Question

# In the problem which i am posting,in this i am unable to understand the code mentioned on line 1 and line 2, As what i know about them is that they are used to set the action listener for the button but the thing which is most confusing for me is that in the syntax sated on the line 1 and line 2 as, {JB1.addActionListener(this)} in this what is the role of “this”…..so please tell the basic behind this.And how this whole syntax works…in detail. #

 import java.awt.*;
    import java.awt.event.ActionEvent;
    import java.awt.event.ActionListener;

    import javax.swing.*;



    public class frametest1_1 implements ActionListener
    {
        JLabel JL;
        public frametest1_1()
        {
            //create a JFrame container
            JFrame JF=new JFrame("A BUTTON");

            //Frame Layout This is contained in Java .awt.*;  "ON USING THIS OPTION OUR BUTTON AND OTHER COMPONENT ARE ADJUSTED IN THE FRAME AUTOMATICALLY"
            JF.setLayout(new FlowLayout());

            //set the size of the container
            JF.setSize(200, 200);

            //set visible
            JF.setVisible(true);

            //make button
            JButton JB1=new JButton("FIRST");
            JButton JB2=new JButton("SECOND");

            //add button to the JFrame container
            JF.add(JB1);
            JF.add(JB2);

            //Create and add Label to the JFrame container
            JL=new JLabel("PRESS A BUTTON");
            JF.add(JL);

            //set action command :now this will help in determining that which button is presses, is it FIRST or SECOND
            JB1.setActionCommand("one");
            JB2.setActionCommand("two");

            //The action responded is added to the actionlistener
            JB1.addActionListener((ActionListener) this); // line 1
            JB2.addActionListener((ActionListener) this); // line 2
        }   

        public void actionPerformed(ActionEvent ae) 
        {

            if(ae.getActionCommand().equals("one"))
                JL.setText("First Button Pressed");     // to set text on the label
            else
                JL.setText("Second button Pressed");    // to set the text on the label

        }
        public static void main(String[] args)
        {
            SwingUtilities.invokeLater(new Runnable()
            {
                public void run()
                {
                    new frametest1_1();
                }
            });
        }
    }

Answer 1

1. Consider that Listener is someone who reacts to some action.

2.ActionListener is an Interface which has a call back method, named actionPerformed, inside which is the code that will run when certain action is done on the controller.

3. This line JB1.addActionListener((ActionListener) this); means as follows

   JB1 - Button
   ActionListener - Interface
   addActionListener - Registering the Button with the Listener.

4. addActionListener will bind/register the Button with the Listener (here its ActionListener).

5. In an MVC architecture Button is the controller, when certain action is done on it, then who all are to be informed to do certain things is done by registering it to the listener.

6. And this Listener will have the callback method which will be overridden by the class that implements the listener.

7. Moreover in your example, you can also do it like this…
JB1.addActionListener(this);