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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 this course. Therefore, forpurposes of this course, you can simply ignore those warnings.
I will provide a concrete example of the specialization that occurs while moving down the interface hierarchy from Collection to List . I will show 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 will illustrate 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 of the program, (in which the add method actually does an insert) makes it possible for the user to control the location of each individual element added to a List . The fact that the location of each element can be controlled in a List is what causes a List to be an ordered collection.
I will illustrate 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 will show that it is not necessary for objects to implement the Comparable interface to make them eligible for inclusion in a List . I will tell you that it is necessary for objects to implement the Comparable interface to make them eligible for inclusion in a SortedSet , although I won't demonstrate that in this module.
Let's begin with a quiz to test your prior knowledge of the Collections Framework.
What output is produced by the program shown in Listing 1 ?
| Listing 1 . The program named Comparable01. |
|---|
//File Comparable01.java
import java.util.*;public class Comparable01{
public static void main(String args[]){
new Worker().doIt();}//end main()
}//end class Comparable01class Worker{
public void doIt(){Iterator iter;
Collection ref;ref = new ArrayList();
Populator.fillIt(ref);iter = ref.iterator();
while(iter.hasNext()){System.out.print(iter.next());
}//end while loopSystem.out.println();
}//end doIt()}// end class Worker
class Populator{public static void fillIt(Collection ref){
ref.add(0,new MyClass(4));ref.add(1,new MyClass(4));
ref.add(2,new MyClass(3));ref.add(3,new MyClass(2));
ref.add(4,new MyClass(1));}//end fillIt()
}//end class Populatorclass MyClass{
int data;MyClass(){
data = 0;}//end noarg constructor
MyClass(int data){this.data = data;
}//end parameterized constructorpublic String toString(){
return "" + data;}//end overridden toString()
}//end MyClass |
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