natural order of the elements (see the Comparable interface) or by a comparator (see the Comparator interface) provided at the time the set is created. This depends on which overloaded constructor is used. I will have moreto say about these alternatives in a future module.
I'm not going to try to explain the details of log(n) time cost here. Suffice it to say that the add , remove , and contains methods execute very fast. (I will have more to say about this is a future module.)
An object of the TreeSet class also is a Set . One of the characteristics of a Java Set (an object that implements the Set interface) is that it can contain no duplicate elements. Therefore, a TreeSet object can contain no duplicate elements. If the add method of a TreeSet object is called in an attempt to add a duplicate element, the element will not be added.
The TreeSet class also implements the SortedSet interface. This guarantees that the contents of a TreeSet object will be in ascending element order, regardless of the order in which the elements are added. (In a future module, I will discuss how comparisons are made to enforce theordering of the elements.)
Because an object of the TreeSet class is a Collection , a reference to such an object can be passed to any method that requires anincoming parameter of type Collection . The receiving method can call anymethod on that reference that is declared in the Collection interface . (I will discuss such methods in detail in future modules.)
The statement in Listing 2 passes the TreeSet object's reference to a method named fillIt , which is a static method of the Populator class. (The Populator class is a class of my own design whose only purpose is to illustrate the polymorphic behavior achieved using the CollectionsFramework.) The behavior of this method is to add elements to the incoming Collection object without regard for the actual type of the object (the class from which the object was instantiated).
| Listing 2 . Populate the collection. |
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Populator.fillIt(ref); |
At this point, I am going to discuss the fillIt method of the Populator class called in Listing 2 . The entire class definition of the Populator class, including the fillit method, is shown in Listing 3 .
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