can`t access outer class modifiers (java)

Question

public class MainFrame extends JFrame {
    MainFrame()
    {
        JButton zeroButton = new JButton("0");
        add(zeroButton);

        Handler handler = new Handler();
        zeroButton.addActionListener(handler);
    }

    private class Handler implements ActionListener
    {

        public void actionPerformed(ActionEvent e) {
              if (e.getSource() == **zeroButton**)
                  JOptionPane.showMessageDialog(null, "hello there!");
        }
    }
}

The code has an error which underlines what I marked as bold in the code. here is the error message: "cannot find symbol"

I thought I can access outer class modifiers from the inner class, but it doesn't work!

Answer 1

zeroButton is not a member of outer class (MainFrame), it's a local variable in MainFrame constructor.

Try something like this

public class MainFrame extends JFrame {
    private final JButton zeroButton;

    MainFrame() {
        zeroButton = new JButton("0");

PS I'm also not sure if you're supposed to compare controls with ==. (never used swing)

Answer 2

You are unable to access it because the button is a local variable in another method.

You have two options: 1. Make the button in a instance variable (a class level variable). See Nikita's answer.

1. Have the handler as an anonymous implementation, in the constructor:

MainFrame() {

MainFrame() {        
        final JButton zeroButton = new JButton("0");  
        add(zeroButton);  
        Handler handler = new Handler();  
        zeroButton.addActionListener(new ActionListener()
        {
           public void actionPerformed(ActionEvent e) { 
              if (e.getSource() == **zeroButton**) 
                  JOptionPane.showMessageDialog(null, "hello there!"); 
        } 

        });  
    }

In this case, the variable should be final because only final local variables are accessible to inner classes.