Adding ActionListener to JComboBox without items not working

Question

Below is the source code for UseTest.java. Why doesn't it work?

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

class Test implements ActionListener{
 JFrame f ;
 JPanel p ;
 JComboBox cb ;
 JLabel l ;

 Test(){
  f = new JFrame("Test") ;
  f.setSize(200, 200) ;

  p = new JPanel() ;
  p.setLayout(new GridLayout(2, 1)) ;

  cb = new JComboBox() ;

  cb.addActionListener(this) ;

  for(int i = 1 ; i <= 20 ; i++)
   cb.addItem(i + "") ;

  //cb.addActionListener(this) ; //doesn't throws exception here


  l = new JLabel() ;
  l.setForeground(Color.red) ;

  p.add(l) ;
  p.add(cb) ;

  f.add(p) ;

  f.setVisible(true) ;
 }

 public void actionPerformed(ActionEvent ae){

  if(cb.getSelectedItem() != null){
   display() ;
  }
 }

 private void display(){
  String str = "" ;
  str = "Combo selection changed to : " + cb.getSelectedItem() ; 
  l.setText(str) ;
  System.out.println(str);
 }
}

public class UseTest{
 public static void main(String args[]){
  Test t = new Test() ;
 }
}

Answer 1

Everything works fine, the listener is correctly added to the JComboBox.

The thing is when you call addItem to the comboBox, the contentsChanged event is fired and your test#actionPerformed method is called which tries to do

l.setText(str);

But l is a JLabel that is initialized AFTER the cb.addItem loop. So when the event handler is called l is still null, thus the NullPointerException.

Answer 2

Running your code, I get a NullPointerException in relation to your JLabel. This has not been initialised by the time the ActionListener is first called upon - when you add your first item is added and selection is therefore changed.

Answer 3

There's a good ComboBoxDemo in How to Use Combo Boxes.

Answer 4

You may want to try adding an ItemListener instead.

Answer 5

Implementing ActionListener as you do is not a common pattern.

You should better code something like:

cb.addActionListener(new ActionListener(){
 public void actionPerformed(ActionEvent ae){

  if((JComboBox)ae.getSource()).getSelectedItem() != null){
   //Do your stuff
  }
 }
});