@Override
    public void actionPerformed(ActionEvent e) {
        if (isGameOver) {
            return;
        }
        JButton currentcell = (JButton) e.getSource();//e in typecasted to jbutton type
        if (currentcell.getText().length() > 0) {
            return;
        }
        currentcell.setText(currentPlayer);
        if (hasWon(currentPlayer)) {
            isGameOver = true;
            JOptionPane.showMessageDialog(this, currentPlayer + " has won.");

            return;
        }
        currentPlayer = currentPlayer.equals(PLAYER1)
                ? PLAYER2
                : PLAYER1;

    }