Mock exam by sreenivasa kumar majji - Q16

[paul wheaton]

If you add() the same button twice, it will appear only once? So the add() method checks to see if that object is already in its collection and the
collection is a Set, yes?

[Anonymous]

yes, you are correct.
Test this code to clarify the concept
import java.awt.*;
public class TestAddTwice extends Frame
{
Panel pnl;
public TestAddTwice()
{
super();
pnl = new Panel();
Button btn = new Button("Test");
pnl.add(btn);
pnl.add(btn);
System.out.println(pnl.getComponentCount());
add(pnl);
}
public static void main(String args[])
{
TestAddTwice test = new TestAddTwice();
test.show();

}
}
the output is
1
Banu

[paul wheaton]

I wonder why the creators of the AWT decided to do this?

[zhang feng]

reply yes
let's write fellow class

<font size="2">  
public class Myclass
{
  public static void main(String[] arg)
  {  Set t=new HashSet();
     t.add(new String("1"));
     t.add(new String("2"));
     String st=new String("3");
     String st2=st;
     System.out.println(t.add(st));
     System.out.println(t.add(st));
     System.out.println(t.add(st2));
     System.out.println(t);
    }
  }
</font>

Guess output what?
true
false
false
[1,2,3]
Set's add() method that will return boolean value , show the object if add successful
i understand this way
i think that the object added the set pass the first reference,before add that pass the object's other reference ,so the objectself cannt the into the set pass the second reference
[ September 07, 2003: Message edited by: zhang feng ]