Passing "this" as a Method Argument - Clarification

Question

I am currently working on Java Swing lessons after finishing my first set of lessons in Java. In this lesson, we are working on communication between different components (buttons,toolbars,etc.) that we have been studying. The problem is, "this" is being passed as a method argument for the addActionListener() method. This works, however, I do not completely understand what it is doing. I did some research regarding "this," and found that the most popular usage for the "this" keyword would be in constructors with variables of the same names. I could not find an example that would fit my case. By looking at the code below, I will explain that parts of the code that I understand.

import java.awt.FlowLayout; Implements FlowLayout class
import java.awt.event.ActionEvent; //Imports ActionEvent Class
import java.awt.event.ActionListener; //Imports ActionListener Interface
import javax.swing.JButton; //Imports JButton class
import javax.swing.JPanel; //Imports JPanel class


public class Toolbar extends JPanel implements ActionListener {
    // ^ Inherits JPanel & Implements ActionListener Interface

    private JButton helloButton; //Creating variable of JButton type
    private JButton goodbyeButton; //Creating variable of JButton type


    public Toolbar() { //Constructor

        helloButton = new JButton("Hello"); //Creates new JButton
        goodbyeButton = new JButton("Goodbye"); //Creates new JButton

        helloButton.addActionListener(this); //Question 1
        goodbyeButton.addActionListener(this); //Question 1

        setLayout(new FlowLayout(FlowLayout.LEFT)); //Sets Layout Type to Left


        add(helloButton); //Adds button to FlowLayout (Layout Manager) Interface
        add(goodbyeButton); //Adds button to FlowLayout (Layout Manager) Interface

    }

    public void setTextPanel(TextPanel textPanel) {
        //No Usage Yet!
    }

    public void actionPerformed(ActionEvent arg0) { //Unimplemented Method from ActionListener
        System.out.println("A button was clicked"); //Prints after button is clicked.

    }

}

Question 1: As you can see, after creating two JButtons, we are adding an addActionListener() method in order to see detect if the button has been clicked. If it has been clicked, then it will do any code typed in actionPerformed that is implemented from the ActionListener interface. In previous lessons, we had a different way of making a button respond to a click. It did the same thing, but it looked like this:

btn.addActionListener(new ActionListener() {

        public void actionPerformed(ActionEvent arg0) {

        textPanel.appendText("Hello\n");

        }


    });

In this example, rather than printing to the console to test to see if the actionListener() worked, we just appended the text to a text area from a customized component. In the code directly above this text, we are working with an anonymous class.

So... The question is, what exactly is "this" doing as it is being passed as a method argument in addActionListener()? I know that it is printing "A button was clicked" to the console, but I don't understand the logic behind what "this" is doing to send that the button has been clicked to actionPerformed().

This is what the applications looks like:

This is the application in motion, and the buttons have already printed to the console after being clicked. I thought it might give you a better idea of what I am working on. I hope somebody can shed some light on this, and explain how "this" is working in this context. Thank you!

Answer 1

this represents the current instance of your class.

Toolbar class implements ActionListener interface, that means it provides an implementation of method actionPerformed.

So, helloButton.addActionListener(this); becomes possible (try to remove implements ActionListener from class declaration, the code won't compile).

Saying this, Toolbar instances can be considered as ActionListener objects and can be passed to button.addActionListener.

When using

btn.addActionListener(new ActionListener() {

        public void actionPerformed(ActionEvent event) {
        }
}

you create a new implementation of the ActionListener interface in the fly. This kind of implementation is named anonymous.

The both solutions are valid. Prefer the anonymous solution if the code in actionPerformed cannot be used from another place.

Answer 2

By implementing ActionListener and then passing this to addActionListener method you are asking to be informed of any actions that are performed through a call to your actionPerformed method.

Or, more simply, you are giving the button your phone number and asking it to call you whenever something happens.

In the earlier lessons you were stating what should happen - print this text or add some text to the textPanel. To do this you were making an ActionListener on-the-fly. Now you implement ActionListener yourself you can request a callback to you anstead of making a listener one on-the-fly.

Answer 3

The addActionListener method for a swing component takes an argument of type ActionListener. A class that implements an ActionListener contains code that specifies what should be done when someone interacts with a swing component.

When you pass this to the addActionListener method, you are passing a reference to the current object that is being instantiated. As it happens, the current object being instantiated is also an ActionListener (as it implements ActionListener) and can therefore be passsed to the addActionListener method.

When you interact with the JButton in the GUI, the actionPerformed method of Toolbar class will be called since that is the ActionListener that you registered the JButton with.

Answer 4

this is just an instance of your Toolbar class. It is used inside instance methods to represent the current instance. Essentially you can imagine your class has another private member variable named this, that points to a Toolbar object.

Since Toolbar implements ActionListener you are assigning the newly instantiated Toolbar object as a listener to the buttons (meaning the Toolbar.actionPerformed() method will be called when the buttons are clicked.)

Answer 5

These two examples would do the same thing (assuming all imports are there, and that capitalization would be correct):

public static void main(String args[]){
    myFrame frame = new myFrame();
    myFrame.addActionListener(new myListener);
}

public myFrame extends JFrame{
    public myFrame(){
        super("myFrame");
    }
}

public myListener implements ActionListener{
    public void ActionPerformed(ActionEvent e){
        //Do Stuff
    }
}

and

public static void main(String args[]){
    myFrame frame = new myFrame();       
}

public myFrame extends JFrame implements ActionListener{
    public myFrame(){
        super("myFrame");
        this.add(this);
    }

    public void ActionPerformed(ActionEvent e){
        //Do Stuff
    }
}

The advantage of having the ActionListener be this is that it would have more direct access to fields and methods, if you want to make the modifier on the methods private. However, if you want to avoid if/else if monstrosities for handling multiple buttons, I recommend either using a separate ActionListener(and providing a means to change what needs to be changed), or taking a look at anonymous classes and lambda expressions.

Answer 6

Often with object-oriented coding, it's worth role-playing the part of an object of the class you're coding. If you do that, "this" means "me".

And because Java is pass-by-reference, "this" is an arrow pointing to "me".

public class Foo implements ActionListener {
      public void actionPerformed(ActionEvent e) {
           // do something useful with e
      }
}

Here we've written a class Foo. Since we've said it implements ActionListener, it must have an actionPerformed() method. Anything can call that:

ActionListener listener = new Foo(...);
ActionEvent event = ...;
foo.actionPerformed(event);

And we can create a Foo and give it to something that generates events:

ActionListener listener = new Foo(...);
button.addListener(listener);

If we role-play the object, you can think of this last line as "Hey button! Tell listener whenever an action happens.".

Now, what if we want the listener to take control of who tells it what on its own? "Hey button, tell me whenever an action happens".

public class Foo implements ActionListener {

      public attachToButton(JButton button) {
           button.addActionListener(this);
      }

      public void actionPerformed(ActionEvent e) {
           // do something useful with e
      }
}

It may help to imagine what the button's addActionListener code might look like:

public class JButton {  // not the real JButton code, but it will be similar
    private List<ActionListener> actionListeners = new ArrayList<ActionListener>();

    public void addActionListener(ActionListener a) {
         actionListeners.add(a);
    }

    // called internally when an event happens
    private void onEvent(ActionEvent e) {
        for(ActionListener listener : actionListeners) {
            listener.actionPerformed(e);
        }
    }
}

So, if you're the listener that called addActionListener(this), then that List contains a reference that points right back at you, which the button uses to tap you on the shoulder every time an action occurs.