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| Listing 4 . Get an Iterator object. |
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Iterator iter = ref.iterator(); |
The call to the iterator method on any Collection object returns an instance of a class that implements the Iterator interface. The Iterator object can be used to traverse the collection, gaining access to each element in order. (The concept of in order means different things for different kinds of collections. For a collection instantiated fromthe TreeSet class, in order means in ascending order.)
Again, the author of the method that uses the Collection object doesn't need to know or care about the internal implementation of thecollection, or the implementation of the methods of the Iterator object. They simply do what they do, and can be used for their intended purpose.
The Iterator interface declares three methods:
You might say that an Iterator object acts as a doorkeeper for the collection object that it represents, providing access to the contents of thecollection in a very specific manner.
The code fragment in Listing 5 below shows how the first two of the above methods can be used to
As mentioned earlier, when the collection is an object instantiated from the TreeSet class, access to the elements is provided in ascending order.
| Listing 5 . Traverse the collection. |
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while(iter.hasNext()){
System.out.print(iter.next());}//end while loop |
At this point, the TreeSet object contains four elements, with no duplicates. Each of the elements is a reference to an object of type Integer . The code in the loop in Listing 5 causes each of those elements to be accessed and displayed in ascending order. This causes the following text to appear on thescreen:
1234
In my opinion, this is the kind of knowledge that a computer science student in a modern data structures course should be learning. This is a far departurefrom courses of the past where CS2 students were required to memorize the intricate details of how to implement various data structures.
Does an architect need to understand the detailed inner workings of an air conditioning compressor to design a cooling system into a building? Ofcourse not!
However, the architect does need to know the tradeoffs among the available cooling systems in terms of initial cost, operating cost, size, efficiency, etc.
Does an audio technician need to understand the detailed inner workings of an electronic audio equalizer in order to construct an integrated audio system?Absolutely not! If that were a requirement, there would likely be very few audio systems in existence.
However, the audio technician does need to understand the tradeoffs among the various available audio equalizers.
Does an OOP software designer need to know the detailed inner workings of the various kinds of collection objects in order to use them effectively? No!
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