User Interface

[swikar.java]

Can anybody tell why use addActionlistner(this); This is like method.
I cannot understand of use ''this'' word.

[Fubarable]

If you look up the API for the addActionListener (I think this can be found in the AbstractButton API), you'll see that it requires that you pass an object into it that implements the ActionListener interface, and this can be any object that implements this interface. In your current situation, the current object, the one whose code you are in, happens to implement this interface (look up at the top of the class and you'll see this). So when you pass "this" to this method, you are passing the current object into the button's action listener list.

Whenever this button is pressed, it iterates through its actionlistener list and calls the actionPerformed() method of each ActionListener object in this list. Since the current object is in that list (and in fact may be the only object in that list), whenever the button is pressed, the current object's actionPerformed() method will be called.

Is that clear as Mudd? Please reply if this doesn't make sense to you and we can try to break this down.

edit: a very simple example:

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

import javax.swing.JButton;
import javax.swing.JFrame;

public class ActionListenerExample implements ActionListener
{
  
  JButton myButton = new JButton("My Button");
  
  public ActionListenerExample()
  {
    myButton.addActionListener(this);
    // now when "myButton" is pressed, "this"'s actionPerformed method will be called
  }
  
  // all classes that implement ActionListener *MUST* have this method
  public void actionPerformed(ActionEvent e)
  {
    System.out.println("Hello from the actionlistener!");
  }
  
  public JButton getMyButton()
  {
    return myButton;
  }

  private static void createAndShowUI()
  {
    JFrame frame = new JFrame("example");
    frame.getContentPane().add(new ActionListenerExample().getMyButton());
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.pack();
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
  }

  public static void main(String[] args)
  {
    java.awt.EventQueue.invokeLater(new Runnable()
    {
      public void run()
      {
        createAndShowUI();
      }
    });
  }
}

[georgemc]

Incidentally, the old addActionListener(this) routine often indicates a poor design: the "telephone switchboard" all-purpose action listener. Doubtless there's a single object handling all UI events, and constantly querying those events to see which widget spewed them out and acting accordingly. Quickly becomes a maintenance nightmare. A far better approach is to add an individual action listener to each widget, typically as an anonymous inner class

[mohkhamis]

"this"is to call another constructor in the same class for calling

[georgemc]

"this"is to call another constructor in the same class for calling

That is only one use of the keyword 'this', and not the one the OP was asking about

[mohkhamis]

sorry im new in java buti m trying

[mohkhamis]

im now learining java how long it will take to devlop a useful program if im everyday studying

[Eranga]

im now learining java how long it will take to devlop a useful program if im everyday studying

It's depends on you lol, depends on your skills/ability to learn new things in short period of time. To build a useful application you should have a useful design. So the answer is in your hand.

Do you familiar with any programming language? It's really helpful to you. And object oriented programming too.

[georgemc]

im now learining java how long it will take to devlop a useful program if im everyday studying

Studying every day is good, but you'll get there quicker if you have something specific in mind you wish to develop. Preferably something related to a hobby or interest you have.

[Eranga]

I agreed with you georgemc. Better to start work on with a complete design, a real one. So you can, you have to learn a lot. It's really valid.

[georgemc]

I needed to learn another language (not Java) quickly once, for a project I'd been dumped on in protest. I have a lot of CDs, and when friends came round at the weekend, we'd be constantly up and down changing CDs all the time. So I wrote a jukebox that held all my CDs as MP3s, and let us just scroll through all the CDs in my collection and choose random songs to put on a playlist, etc etc, via a numeric keypad.

Point is, because it was something I was actually interested in, I learnt a lot quicker than if I just plodded along doing generic exercises somebody else had set me, plus, I had something useful at the end of it

[Eranga]

Nice, lots of people worry about Hello World!

[mohkhamis]

which book do u prefer to learn java on it u recomment im reading now th ebook for study guide is it useful or not

[georgemc]

which book do u prefer to learn java on it u recomment im reading now th ebook for study guide is it useful or not

I bought "The Java Programming Language" by James Gosling, got a Java job and jumped right in the deep end, but I wouldn't recommend that to anybody!

The "Head First" series of books are very good by all accounts

[Eranga]

which book do u prefer to learn java on it u recomment im reading now th ebook for study guide is it useful or not

I suggest you the Suns' tutorial. It's really helpful to me at the time I start work on Swing/AWT in Java.

Here is the the link. Start from the first page.

[swikar.java]

1 for i:=1 to n do
2 for j:=1 to n/3 do
3 x:=x+1

How many times third line execute?
This is actually Psedocode.

[Eranga]

1 for i:=1 to n do
2 for j:=1 to n/3 do
3 x:=x+1

How many times third line execute?
This is actually Psedocode.

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