addActionLister parameter

Question

I am getting expected output when i am using "this" reference in addActionListener() call, how can I achieve it using the object call directly ? When i am using jb.addActionListener(new EventHandling1());

I am getting error -"Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException".

Also which object needs to be passed to this method addActionListener() ?

Below is the code which is failing with the above mentioned error.

import javax.swing.*;

import java.awt.event.*;

public class EventHandling1 implements ActionListener {

    JButton jb;

    public static void main(String[] args) {

        EventHandling1 eh = new EventHandling1();
        eh.go();
    }

    public void go() {
        JFrame jf = new JFrame();
        jb = new JButton("New button");
        jb.addActionListener(new EventHandling1());

        jf.getContentPane().add(jb);
        jf.setVisible(true);
        jf.setSize(700, 500);

    }

    public void actionPerformed(ActionEvent event) {
        jb.setText("Pressed!!");

    }

}

Answer 1

You're getting the NPE because the jb variable in your new EventHandling1 object is null since you never call go() on the new object (you do call it on the original, but that has no effect on the new one). Even if you get rid of the need to call go() say by folding all into the EventHandling1 constructor, you're still stuck up a tree, since the jb variable then will not refer to the same object as the displayed GUI.

I suggest that instead of creating a new EventHandling1 object, change EventHandling1 so that it does not implement ActionListener and then create a new ActionListener class, either an anonymous inner class, or else a private inner class, and instantiate that object in your parameter.

e.g.,

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

public class EventHandling1  {

    JButton jb;

    public static void main(String[] args) {
        EventHandling1 eh = new EventHandling1();
        eh.go();
    }

    public void go() {
        JFrame jf = new JFrame();
        jb = new JButton("New button");
        jb.addActionListener(new MyEventHandler());

        jf.getContentPane().add(jb);
        jf.setVisible(true);
        jf.setSize(700, 500);
    }

    private class MyEventHandler implements ActionListener {
        public void actionPerformed(ActionEvent event) {
            jb.setText("Pressed!!");
        }
    }
}

Or if your listeners become more complex, then use a stand-alone outer class, or better still, a Control object. At times I have used a combination of anonymous inner classes and control objects like so:

myButton.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        if (myControl != null) {
            myControl.myButtonAction(e);
        }
    }
});

---

You ask:

in my code when I am using this reference, is it referring EventHandling1 object with reference jb ? >

When you use this as a parameter, you are using the current displayed object, one with a fully initialized JButton, jb. When you create and pass in a new EventHandling1 object, you are passing a distinct and new object (the key term being new), one that is not displayed, one without an initialized jb variable -- not good.

Also, which all objects can be passed to addActionListener() ? in your code its taking MyEventHandler

You can pass in any object that implements the ActionListener interface.