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The code in this series of modules is written with no thought given to Generics . As a result, if you copy and compile the code, you will probably get warnings about unchecked or unsafe operations .
While you will ultimately need to understand how to use Generics, that is a very complex topic. An understanding of Generics is beyond the scope of thiscourse. Therefore, for purposes of this course, you can simply ignore those warnings.
We will begin this module with a little quiz. Take a look at the program shown in Listing 1 and see if you can answer the following question.
What output does the program in Listing 1 produce?
| Listing 1 . The program named Ap401. |
|---|
import java.util.TreeSet;
import java.util.ArrayList;import java.util.Collection;
import java.util.Iterator;public class Ap401{
public static void main(String args[]){
new Worker().doIt();}//end main()
}//end class Ap401class Worker{
public void doIt(){Collection ref = new TreeSet();
Populator.fillIt(ref);Iterator iter = ref.iterator();
while(iter.hasNext()){System.out.print(iter.next());
}//end while loopSystem.out.print(" ");
ref = new ArrayList();Populator.fillIt(ref);
iter = ref.iterator();while(iter.hasNext()){
System.out.print(iter.next());}//end while loop
System.out.println();}//end doIt()
}// end class Workerclass Populator{
public static void fillIt(Collection ref){ref.add(new Integer(4));
ref.add(new Integer(4));ref.add(new Integer(3));
ref.add(new Integer(2));ref.add(new Integer(1));
}//end fillIt()}//end class populator |
If you selected the following answer, then you are correct.
E. 1234 44321
The program in Listing 1 illustrates the basic purpose of the core collection interfaces in the Java Collections Framework. That purposeis to allow collections to be manipulated without regard for how the collections are implemented.
For example, there is more than one way to implement a list. Two common ways involve arrays and linked structures. If two lists are implemented indifferent ways, but both satisfy the requirements of the core collection interfaces, they can each be manipulated the same way regardless of the detailsof their implementation.
A collection of type TreeSet and a collection of type ArrayList are instantiated in the program in Listing 1 . Each of the collections is viewed as being of the interface type Collection . A method named add is used to populate each collection with the same values in the same order.
The behavior of the add method is appropriate, and different in each of the two cases, with the final contents of each collection being determined bythe respective behavior of the add method for that type of collection.
The code in the fragment shown in Listing 2 defines a static method named fillIt of the class named Populator . This is a class of my own design intended solely to illustrate the primary point of this program.
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