Callbacks

Does Java have a facility to execute callback functions? :ehh:

what do you mean by 'callback functions'?

Not that I know of. You could encapsulate your callback function in an object and pass that as an argument.


NOTE: You could take a look at JNI. It might have them, it might not. However, I'm sure you can download a 3rd party package that supports it, but I wouldn't see the point in doing that.

Java doesn't have callbacks or delegates. It has listeners: usually, these are tiny interfaces (1-4 methods) that you implement and pass that object to whoever should call you back.

Code:

---

JButton button = new JButton("Hit Me");
button.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        JOptionPane.showMessageBox("Whee!");
    }
});

---

Quote:

---

Originally Posted by CornedBee

Java doesn't have callbacks or delegates. It has listeners: usually, these are tiny interfaces (1-4 methods) that you implement and pass that object to whoever should call you back.

Code:

---

JButton button = new JButton("Hit Me");
button.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        JOptionPane.showMessageBox("Whee!");
    }
});

---

---

Reading about the security within Java and part of it uses "javax.security.auth.callback" Cannot something like this be translated across?

http://java.sun.com/developer/techni...curity/jaasv2/

Or is this just something that is only implemented in the security modules of Java?

The reason I ask, is becasuse I am making a menu generator for the console. Each Menu, will have one of more MenuItem. It would be nice if the Menu object, could receive a response from the user and ivoke a callback associated with the particular MenuItem for the choice made.

Would it be possible to acheive this with listeners?

If I understand correctly it should be. Simply add an actionlistener to the menu items, and process them in an actionperformed method.

JMenu fileMenu;
JMenuItem openItem;
JMenuItem saveItem;
JMenuItem exitItem;

//add the actionlistener

Code:

---

public void actionPerformed(ActionEvent ae)
{
  if (ae.getSource() == openItem)
  {
    //process open
  }
  else if (ae.getSource() == saveItem)
  {
  //process save
  }
  else if (ae.getSource() == exitItem)
  {
    //process exit
  }
}

---

Something like that? If not, maybe CB will help you.

I fail to see the problem. I suggest you do a Swing tutorial to learn how listeners work. They're practically identical to normal callbacks.

For the moment I have created a mini interface like a listener to pass to the MenuItem object as a callback. I did have a quick look at swing, but decided against it for this as I only need a one callback function and given the time constraints I have on this project I would not have time to fully understand it before using it.

Code:

---

interface MenuItemCallback
{
        public void callback();
}

public class MenuItem
{
        private Menu owner;
        private Menu subMenu;
        private String displayName;
        private char accessChar;
        private MenuItemCallback callback;
       
        MenuItem(Menu owner, String displayName, char accessChar)
        {
                this(owner, displayName, accessChar, (Menu) null, (MenuItemCallback) null);
        }
       
        MenuItem(Menu owner, String displayName, char accessChar, Menu subMenu)
        {
                this(owner, displayName, accessChar, subMenu, (MenuItemCallback) null);
        }


        MenuItem(Menu owner, String displayName, char accessChar, MenuItemCallback callback)
        {
                this(owner, displayName, accessChar, (Menu) null, callback);
        }
       
        MenuItem(Menu owner, String displayName, char accessChar, Menu subMenu, MenuItemCallback callback)
        {
                this.owner = owner;
                this.displayName = displayName;
                this.accessChar = accessChar;
                this.subMenu = subMenu;
                this.callback = callback;
        }

        .....
}

---

Alright. The short version.

This hypothetical code (C function pointer syntax, disregarding the this pointer problem):

Code:

---

class WithCallback
{
  private void (*actionPerformedHandler)(ActionEvent);

  public void setCallback(void (*handler)(ActionEvent)) {
    actionPerformedHandler = handler;
  }

  protected void fireCallback()  {
    actionPerformedHandler(new ActionEvent(this));
  }
}

class SuppliesCallback
{
  public void onClick(ActionEvent e) {
    // Whatever
  }

  public void init() {
    wantsCallback.setCallback(onClick);
  }
}

---

is conceptually equivalent to this code:

Code:

---

interface ActionListener
{
  public void actionPerformed(ActionEvent e);
}

class WithCallback
{
  private ActionListener actionPerformedHandler;

  public void setCallback(ActionListener handler) {
    actionPerformedHandler = handler;
  }

  protected void fireCallback()  {
    actionPerformedHandler.actionPerformed(new ActionEvent(this));
  }
}

class SuppliesCallback
{
  public void onClick(ActionEvent e) {
    // Whatever
  }

  public void init() {
    wantsCallback.setCallback(new ActionListener() {
      public void actionPerformed(ActionEvent e) {
        onClick(e);
      }
    });
  }
}

---