Why is itemStateChanged on JComboBox is called twice when changed?

Question

I'm using a JComboBox with an ItemListener on it. When the value is changed, the itemStateChanged event is called twice. The first call, the ItemEvent is showing the original item selected. On the second time, it is showing the item that has been just selected by the user. Here's some tester code:

public Tester(){

    JComboBox box = new JComboBox();
    box.addItem("One");
    box.addItem("Two");
    box.addItem("Three");
    box.addItem("Four");

    box.addItemListener(new ItemListener(){
        public void itemStateChanged(ItemEvent e){
            System.out.println(e.getItem());
        }
    });

    JFrame frame = new JFrame();
    frame.getContentPane().add(box);
    frame.pack();
    frame.setVisible(true);
}

So when I changed the Combo box once from "One" to "Three" the console shows:

One
Three

Is there a way I can tell using the ItemEvent maybe, that it's the second item (ie. the user selected item)? And if someone can explain why it gets called twice, that would be nice too!

Thanks

Answer 1

Have a look at this source:

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class Tester {

    public Tester(){

        JComboBox box = new JComboBox();
        box.addItem("One");
        box.addItem("Two");
        box.addItem("Three");
        box.addItem("Four");

        box.addItemListener(new ItemListener(){
            public void itemStateChanged(ItemEvent e){
                System.out.println(e.getItem() + " " + e.getStateChange() );
            }
        });

        JFrame frame = new JFrame();
        frame.getContentPane().add(box);
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String [] args) {
        Tester tester = new Tester();
    }
}

Use the getStateChange to determine if an item is selected or deselected

Answer 2

According to this thread,

It gets tripped when you leave one result and then called again when set to another result

Don't listen for itemStateChanged. Use an ActionListener instead, which is good for handling events of the combo.
You need a ItemStateListener if you need to separately handle deselection / selection depending on the item involved.

Changing the state of the item within itemStateChanged causes itemStateChanged to be fired... this called "reentrance".

Answer 3

Yo can do it like this: you will need this

import java.awt.event.*;

The code is:

jComboBox1.addActionListener(new ActionListener() {
  @Override
  public void actionPerformed(ActionEvent e) {
   System.out.println("Hello");
  }
 });