如何检查此JButton变量是否已设置？

Question

我正在尝试使用Swing在Java中制作一个内存游戏。但是我被困住了，因为我不知道如何检查是否设置了previousButton变量。

我尝试检查文本是否为该变量的空白，但是它给了我NullPointerException。我知道为什么了，因为它正在尝试检查不存在的JButton的文本。

        @Override
        public void actionPerformed(ActionEvent e) {
            if (e.getSource() == gameView.button1) {
                currentButton = gameView.button1;
                gameView.setFrontOfCard(gameView.button1);
            } else if (e.getSource() == gameView.button2) {
                currentButton = gameView.button2;
                gameView.setFrontOfCard(gameView.button2);
                System.out.println(e.getActionCommand());
            } else if (e.getSource() == gameView.button3) {
                currentButton = gameView.button3;
                gameView.setFrontOfCard(gameView.button3);
                System.out.println(e.getActionCommand());
            } else if (e.getSource() == gameView.button4) {
                currentButton = gameView.button4;
                gameView.setFrontOfCard(gameView.button4);
                System.out.println(e.getActionCommand());
            } else if (e.getSource() == gameView.button5) {
                currentButton = gameView.button5;
                gameView.setFrontOfCard(gameView.button5);
                System.out.println(e.getActionCommand());
            } else if (e.getSource() == gameView.button6) {
                currentButton = gameView.button6;
                gameView.setFrontOfCard(gameView.button6);
                System.out.println(e.getActionCommand());
            } else if (e.getSource() == gameView.button7) {
                currentButton = gameView.button7;
                gameView.setFrontOfCard(gameView.button7);
                System.out.println(e.getActionCommand());
            } else if (e.getSource() == gameView.button8) {
                currentButton = gameView.button8;
                gameView.setFrontOfCard(gameView.button8);
                System.out.println(e.getActionCommand());
            } else if (e.getSource() == gameView.button9) {
                currentButton = gameView.button9;
                gameView.setFrontOfCard(gameView.button9);
                System.out.println(e.getActionCommand());
            } else if (e.getSource() == gameView.button10) {
                currentButton = gameView.button10;
                gameView.setFrontOfCard(gameView.button10);
                System.out.println(e.getActionCommand());
            } else if (e.getSource() == gameView.button11) {
                currentButton = gameView.button11;
                gameView.setFrontOfCard(gameView.button11);
                System.out.println(e.getActionCommand());
            } else if (e.getSource() == gameView.button12) {
                currentButton = gameView.button12;
                gameView.setFrontOfCard(gameView.button12);
                System.out.println(e.getActionCommand());
            } else if (e.getSource() == gameView.button13) {
                currentButton = gameView.button13;
                gameView.setFrontOfCard(gameView.button13);
                System.out.println(e.getActionCommand());
            } else if (e.getSource() == gameView.button14) {
                currentButton = gameView.button14;
                gameView.setFrontOfCard(gameView.button14);
                System.out.println(e.getActionCommand());
            } else if (e.getSource() == gameView.button15) {
                currentButton = gameView.button15;
                gameView.setFrontOfCard(gameView.button15);
                System.out.println(e.getActionCommand());
            } else if (e.getSource() == gameView.button16) {
                currentButton = gameView.button16;
                gameView.setFrontOfCard(gameView.button16);
                System.out.println(e.getActionCommand());
            }

            if (currentButton == previousButton) {
                gameView.setScoreLabel(gameModel.incrementScore());

            } else if (!previousButton.getText().isEmpty()) {
                gameView.setBackOfCard(currentButton);
                gameView.setBackOfCard(previousButton);
            }

            previousButton = currentButton;
        }
    }

我希望它检查前一个按钮是否不为空，因此我可以“转动”卡。但这给了我NullPointerException。