Question

I have started JAVA and doing serial communication using RxTx.

Referring to:
http://rxtx.qbang.org/wiki/index.php/Event_based_two_way_Communication
http://henrypoon.wordpress.com/2011/01/01/serial-communication-in-java-with-example-program/

In the 2nd link I am not able to decipher usage of ‘this’:
Can anyone please explain:

Communicator.java

public class Communicator implements SerialPortEventListener
{
  GUI window = null;
   ..
   ..
 public Communicator(GUI window)
    {
        this.window = window;
    }

...
..
}

In GUI.java

public class GUI extends javax.swing.JFrame {
Communicator communicator = null;
Communicator communicator = null;
    //KeybindingController object
    KeybindingController keybindingController = null;

    /** Creates new form GUI */
    public GUI() {
        initComponents();
        createObjects();
        communicator.searchForPorts();
        keybindingController.toggleControls();
        keybindingController.bindKeys();
    }

    private void createObjects()
    {
        **communicator = new Communicator(this);**
        keybindingController = new KeybindingController(this);
    }
...
..}

I am confused how this is used to create an object of Communicator class, as highlighted in above code(appearing communicator = new Communicator(this);)

Another confusion is:
Communicator.java

public class Communicator implements SerialPortEventListener
{ 
...
...
 public void connect()
    {
        String selectedPort = (String)window.cboxPorts.getSelectedItem();
        selectedPortIdentifier = (CommPortIdentifier)portMap.get(selectedPort);

        CommPort commPort = null;

        try
        {
            //the method below returns an object of type CommPort
            commPort = selectedPortIdentifier.open("TigerControlPanel", TIMEOUT);
            //the CommPort object can be casted to a SerialPort object
            serialPort = (SerialPort)commPort;
....
...}


 public void initListener()
    {
        try
        {
            **serialPort.addEventListener(this);**
            serialPort.notifyOnDataAvailable(true);
        }
        catch (TooManyListenersException e)
        {
            logText = "Too many listeners. (" + e.toString() + ")";
            window.txtLog.setForeground(Color.red);
            window.txtLog.append(logText + "\n");
        }
    }
....
}

Again I am confused with the use of ‘this’ here (serialPort.addEventListener(this);)

I compared with the code at
http://rxtx.qbang.org/wiki/index.php/Event_based_two_way_Communication

there it suggests

...
InputStream in = serialPort.getInputStream();
**serialPort.addEventListener(new SerialReader(in));**
...

 public static class SerialReader implements SerialPortEventListener 
    {
        private InputStream in;
        private byte[] buffer = new byte[1024];

        public SerialReader ( InputStream in )
        {
            this.in = in;
        }

        public void serialEvent(**SerialPortEvent arg0**) {
            int data;

            try
            {
                int len = 0;
                while ( ( data = in.read()) > -1 )
                {
                    if ( data == '\n' ) {
                        break;
                    }
                    buffer[len++] = (byte) data;
                }
                System.out.print(new String(buffer,0,len));
            }
            catch ( IOException e )
            {
                e.printStackTrace();
                System.exit(-1);
            }             
        }

    }

The description for addEventListener
http://docs.oracle.com/cd/E17802_01/products/products/javacomm/reference/api/

addEventListener

public abstract void addEventListener(SerialPortEventListener lsnr)
throws java.util.TooManyListenersException
Registers a SerialPortEventListener object to listen for SerialEvents. Interest in specific events may be expressed using the notifyOnXXX calls. The serialEvent method of SerialPortEventListener will be called with a SerialEvent object describing the event.

I want to know usage of this as how it is passing ‘SerialPortEventListener lsnr’ as the parameter to addEventListener in above code.

Thanks

Answer 1

this keyword is a reference to the current instance for which the code is being executed. So, since this is a reference, you can use it as any other reference. No problem in that.

Now let’s take a look at your usage: –

new Communicator(this);

Since this statement is used inside the method of GUI class, so, this refers to the instance of GUI, currently executing the code. Now, by passing it to the constructor, you are simply passing the reference of current instance to it. And it’s quite valid, since Communicator constructor takes a reference of type GUI: –

public Communicator(GUI window)
{
    this.window = window;
}

---

Now let’s move ahead with the next statement:

serialPort.addEventListener(this);

Here, you are registering the serialPort with an EventListener which is referenced by this. Since, this is used inside the class – Communicator, which implements SerialPortEventListener, so basically you are registering to a Communicator instance, which is nothing but a SerialPortEventListener. So, you are registering to that event.

As far as your other code is concerned:

serialPort.addEventListener(new SerialReader(in));

Here, you have just used a new instance instead of this, since you are not inside SerialReader class. So, you don’t have this reference to any SerialReader instance, and hence you need to create an object manually of that class.

So, there is no difference. Because, in any case, you are registering the class that implements SerialPortEventListener only.