Why is my JComboBox returning null?

Question

Note: This is a shortened version of my actual code, but pretty much the same in terms of structure. I basically cut out the panel connecting to component code and panel connecting to frame code.

On a Display.java , I have the following. Note that I did not add any listener to the targetEnvironmentComboBox . Not sure if that can be a problem:

public class Display extends JFrame {

    private static JButton executeButton;
    private static JComboBox<String> commandOptionsComboBox, targetEnvironmentComboBox;

    //getters
    public static JButton getExecuteButton()    {   return executeButton;               }
    public static JComboBox<String> getCommandOptionsComboBox()    {    return commandOptionsComboBox;      }
    public static JComboBox<String> getTargetEnvironmentComboBox()      {   return targetEnvironmentComboBox;   }

    public Display() {
          super("Display");
          setLayout(new BorderLayout());
          setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

          commandOptionsComboBox = new JComboBox(commandOptions.toArray());
          commandOptionsComboBox.addActionListener(new CommandListener());

          executeButton = new JButton("Execute");
          executeButton.addActionListener(new CommandListener());

          targetEnvironmentComboBox = new JComboBox(targetEnvironments.toArray());


        //main method that gets executed at the start of program
        public static void main(String[] args) {
        new Display();

        }
  }

On a separate CommandListener.java , I have the following:

 public class CommandListener implements ActionListener {

JButton executeButton = Display.getExecuteButton();
JComboBox<String> commandOptionsComboBox = Display.getCommandOptionsComboBox();
JComboBox<String> targetEnvironmentComboBox = Display.getTargetEnvironmentComboBox();

@Override
public void actionPerformed(ActionEvent event) {

    if(event.getSource() == executeButton) {
        System.out.println("HGello world");
        executeCommand(event);

    }else if (event.getSource() == commandOptionsComboBox) {
        System.out.println("commandline");
        disableUnusedComponents(event);
    }
}

private void disableUnusedComponents(ActionEvent event) {
     **JComboBox<String> targetEnvironmentComboBox = Display.getTargetEnvironmentComboBox();**
     String command = (String) commandOptionsComboBox.getSelectedItem();
    switch(command) {
          case "-duplicate":
            targetEnvironmentComboBox.setEnabled(false);
            targetEnvironmentComboBox.setVisible(false);
            break;
          default: break;
     }

My question is that when I get the targetEnvironmentComboBox in the commandListener class right before the actionPerformed() method, it throws a Null Pointer Exception . If I remove that I get the targetEnvironmentComboBox in the disableUsedComponents() method where the **, it was able to successfully get the combobox .

Also if I execute the executeButton listener, it was able to get references to all the components in the CommandListener class, but if I execute the listener for the getCommandOptionsComboBox , it returns null for executeButton and targetEnvironmentComboBox .

Anyone can explain why this is the case?

Secondly, I know this is probably not the best implementation. Any suggestions on things I can change to follow better practice?

Answer 1

You're getting a null because the targetEnvironmentComboBox doesn't get initialized until after you try to access it in CommandListener . When a new CommandListener is created, it reads the targetEnvironmentComboBox and stores it to a local variable. And look at where you create the CommandListener :

//Creating CommandListeners, which take the reference to targetEnvironmentComboBox (null)
//Although targetEnvironmentComboBox is later set to a usable value, the local copies still
//have the null reference which is assigned here
commandOptionsComboBox.addActionListener(new CommandListener());
executeButton = new JButton("Execute");
executeButton.addActionListener(new CommandListener());

//initializing targetEnvironmentComboBox, which is still null (the first time at least)
targetEnvironmentComboBox = new JComboBox(targetEnvironments.toArray());

If you put Display.getTargetEnvironmentComboBox() inside the method, it doesn't take a copy of the reference until it's needed, at which point it's been properly initialized.

An easy fix, although not the right one, is to initialize it first:

targetEnvironmentComboBox = new JComboBox(targetEnvironments.toArray());

commandOptionsComboBox.addActionListener(new CommandListener());
executeButton = new JButton("Execute");
executeButton.addActionListener(new CommandListener());

A better solution is to use proper encapsulation, and pass in what you need to access the correct objects in a constructor to the CommandListener s. I'd propose modifying your CommandListener class with something like this:

public class CommandListener implements ActionListener {
    JButton executeButton;
    JComboBox<String> commandOptionsComboBox;
    JComboBox<String> targetEnvironmentComboBox;

    public CommandListener(JButton executeButton,
                           JComboBox<String> commandOptionsComboBox,
                           JComboBox<String> targetEnvironmentComboBox){
        this.executeButton = executeButton;
        this.commandOptionsComboBox = commandOptionsComboBox;
        this.targetEnvironmentComboBox = targetEnvironmentComboBox;
    }

    /* The rest of your code */
}

Then passing those field in (making sure you're passing non-null values), and doing away with all the static variables (or refactoring to something more manageable).

Answer 2

Instead of Using static methods to access an instance, use encapsulation and access it through setters and getters .

The only way you can get the INSTANCE of your targetEnvironmentComboBox object is by having the instance of your Display also "pointing" at the same space in memory , try:

public JComboBox getComobo(){
   return [your_object];
}

instead of:

public static JComboBox getTargetEnvironmentComboBox(){
  // your code . . .
}

and pass the Display instance through the CommandListener constructor.

Answer 3

Here's a simplified example of how to set this sort of thing up in a way that both greatly reduces the amount of boilerplate code and is essentially immune to the kind of initialization problem you're encountering. This is also a fairly typical approach in Swing UI code and, in fact, one of the drivers for the introduction of anonymous inner classes.

public class Display extends JPanel {

    private JButton executeButton;
    private JComboBox<String> commandOptionsComboBox;

    public Display() {

        executeButton = new JButton("Exec");
        commandOptionsComboBox = new JComboBox<>();

        executeButton.addActionListener(new ActionListener()
        {
            @Override
            public void actionPerformed(ActionEvent e)
            {
                System.out.println("Action performed");
                commandOptionsComboBox.setEnabled(false);
            }
        });
    }