calling a method when content of clipboard is changed

You can call Clipboard.addFlavorListener to listen for clipboard updates from the OS:

Toolkit.getDefaultToolkit().getSystemClipboard().addFlavorListener(new FlavorListener() { 
   @Override 
   public void flavorsChanged(FlavorEvent e) {

      System.out.println("ClipBoard UPDATED: " + e.getSource() + " " + e.toString());
   } 
}); 

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Some Side Notes:

• For launching your application, consider using initial threads.
• Call JFrame.pack to set the frame size.
• Key Bindings are preferred over KeyListeners for mapping KeyEvents in Swing.

I use this. The whole class.

public class ClipBoardListener extends Thread implements ClipboardOwner{
Clipboard sysClip = Toolkit.getDefaultToolkit().getSystemClipboard();  


    @Override
  public void run() {
    Transferable trans = sysClip.getContents(this);  
    TakeOwnership(trans);       

  }  

    @Override
  public void lostOwnership(Clipboard c, Transferable t) {  

  try {  
    ClipBoardListener.sleep(250);  //waiting e.g for loading huge elements like word's etc.
  } catch(Exception e) {  
    System.out.println("Exception: " + e);  
  }  
  Transferable contents = sysClip.getContents(this);  
    try {
        process_clipboard(contents, c);
    } catch (Exception ex) {
        Logger.getLogger(ClipBoardListener.class.getName()).log(Level.SEVERE, null, ex);
    }
  TakeOwnership(contents);


}  

  void TakeOwnership(Transferable t) {  
    sysClip.setContents(t, this);  
  }  

public void process_clipboard(Transferable t, Clipboard c) { //your implementation
    String tempText;
    Transferable trans = t;

    try {
        if (trans != null?trans.isDataFlavorSupported(DataFlavor.stringFlavor):false) {
            tempText = (String) trans.getTransferData(DataFlavor.stringFlavor);
            System.out.println(tempText);  
        }

    } catch (Exception e) {
    }
}

}

When other program takes ownership of the clipboard it waits 250 ms and takes back clipboard's ownership with updated content.