When an item is set on the system clipboard, it is possible to be notified when that item is no longer on the clipboard. This is done by including a clipboard owner object when setting the item. 
    // This class serves as the clipboard owner.
    class MyClipboardOwner implements ClipboardOwner {
        // This method is called when this object is no longer
        // the owner of the item on the system clipboard.
        public void lostOwnership(Clipboard clipboard, Transferable contents) {
            // To retrieve the contents, see
            // e637 Getting and Setting Text on the System Clipboard
        }
    }

Here's some code that uses the clipboard owner: 
    // Create a clipboard owner
    ClipboardOwner owner = new MyClipboardOwner();
    
    // Set a string on the system clipboard and include the owner object
    StringSelection ss = new StringSelection("A String");
    Toolkit.getDefaultToolkit().getSystemClipboard().setContents(ss, owner);