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ItemEvent and ItemListener




An event of type ItemEvent is generated when a source such as a checkbox is clicked to check/uncheck it or when a list item is clicked.




Some methods of ItemEvent class


Method Description
public Object getItem() Returns the item which was clicked and triggered the ItemEvent.
public ItemSelectable getItemSelectable() Returns the item which was clicked.
public int getStateChange() Returns the current state of item which which was clicked.





A class to listen and respond to ItemEvent must perform the next two steps -





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Handling an ItemEvent by implementing ItemListener interface


In the upcoming code, we are going to create a class that will listen to ItemEvent, by implementing ItemListener interface. In this code, ItemEvent is generated when a source, i.e. checkbox, is clicked to check/uncheck it.

import java.awt.*;
import java.awt.event.*;


public class ItemEx1 implements ItemListener
{
Frame jf;
Checkbox chk1, chk2;
Label label1;


ItemEx1()
{
jf= new Frame("Checkbox");
chk1 = new Checkbox("Happy");
chk2 = new Checkbox("Sad");
label1 = new Label();

jf.add(chk1);
jf.add(chk2);

chk1.addItemListener(this);
chk2.addItemListener(this);

jf.setLayout(new FlowLayout());
jf.setSize(220,150);
jf.setVisible(true);
}

public void itemStateChanged(ItemEvent ie)
{
Checkbox ch =(Checkbox)ie.getItemSelectable();
if(ch.getState()==true)
{
label1.setText(ch.getLabel()+ " is checked");
jf.add(label1);
jf.setVisible(true);
}
else
{
label1.setText(ch.getLabel()+ " is unchecked");
jf.add(label1);
jf.setVisible(true);
}
}

public static void main(String... ar)
{
new ItemEx1();
}

}

When you run the code, you are presented a window shown in the Figure2 below -:

Figure 1


When you check a checkbox an event ItemEvent is fired and you are presented a message to display which checkbox is last checked/unchecked. For example, when you check the checkbox with label- happy, you are notified like -

Figure 2


When you uncheck this checkbox, an event ItemEvent is fired you are notified with a message that you've unchecked the checkbox(including its name).

Figure 3




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