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"Returns an iterator over the elements in this list in proper sequence."
As a result, the code shown in Listing 5 , along with the overridden toString method of the MyClass class causes the program to display the elements in the following order:
443521 .
| Listing 5 . Display using an iterator. |
|---|
iter = ref.iterator();
while(iter.hasNext()){System.out.print(iter.next());
}//end while loop |
One final thing that is worthy of note in this program is that a List objects allows duplicates. Hence, the populated collection contains referencesto two separate objects that are equal to one another in the sense that they both contain thesame values in their instance variables.
Let's take a look at one more sample program. What output is produced by the program shown in Listing 6 ?
| Listing 6 . The program named Comparable03. |
|---|
//File Comparable03.java
import java.util.*;public class Comparable03{
public static void main(String args[]){
new Worker().doIt();}//end main()
}//end class Comparable03class 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(new MyClass(4));ref.add(new MyClass(4));
ref.add(new MyClass(3));ref.add(new MyClass(2));
ref.add(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 |
If you selected C. 44321 , you are correct.
As shown in Listing 7 , this program takes a different approach to solving the problem originally exposed in the program shown in Listing 1 .
| Listing 7 . No need to cast to type List. |
|---|
class Populator{
public static void fillIt(Collection ref){
ref.add(new MyClass(4));ref.add(new MyClass(4));
ref.add(new MyClass(3));ref.add(new MyClass(2));
ref.add(new MyClass(1));}//end fillIt()
}//end class populator |
This program does not change the type of the incoming reference to the ArrayList object in the fillIt method. Rather, it continues to treat the incoming reference as type Collection , and calls the version of the add method that is declared in the Collection interface. This avoids the requirement to cast the incoming reference to type List .
The contract for this version of the add method in the List interface is
"Appends the specified element to the end of this list (optional operation)."
As a result, the new elements are added to the collection in increasing index order. Since an iterator on a List returns the elements in increasing index order, this program displays the elements in the same order that they areadded.
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