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The Collection interface declares the following two overloaded versions of the toArray method:
public Object[] toArray()public Object[] toArray(Object[]a) In the previous module, I taught you how to use the first (simpler) of the two methods. I also discussed the need to exercise care when using theelements stored in the returned array to avoid corrupting the state of the objects referred to by elements in the collection.
In this module, I will teach you how to use the second (more-complex) version of the toArray method declared in the Collection interface. I will discuss issues regarding the type of the array and the typesof the objects referred to by the elements in the collection. I will also discuss issues regarding the relative sizes of the array and the collection.
Finally, I will reaffirm that you need to exercise care when using the elements stored in the array, to avoid corrupting the state of the objectsreferred to by the elements in the collection.
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.
As has been the case in the last few modules, let's begin with a quiz to test your prior knowledge of the Collections Framework. To take this quiz,examine the program shown in Listing 1 and write down the output produced by that program.
| Listing 1 . The program named ToArray02 . |
|---|
//File ToArray02.java
//Copyright 2001, R.G.Baldwinimport java.util.*;
import javax.swing.*;public class ToArray02{
public static void main(String args[]){
new Worker().doIt();}//end main()
}//end class ToArray02//===================================//
class Worker{public void doIt(){
Collection ref;//Create, populate, and display
// the contents of an arrayJComponent[] array = new JComponent[8];
for(int cnt=0;cnt<8;cnt++){
array[cnt]= new JButton();
array[cnt].setToolTipText("" + (cnt+10));
}//end for loopSystem.out.println();
showArray(array,"Original array contents");//Create, populate, and display the
// contents ofa collection
ref = new LinkedList();Populator.fillIt(ref);
showCollection(ref,"Collection contents");//Get collection contents into the
// array and display the new// contents of the array.
array = (JComponent[])ref.toArray(array);
showArray(array,"New array contents");//Modify a property of an object
// referred to by one of the// elements in the array. Display
// array contents after// modification
((JComponent)array[0]).setToolTipText("XX");
showArray(array,"Modified array contents");//Display the contents of the collection
showCollection(ref,"Collection contents");}//end doIt()
//-----------------------------------////Utility method for displaying
// array contentsvoid showArray(Object[] array,String title){System.out.println(title);
for(int i = 0; i<array.length;i++){
if(array[i]== null){
System.out.print("null ");}else{
System.out.print(((JComponent)array[i]).
getToolTipText() + " ");}//end else
}//end for loopSystem.out.println();
}//end showArray()//-----------------------------------//
//Utility method for displaying// collection contents
void showCollection(Collection ref,String title){System.out.println(title);
Iterator iter = ref.iterator();while(iter.hasNext()){
System.out.print(((JComponent)iter.next()).getToolTipText() + " ");
}//end while loopSystem.out.println();
}//end showCollection}// end class Worker
//===================================//class Populator{
public static void fillIt(Collection ref){ref.add(new JButton());
ref.add(new JButton());ref.add(new JLabel());
ref.add(new JButton());ref.add(new JButton());
ref.add(new JLabel());Iterator iter = ref.iterator();
int cnt = 0;JComponent refVar;
while(iter.hasNext()){refVar = (JComponent)iter.next();
if(refVar instanceof JButton){refVar.setToolTipText("B"+cnt++);
}else{refVar.setToolTipText("L" + cnt++);
}//end else}//end while loop
}//end fillIt()}//end class Populator |
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