@Override
        
        /**
         * A button has been clicked. 
         */
        public void actionPerformed( ActionEvent e ) {
            
            // get the action string
            String action = e.getActionCommand();
            
            // the user has chosen to add a file
            if( action.equalsIgnoreCase( ADD_FILE ) ) {
                addFileToIndex();
            }
            // the user has chosen to show files parsed
            else if( action.equalsIgnoreCase( SHOW_FILES ) ) {
                showFiles();
            }
            // the user has chosen to remove a file
            else if( action.equalsIgnoreCase( REMOVE_FILE ) ) {
                removeFile();
            }
            // the user has chosen to list the index
            else if( action.equalsIgnoreCase( LIST_INDEX ) ) {
                listIndex();
            }
            // the user has chosen to search the index
            else if( action.equals( SEARCH_INDEX ) ) {
                searchIndex();
            }
            // the user has chosen to exit the program
            else if( action.equalsIgnoreCase( EXIT_PROGRAM ) ) {
                exitProgram();
            }
        }